XML Schema Part 1: Structures Second Edition (original) (raw)

1 Introduction

This document sets out the structural part (XML Schema: Structures) of the XML Schema definition language.

Chapter 2 presents a Conceptual Framework (§2) for XML Schemas, including an introduction to the nature of XML Schemas and an introduction to the XML Schema abstract data model, along with other terminology used throughout this document.

Chapter 3, Schema Component Details (§3), specifies the precise semantics of each component of the abstract model, the representation of each component in XML, with reference to a DTD and XML Schema for an XML Schema document type, along with a detailed mapping between the elements and attribute vocabulary of this representation and the components and properties of the abstract model.

Chapter 4 presents Schemas and Namespaces: Access and Composition (§4), including the connection between documents and schemas, the import, inclusion and redefinition of declarations and definitions and the foundations of schema-validity assessment.

Chapter 5 discusses Schemas and Schema-validity Assessment (§5), including the overall approach to schema-validity assessment of documents, and responsibilities of schema-aware processors.

The normative appendices include a Schema for Schemas (normative) (§A) for the XML representation of schemas andReferences (normative) (§B).

The non-normative appendices include the DTD for Schemas (non-normative) (§G) and a Glossary (non-normative) (§F).

This document is primarily intended as a language definition reference. As such, although it contains a few examples, it is not primarily designed to serve as a motivating introduction to the design and its features, or as a tutorial for new users. Rather it presents a careful and fully explicit definition of that design, suitable for guiding implementations. For those in search of a step-by-step introduction to the design, the non-normative [XML Schema: Primer] is a much better starting point than this document.

1.1 Purpose

The purpose of XML Schema: Structures is to define the nature of XML schemas and their component parts, provide an inventory of XML markup constructs with which to represent schemas, and define the application of schemas to XML documents.

The purpose of an XML Schema: Structures schema is to define and describe a class of XML documents by using schema components to constrain and document the meaning, usage and relationships of their constituent parts: datatypes, elements and their content and attributes and their values. Schemas may also provide for the specification of additional document information, such as normalization and defaulting of attribute and element values. Schemas have facilities for self-documentation. Thus, XML Schema: Structures can be used to define, describe and catalogue XML vocabularies for classes of XML documents.

Any application that consumes well-formed XML can use the XML Schema: Structures formalism to express syntactic, structural and value constraints applicable to its document instances. The XML Schema: Structures formalism allows a useful level of constraint checking to be described and implemented for a wide spectrum of XML applications. However, the language defined by this specification does not attempt to provide_all_ the facilities that might be needed by any application. Some applications may require constraint capabilities not expressible in this language, and so may need to perform their own additional validations.

1.3 Documentation Conventions and Terminology

The section introduces the highlighting and typography as used in this document to present technical material.

Special terms are defined at their point of introduction in the text. For example [Definition:] a term is something used with a special meaning. The definition is labeled as such and the term it defines is displayed in boldface. The end of the definition is not specially marked in the displayed or printed text. Uses of defined terms are links to their definitions, set off with middle dots, for instance ·term·.

Non-normative examples are set off in boxes and accompanied by a brief explanation:

And an explanation of the example.

The definition of each kind of schema component consists of a list of its properties and their contents, followed by descriptions of the semantics of the properties:

References to properties of schema components are links to the relevant definition as exemplified above, set off with curly braces, for instance {example property}.

The correspondence between an element information item which is part of the XML representation of a schema and one or more schema components is presented in a tableau which illustrates the element information item(s) involved. This is followed by a tabulation of the correspondence between properties of the component and properties of the information item. Where context may determine which of several different components may arise, several tabulations, one per context, are given. The property correspondences are normative, as are the illustrations of the XML representation element information items.

In the XML representation, bold-face attribute names (e.g. count below) indicate a required attribute information item, and the rest are optional. Where an attribute information item has an enumerated type definition, the values are shown separated by vertical bars, as forsize below; if there is a default value, it is shown following a colon. Where an attribute information item has a built-in simple type definition defined in [XML Schemas: Datatypes], a hyperlink to its definition therein is given.

The allowed content of the information item is shown as a grammar fragment, using the Kleene operators ?,* and +. Each element name therein is a hyperlink to its own illustration.

Note: The illustrations are derived automatically from the Schema for Schemas (normative) (§A). In the case of apparent conflict, the Schema for Schemas (normative) (§A) takes precedence, as it, together with the ·Schema Representation Constraints·, provide the normative statement of the form of XML representations.

<example
**count** = integer
size = (large | medium | small) : medium>
Content: (all | any*)

References to elements in the text are links to the relevant illustration as exemplified above, set off with angle brackets, for instance .

References to properties of information items as defined in [XML-Infoset] are notated as links to the relevant section thereof, set off with square brackets, for example [children].

Properties which this specification defines for information items are introduced as follows:

References to properties of information items defined in this specification are notated as links to their introduction as exemplified above, set off with square brackets, for example [new property].

The following highlighting is used for non-normative commentary in this document:

Note: General comments directed to all readers.

Following [XML 1.0 (Second Edition)], within normative prose in this specification, the words_may_ and must are defined as follows:

may

Conforming documents and XML Schema-aware processors are permitted to but need not behave as described.

must

Conforming documents and XML Schema-aware processors are required to behave as described; otherwise they are in error.

Note however that this specification provides a definition of error and of conformant processors' responsibilities with respect to errors (see Schemas and Schema-validity Assessment (§5)) which is considerably more complex than that of [XML 1.0 (Second Edition)].

2 Conceptual Framework

This chapter gives an overview of XML Schema: Structures at the level of its abstract data model. Schema Component Details (§3) provides details on this model, including a normative representation in XML for the components of the model. Readers interested primarily in learning to write schema documents may wish to first read [XML Schema: Primer] for a tutorial introduction, and only then consult the sub-sections of Schema Component Details (§3) named XML Representation of ... for the details.

2.1 Overview of XML Schema

An XML Schema consists of components such as type definitions and element declarations. These can be used to assess the validity of well-formed element and attribute information items (as defined in [XML-Infoset]), and furthermore may specify augmentations to those items and their descendants. This augmentation makes explicit information which may have been implicit in the original document, such as normalized and/or default values for attributes and elements and the types of element and attribute information items. [Definition:] We refer to the augmented infoset which results from conformant processing as defined in this specification as the post-schema-validation infoset, or PSVI.

Schema-validity assessment has two aspects:

1 Determining local schema-validity, that is whether an element or attribute information item satisfies the constraints embodied in the relevant components of an XML Schema;

2 Synthesizing an overall validation outcome for the item, combining local schema-validity with the results of schema-validity assessments of its descendants, if any, and adding appropriate augmentations to the infoset to record this outcome.

Throughout this specification, [Definition:] the word valid and its derivatives are used to refer to clause 1 above, the determination of local schema-validity.

Throughout this specification, [Definition:] the word assessment is used to refer to the overall process of local validation, schema-validity assessment and infoset augmentation.

2.2 XML Schema Abstract Data Model

This specification builds on [XML 1.0 (Second Edition)] and[XML-Namespaces]. The concepts and definitions used herein regarding XML are framed at the abstract level of information items as defined in [XML-Infoset]. By definition, this use of the infoset provides a priori guarantees of well-formedness(as defined in [XML 1.0 (Second Edition)]) and namespace conformance (as defined in [XML-Namespaces]) for all candidates for ·assessment· and for all ·schema documents·.

Just as [XML 1.0 (Second Edition)] and[XML-Namespaces] can be described in terms of information items, XML Schemas can be described in terms of an abstract data model. In defining XML Schemas in terms of an abstract data model, this specification rigorously specifies the information which must be available to a conforming XML Schema processor. The abstract model for schemas is conceptual only, and does not mandate any particular implementation or representation of this information. To facilitate interoperation and sharing of schema information, a normative XML interchange format for schemas is provided.

[Definition:] Schema component is the generic term for the building blocks that comprise the abstract data model of the schema. [Definition:] An XML Schema is a set of ·schema components·. There are 13 kinds of component in all, falling into three groups. The primary components, which may (type definitions) or must (element and attribute declarations) have names are as follows:

• Simple type definitions
• Complex type definitions
• Attribute declarations
• Element declarations

The secondary components, which must have names, are as follows:

• Attribute group definitions
• Identity-constraint definitions
• Model group definitions
• Notation declarations

Finally, the "helper" components provide small parts of other components; they are not independent of their context:

• Annotations
• Model groups
• Particles
• Wildcards
• Attribute Uses

During ·validation·, [Definition:] declaration components are associated by (qualified) name to information items being ·validated·.

On the other hand, [Definition:] definition components define internal schema components that can be used in other schema components.

[Definition:] Declarations and definitions may have and be identified by names, which are NCNames as defined by [XML-Namespaces].

[Definition:] Several kinds of component have a target namespace, which is either·absent· or a namespace name, also as defined by [XML-Namespaces]. The ·target namespace· serves to identify the namespace within which the association between the component and its name exists. In the case of declarations, this in turn determines the namespace name of, for example, the element information items it may ·validate·.

Note: At the abstract level, there is no requirement that the components of a schema share a·target namespace·. Any schema for use in·assessment· of documents containing names from more than one namespace will of necessity include components with different ·target namespaces·. This contrasts with the situation at the level of the XML representation of components, in which each schema document contributes definitions and declarations to a single target namespace.

·Validation·, defined in detail in Schema Component Details (§3), is a relation between information items and schema components. For example, an attribute information item may ·validate· with respect to an attribute declaration, a list of element information items may ·validate· with respect to a content model, and so on. The following sections briefly introduce the kinds of components in the schema abstract data model, other major features of the abstract model, and how they contribute to ·validation·.

2.2.1 Type Definition Components

The abstract model provides two kinds of type definition component: simple and complex.

[Definition:] This specification uses the phrase type definition in cases where no distinction need be made between simple and complex types.

Type definitions form a hierarchy with a single root. The subsections below first describe characteristics of that hierarchy, then provide an introduction to simple and complex type definitions themselves.

2.2.1.1 Type Definition Hierarchy

[Definition:] Except for a distinguished ·ur-type definition·, every ·type definition· is, by construction, either a·restriction· or an ·extension· of some other type definition. The graph of these relationships forms a tree known as the Type Definition Hierarchy.

[Definition:] A type definition whose declarations or facets are in a one-to-one relation with those of another specified type definition, with each in turn restricting the possibilities of the one it corresponds to, is said to be a restriction. The specific restrictions might include narrowed ranges or reduced alternatives. Members of a type, A, whose definition is a ·restriction· of the definition of another type, B, are always members of type B as well.

[Definition:] A complex type definition which allows element or attribute content in addition to that allowed by another specified type definition is said to be an extension.

[Definition:] A distinguished complex type definition, the ur-type definition, whose name is anyType in the XML Schema namespace, is present in each ·XML Schema·, serving as the root of the type definition hierarchy for that schema.

[Definition:] A type definition used as the basis for an ·extension· or·restriction· is known as the base type definition of that definition.

2.2.1.2 Simple Type Definition

A simple type definition is a set of constraints on strings and information about the values they encode, applicable to the ·normalized value· of an attribute information item or of an element information item with no element children. Informally, it applies to the values of attributes and the text-only content of elements.

Each simple type definition, whether built-in (that is, defined in [XML Schemas: Datatypes]) or user-defined, is a ·restriction· of some particular simple ·base type definition·. For the built-in primitive type definitions, this is [Definition:] the simple ur-type definition, a special restriction of the·ur-type definition·, whose name is anySimpleType in the XML Schema namespace. The ·simple ur-type definition· is considered to have an unconstrained lexical space, and a value space consisting of the union of the value spaces of all the built-in primitive datatypes and the set of all lists of all members of the value spaces of all the built-in primitive datatypes.

The mapping from lexical space to value space is unspecified for items whose type definition is the·simple ur-type definition·. Accordingly this specification does not constrain processors' behaviour in areas where this mapping is implicated, for example checking such items against enumerations, constructing default attributes or elements whose declared type definition is the·simple ur-type definition·, checking identity constraints involving such items.

Note: The Working Group expects to return to this area in a future version of this specification.

Simple types may also be defined whose members are lists of items themselves constrained by some other simple type definition, or whose membership is the union of the memberships of some other simple type definitions. Such list and union simple type definitions are also restrictions of the ·simple ur-type definition·.

For detailed information on simple type definitions, see Simple Type Definitions (§3.14) and [XML Schemas: Datatypes]. The latter also defines an extensive inventory of pre-defined simple types.

2.2.1.3 Complex Type Definition

A complex type definition is a set of attribute declarations and a content type, applicable to the [attributes] and[children] of an element information item respectively. The content type may require the [children] to contain neither element nor character information items (that is, to be empty), to be a string which belongs to a particular simple type or to contain a sequence of element information items which conforms to a particular model group, with or without character information items as well.

Each complex type definition other than the·ur-type definition· is either

• a restriction of a complex ·base type definition·

or

• an ·extension· of a simple or complex ·base type definition·.

A complex type which extends another does so by having additional content model particles at the end of the other definition's content model, or by having additional attribute declarations, or both.

Note: This specification allows only appending, and not other kinds of extensions. This decision simplifies application processing required to cast instances from derived to base type. Future versions may allow more kinds of extension, requiring more complex transformations to effect casting.

For detailed information on complex type definitions, see Complex Type Definitions (§3.4).

2.2.2 Declaration Components

There are three kinds of declaration component: element, attribute, and notation. Each is described in a section below. Also included is a discussion of element substitution groups, which is a feature provided in conjunction with element declarations.

2.2.2.2 Element Substitution Group

In XML 1.0, the name and content of an element must correspond exactly to the element type referenced in the corresponding content model.

[Definition:] Through the new mechanism of element substitution groups, XML Schemas provides a more powerful model supporting substitution of one named element for another. Any top-level element declaration can serve as the defining member, or head, for an element substitution group. Other top-level element declarations, regardless of target namespace, can be designated as members of the substitution group headed by this element. In a suitably enabled content model, a reference to the head ·validates· not just the head itself, but elements corresponding to any other member of the substitution group as well.

All such members must have type definitions which are either the same as the head's type definition or restrictions or extensions of it. Therefore, although the names of elements can vary widely as new namespaces and members of the substitution group are defined, the content of member elements is strictly limited according to the type definition of the substitution group head.

Note that element substitution groups are not represented as separate components. They are specified in the property values for element declarations (see Element Declarations (§3.3)).

2.3 Constraints and Validation Rules

The [XML 1.0 (Second Edition)] specification describes two kinds of constraints on XML documents: well-formedness and_validity_ constraints. Informally, the well-formedness constraints are those imposed by the definition of XML itself (such as the rules for the use of the < and > characters and the rules for proper nesting of elements), while validity constraints are the further constraints on document structure provided by a particular DTD.

The preceding section focused on ·validation·, that is the constraints on information items which schema components supply. In fact however this specification provides four different kinds of normative statements about schema components, their representations in XML and their contribution to the·validation· of information items:

Schema Component Constraint

[Definition:] Constraints on the schema components themselves, i.e. conditions components must satisfy to be components at all. Located in the sixth sub-section of the per-component sections of Schema Component Details (§3)and tabulated in Schema Component Constraints (§C.4).

Schema Representation Constraint

[Definition:] Constraints on the representation of schema components in XML beyond those which are expressed in Schema for Schemas (normative) (§A). Located in the third sub-section of the per-component sections of Schema Component Details (§3)and tabulated in Schema Representation Constraints (§C.3).

Validation Rules

[Definition:] Contributions to ·validation· associated with schema components. Located in the fourth sub-section of the per-component sections of Schema Component Details (§3)and tabulated in Validation Rules (§C.1).

Schema Information Set Contribution

[Definition:] Augmentations to ·post-schema-validation infoset·s expressed by schema components, which follow as a consequence of ·validation· and/or ·assessment·. Located in the fifth sub-section of the per-component sections of Schema Component Details (§3)and tabulated in Contributions to the post-schema-validation infoset (§C.2).

The last of these, schema information set contributions, are not as new as they might at first seem. XML 1.0 validation augments the XML 1.0 information set in similar ways, for example by providing values for attributes not present in instances, and by implicitly exploiting type information for normalization or access. (As an example of the latter case, consider the effect of NMTOKENS on attribute white space, and the semantics ofID and IDREF.) By including schema information set contributions, this specification makes explicit some features that XML 1.0 left implicit.

2.4 Conformance

This specification describes three levels of conformance for schema aware processors. The first is required of all processors. Support for the other two will depend on the application environments for which the processor is intended.

[Definition:] Minimally conforming processors must completely and correctly implement the ·Schema Component Constraints·, ·Validation Rules·, and ·Schema Information Set Contributions· contained in this specification.

[Definition:] ·Minimally conforming· processors which accept schemas represented in the form of XML documents as described in Layer 2: Schema Documents, Namespaces and Composition (§4.2) are additionally said to provide conformance to the XML Representation of Schemas.Such processors must, when processing schema documents, completely and correctly implement all ·Schema Representation Constraints· in this specification, and must adhere exactly to the specifications in Schema Component Details (§3) for mapping the contents of such documents to ·schema components· for use in ·validation· and ·assessment·.

Note: By separating the conformance requirements relating to the concrete syntax of XML schema documents, this specification admits processors which use schemas stored in optimized binary representations, dynamically created schemas represented as programming language data structures, or implementations in which particular schemas are compiled into executable code such as C or Java. Such processors can be said to be ·minimally conforming· but not necessarily in ·conformance to the XML Representation of Schemas·.

[Definition:] Fully conformingprocessors are network-enabled processors which are not only both ·minimally conforming· and ·in conformance to the XML Representation of Schemas·, but which additionally must be capable of accessing schema documents from the World Wide Web according to Representation of Schemas on the World Wide Web (§2.7) and How schema definitions are located on the Web (§4.3.2)..

Note: Although this specification provides just these three standard levels of conformance, it is anticipated that other conventions can be established in the future. For example, the World Wide Web Consortium is considering conventions for packaging on the Web a variety of resources relating to individual documents and namespaces. Should such developments lead to new conventions for representing schemas, or for accessing them on the Web, new levels of conformance can be established and named at that time. There is no need to modify or republish this specification to define such additional levels of conformance.

See Schemas and Namespaces: Access and Composition (§4) for a more detailed explanation of the mechanisms supporting these levels of conformance.

2.5 Names and Symbol Spaces

As discussed in XML Schema Abstract Data Model (§2.2), most schema components (may) have ·names·. If all such names were assigned from the same "pool", then it would be impossible to have, for example, a simple type definition and an element declaration both with the name "title" in a given ·target namespace·.

Therefore [Definition:] this specification introduces the termsymbol space to denote a collection of names, each of which is unique with respect to the others. A symbol space is similar to the non-normative concept of namespace partition introduced in [XML-Namespaces]. There is a single distinct symbol space within a given ·target namespace· for each kind of definition and declaration component identified in XML Schema Abstract Data Model (§2.2), except that within a target namespace, simple type definitions and complex type definitions share a symbol space. Within a given symbol space, names are unique, but the same name may appear in more than one symbol space without conflict. For example, the same name can appear in both a type definition and an element declaration, without conflict or necessary relation between the two.

Locally scoped attribute and element declarations are special with regard to symbol spaces. Every complex type definition defines its own local attribute and element declaration symbol spaces, where these symbol spaces are distinct from each other and from any of the other symbol spaces. So, for example, two complex type definitions having the same target namespace can contain a local attribute declaration for the unqualified name "priority", or contain a local element declaration for the name "address", without conflict or necessary relation between the two.

2.6 Schema-Related Markup in Documents Being Validated

The XML representation of schema components uses a vocabulary identified by the namespace name http://www.w3.org/2001/XMLSchema. For brevity, the text and examples in this specification use the prefixxs: to stand for this namespace; in practice, any prefix can be used.

XML Schema: Structures also defines several attributes for direct use in any XML documents. These attributes are in a different namespace, which has the namespace name http://www.w3.org/2001/XMLSchema-instance. For brevity, the text and examples in this specification use the prefixxsi: to stand for this latter namespace; in practice, any prefix can be used. All schema processors have appropriate attribute declarations for these attributes built in, see Attribute Declaration for the 'type' attribute (§3.2.7),Attribute Declaration for the 'nil' attribute (§3.2.7), Attribute Declaration for the 'schemaLocation' attribute (§3.2.7) and Attribute Declaration for the 'noNamespaceSchemaLocation' attribute (§3.2.7).

2.6.1 xsi:type

The Simple Type Definition (§2.2.1.2) or Complex Type Definition (§2.2.1.3) used in ·validation· of an element is usually determined by reference to the appropriate schema components. An element information item in an instance may, however, explicitly assert its type using the attribute xsi:type. The value of this attribute is a ·QName·; see QName Interpretation (§3.15.3) for the means by which the ·QName· is associated with a type definition.

3 Schema Component Details

3.1 Introduction

The following sections provide full details on the composition of all schema components, together with their XML representations and their contributions to ·assessment·. Each section is devoted to a single component, with separate subsections for

1. properties: their values and significance
2. XML representation and the mapping to properties
3. constraints on representation
4. validation rules
5. ·post-schema-validation infoset· contributions
6. constraints on the components themselves

The sub-sections immediately below introduce conventions and terminology used throughout the component sections.

3.1.1 Components and Properties

Components are defined in terms of their properties, and each property in turn is defined by giving its range, that is the values it may have. This can be understood as defining a schema as a labeled directed graph, where the root is a schema, every other vertex is a schema component or a literal (string, boolean, number) and every labeled edge is a property. The graph is not acyclic: multiple copies of components with the same name in the same ·symbol space· may not exist, so in some cases re-entrant chains of properties must exist. Equality of components for the purposes of this specification is always defined as equality of names (including target namespaces) within symbol spaces.

Note: A schema and its components as defined in this chapter are an idealization of the information a schema-aware processor requires: implementations are not constrained in how they provide it. In particular, no implications about literal embedding versus indirection follow from the use below of language such as "properties . . . having . . . components as values".

[Definition:] Throughout this specification, the term absent is used as a distinguished property value denoting absence.

Any property not identified as optional is required to be present; optional properties which are not present are taken to have ·absent· as their value. Any property identified as a having a set, subset or list value may have an empty value unless this is explicitly ruled out: this is not the same as ·absent·. Any property value identified as a superset or subset of some set may be equal to that set, unless a proper superset or subset is explicitly called for. By 'string' in Part 1 of this specification is meant a sequence of ISO 10646 characters identified as legal XML charactersin [XML 1.0 (Second Edition)].

3.1.3 The Mapping between XML Representations and Components

For each kind of schema component there is a corresponding normative XML representation. The sections below describe the correspondences between the properties of each kind of schema component on the one hand and the properties of information items in that XML representation on the other, together with constraints on that representation above and beyond those implicit in theSchema for Schemas (normative) (§A).

The language used is as if the correspondences were mappings from XML representation to schema component, but the mapping in the other direction, and therefore the correspondence in the abstract, can always be constructed therefrom.

In discussing the mapping from XML representations to schema components below, the value of a component property is often determined by the value of an attribute information item, one of the [attributes] of an element information item. Since schema documents are constrained by theSchema for Schemas (normative) (§A), there is always a simple type definition associated with any such attribute information item. [Definition:] The phrase actual value is used to refer to the member of the value space of the simple type definition associated with an attribute information item which corresponds to its ·normalized value·. This will often be a string, but may also be an integer, a boolean, a URI reference, etc. This term is also occasionally used with respect to element or attribute information items in a document being ·validated·.

Many properties are identified below as having other schema components or sets of components as values. For the purposes of exposition, the definitions in this section assume that (unless the property is explicitly identified as optional) all such values are in fact present. When schema components are constructed from XML representations involving reference by name to other components, this assumption may be violated if one or more references cannot be resolved. This specification addresses the matter of missing components in a uniform manner, described in Missing Sub-components (§5.3): no mention of handling missing components will be found in the individual component descriptions below.

Forward reference to named definitions and declarations _is_allowed, both within and between ·schema documents·. By the time the component corresponding to an XML representation which contains a forward reference is actually needed for ·validation· an appropriately-named component may have become available to discharge the reference: see Schemas and Namespaces: Access and Composition (§4) for details.

3.1.4 White Space Normalization during Validation

Throughout this specification, [Definition:] theinitial value of some attribute information item is the value of the[normalized value] property of that item. Similarly, the initial value of an element information item is the string composed of, in order, the[character code] of each character information item in the [children] of that element information item.

The above definition means that comments and processing instructions, even in the midst of text, are ignored for all ·validation· purposes.

[Definition:] Thenormalized value of an element or attribute information item is an ·initial value· whose white space, if any, has been normalized according to the value of the whiteSpace facet of the simple type definition used in its ·validation·:

preserve

No normalization is done, the value is the ·normalized value·

replace

All occurrences of #x9 (tab), #xA (line feed) and#xD (carriage return) are replaced with #x20 (space).

collapse

Subsequent to the replacements specified above under replace, contiguous sequences of #x20s are collapsed to a single#x20, and initial and/or final #x20s are deleted.

If the simple type definition used in an item's ·validation· is the ·simple ur-type definition·, the ·normalized value· must be determined as in the preserve case above.

There are three alternative validation rules which may supply the necessary background for the above: Attribute Locally Valid (§3.2.4) (clause 3), Element Locally Valid (Type) (§3.3.4) (clause 3.1.3) or Element Locally Valid (Complex Type) (§3.4.4) (clause 2.2).

These three levels of normalization correspond to the processing mandated in XML 1.0 for element content, CDATA attribute content and tokenized attributed content, respectively. See Attribute Value Normalization in [XML 1.0 (Second Edition)] for the precedent for replace and collapse for attributes. Extending this processing to element content is necessary to ensure a consistent ·validation· semantics for simple types, regardless of whether they are applied to attributes or elements. Performing it twice in the case of attributes whose [normalized value] has already been subject to replacement or collapse on the basis of information in a DTD is necessary to ensure consistent treatment of attributes regardless of the extent to which DTD-based information has been made use of during infoset construction.

Note: Even when DTD-based information has been appealed to, andAttribute Value Normalization has taken place, the above definition of ·normalized value· may mean further normalization takes place, as for instance when character entity references in attribute values result in white space characters other than spaces in their ·initial value·s.

3.2 Attribute Declarations

Attribute declarations provide for:

• Local ·validation· of attribute information item values using a simple type definition;
• Specifying default or fixed values for attribute information items.

<xs:attribute name="age" type="xs:positiveInteger" use="required"/>

The XML representation of an attribute declaration.

3.2.1 The Attribute Declaration Schema Component

The attribute declaration schema component has the following properties:

The {name} property must match the local part of the names of attributes being ·validated·.

The value of the attribute must conform to the supplied {type definition}.

A non-·absent· value of the {target namespace} property provides for ·validation· of namespace-qualified attribute information items (which must be explicitly prefixed in the character-level form of XML documents). ·Absent· values of{target namespace} ·validate· unqualified (unprefixed) items.

A {scope} of global identifies attribute declarations available for use in complex type definitions throughout the schema. Locally scoped declarations are available for use only within the complex type definition identified by the {scope} property. This property is ·absent· in the case of declarations within attribute group definitions: their scope will be determined when they are used in the construction of complex type definitions.

{value constraint} reproduces the functions of XML 1.0 default and #FIXEDattribute values. default specifies that the attribute is to appear unconditionally in the ·post-schema-validation infoset·, with the supplied value used whenever the attribute is not actually present; fixed indicates that the attribute value if present must equal the supplied constraint value, and if absent receives the supplied value as for_default_. Note that it is values that are supplied and/or checked, not strings.

See Annotations (§3.13) for information on the role of the{annotation} property.

Note: A more complete and formal presentation of the semantics of {name}, {target namespace} and {value constraint} is provided in conjunction with other aspects of complex type ·validation· (see Element Locally Valid (Complex Type) (§3.4.4).)

[XML-Infoset] distinguishes attributes with names such as xmlns or xmlns:xsl from ordinary attributes, identifying them as [namespace attributes]. Accordingly, it is unnecessary and in fact not possible for schemas to contain attribute declarations corresponding to such namespace declarations, see xmlns Not Allowed (§3.2.6). No means is provided in this specification to supply a default value for a namespace declaration.

3.2.2 XML Representation of Attribute Declaration Schema Components

The XML representation for an attribute declaration schema component is an element information item. It specifies a simple type definition for an attribute either by reference or explicitly, and may provide default information. The correspondences between the properties of the information item and properties of the component are as follows:

<attribute
default = string
fixed = string
form = (qualified | unqualified)
id = ID
name = NCName
ref = QName
type = QName
use = (optional | prohibited | required) : optional
_{any attributes with non-schema namespace . . .}_>
Content: (annotation?, simpleType?)

If the element information item has as its parent, the corresponding schema component is as follows:

otherwise if the element information item has or as an ancestor and the ref [attribute] is absent, it corresponds to an attribute use with properties as follows (unless use='prohibited', in which case the item corresponds to nothing at all):

otherwise (the element information item has or as an ancestor and theref [attribute] is present), it corresponds to an attribute use with properties as follows (unless use='prohibited', in which case the item corresponds to nothing at all):

Attribute declarations can appear at the top level of a schema document, or within complex type definitions, either as complete (local) declarations, or by reference to top-level declarations, or within attribute group definitions. For complete declarations, top-level or local, the type attribute is used when the declaration can use a built-in or pre-declared simple type definition. Otherwise an anonymous is provided inline.

The default when no simple type definition is referenced or provided is the ·simple ur-type definition·, which imposes no constraints at all.

Attribute information items ·validated· by a top-level declaration must be qualified with the{target namespace} of that declaration (if this is ·absent·, the item must be unqualified). Control over whether attribute information items·validated· by a local declaration must be similarly qualified or not is provided by the form [attribute], whose default is provided by the attributeFormDefault [attribute] on the enclosing , via its determination of {target namespace}.

The names for top-level attribute declarations are in their own·symbol space·. The names of locally-scoped attribute declarations reside in symbol spaces local to the type definition which contains them.

3.2.5 Attribute Declaration Information Set Contributions

Schema Information Set Contribution: Attribute Validated by Type

If clause 3 of Attribute Locally Valid (§3.2.4) applies with respect to an attribute information item, in the ·post-schema-validation infoset· the attribute information item has a property:

Furthermore, the item has one of the following alternative sets of properties:

Either

or

If the ·type definition· has {variety} union, then calling[Definition:] that member of the {member type definitions} which actually·validated· the attribute item's ·normalized value· theactual member type definition, there are three additional properties:

The first (·item isomorphic·) alternative above is provided for applications such as query processors which need access to the full range of details about an item's·assessment·, for example the type hierarchy; the second, for lighter-weight processors for whom representing the significant parts of the type hierarchy as information items might be a significant burden.

Also, if the declaration has a {value constraint}, the item has a property:

If the attribute information item was not ·strictly assessed·, then instead of the values specified above,

1 The item's [schema normalized value]property has the ·initial value· of the item as its value;

2 The [type definition] and[member type definition] properties, or their alternatives, are based on the ·simple ur-type definition·.

3.3 Element Declarations

Element declarations provide for:

• Local ·validation· of element information item values using a type definition;
• Specifying default or fixed values for an element information items;
• Establishing uniquenesses and reference constraint relationships among the values of related elements and attributes;
• Controlling the substitutability of elements through the mechanism of ·element substitution groups·.

<xs:element name="PurchaseOrder" type="PurchaseOrderType"/>

<xs:element name="gift"> xs:complexType xs:sequence <xs:element name="birthday" type="xs:date"/> <xs:element ref="PurchaseOrder"/>

XML representations of several different types of element declaration

3.3.1 The Element Declaration Schema Component

The element declaration schema component has the following properties:

The {name} property must match the local part of the names of element information items being ·validated·.

A {scope} of global identifies element declarations available for use in content models throughout the schema. Locally scoped declarations are available for use only within the complex type identified by the {scope} property. This property is ·absent· in the case of declarations within named model groups: their scope is determined when they are used in the construction of complex type definitions.

A non-·absent· value of the {target namespace} property provides for ·validation· of namespace-qualified element information items. ·Absent· values of{target namespace} ·validate· unqualified items.

An element information item is ·valid·if it satisfies the {type definition}. For such an item, schema information set contributions appropriate to the {type definition} are added to the corresponding element information item in the ·post-schema-validation infoset·.

If {nillable} is true, then an element may also be ·valid· if it carries the namespace qualified attribute with [local name] nil from namespace http://www.w3.org/2001/XMLSchema-instance and value true (see xsi:nil (§2.6.2)) even if it has no text or element content despite a {content type} which would otherwise require content. Formal details of element ·validation· are described in Element Locally Valid (Element) (§3.3.4).

{value constraint} establishes a default or fixed value for an element. If default is specified, and if the element being ·validated· is empty, then the canonical form of the supplied constraint value becomes the [schema normalized value] of the ·validated· element in the ·post-schema-validation infoset·. If fixed is specified, then the element's content must either be empty, in which case fixed behaves as default, or its value must match the supplied constraint value.

Note: The provision of defaults for elements goes beyond what is possible in XML 1.0 DTDs, and does not exactly correspond to defaults for attributes. In particular, an element with a non-empty {value constraint} whose simple type definition includes the empty string in its lexical space will nonetheless never receive that value, because the {value constraint} will override it.

{identity-constraint definitions} express constraints establishing uniquenesses and reference relationships among the values of related elements and attributes. See Identity-constraint Definitions (§3.11).

Element declarations are potential members of the substitution group, if any, identified by {substitution group affiliation}. Potential membership is transitive but not symmetric; an element declaration is a potential member of any group of which its {substitution group affiliation} is a potential member. Actual membership may be blocked by the effects of {substitution group exclusions} or {disallowed substitutions}, see below.

An empty {substitution group exclusions} allows a declaration to be nominated as the {substitution group affiliation} of other element declarations having the same {type definition} or types derived therefrom. The explicit values of {substitution group exclusions} rule out element declarations having types which are _extension_s or _restriction_s respectively of {type definition}. If both values are specified, then the declaration may not be nominated as the{substitution group affiliation} of any other declaration.

The supplied values for {disallowed substitutions} determine whether an element declaration appearing in a ·content model· will be prevented from additionally·validating· elements (a) with an xsi:type (§2.6.1) that identifies an_extension_ or restriction of the type of the declared element, and/or (b) from ·validating· elements which are in the substitution group headed by the declared element. If {disallowed substitutions} is empty, then all derived types and substitution group members are allowed.

Element declarations for which {abstract} is true can appear in content models only when substitution is allowed; such declarations may not themselves ever be used to ·validate· element content.

See Annotations (§3.13) for information on the role of the{annotation} property.

3.3.2 XML Representation of Element Declaration Schema Components

The XML representation for an element declaration schema component is an element information item. It specifies a type definition for an element either by reference or explicitly, and may provide occurrence and default information. The correspondences between the properties of the information item and properties of the component(s) it corresponds to are as follows:

<element
abstract = boolean : false
block = (#all | List of (extension | restriction | substitution))
default = string
final = (#all | List of (extension | restriction))
fixed = string
form = (qualified | unqualified)
id = ID
maxOccurs = (nonNegativeInteger | unbounded) : 1
minOccurs = nonNegativeInteger : 1
name = NCName
nillable = boolean : false
ref = QName
substitutionGroup = QName
type = QName
_{any attributes with non-schema namespace . . .}_>
Content: (annotation?, ((simpleType | complexType)?, (unique | key | keyref)*))

If the element information item has as its parent, the corresponding schema component is as follows:

otherwise if the element information item has or as an ancestor and theref [attribute] is absent, the corresponding schema components are as follows (unless minOccurs=maxOccurs=0, in which case the item corresponds to no component at all):

An element declaration as in the first case above, with the exception of its {target namespace} and {scope} properties, which are as below:

otherwise (the element information item has or as an ancestor and theref [attribute] is present), the corresponding schema component is as follows (unless minOccurs=maxOccurs=0, in which case the item corresponds to no component at all):

corresponds to an element declaration, and allows the type definition of that declaration to be specified either by reference or by explicit inclusion.

s within produce_global_ element declarations; s within or produce either particles which contain global element declarations (if there's a ref attribute) or local declarations (otherwise). For complete declarations, top-level or local, the type attribute is used when the declaration can use a built-in or pre-declared type definition. Otherwise an anonymous or is provided inline.

Element information items ·validated· by a top-level declaration must be qualified with the{target namespace} of that declaration (if this is ·absent·, the item must be unqualified). Control over whether element information items ·validated· by a local declaration must be similarly qualified or not is provided by the form [attribute], whose default is provided by the elementFormDefault [attribute] on the enclosing , via its determination of {target namespace}.

As noted above the names for top-level element declarations are in a separate·symbol space· from the symbol spaces for the names of type definitions, so there can (but need not be) a simple or complex type definition with the same name as a top-level element. As with attribute names, the names of locally-scoped element declarations with no {target namespace} reside in symbol spaces local to the type definition which contains them.

Note that the above allows for two levels of defaulting for unspecified type definitions. An with no referenced or included type definition will correspond to an element declaration which has the same type definition as the head of its substitution group if it identifies one, otherwise the ·ur-type definition·. This has the important consequence that the minimum valid element declaration, that is, one with only a name attribute and no contents, is also (nearly) the most general, validating any combination of text and element content and allowing any attributes, and providing for recursive validation where possible.

See below at XML Representation of Identity-constraint Definition Schema Components (§3.11.2) for , and .

<xs:element name="unconstrained"/>

<xs:element name="emptyElt"> xs:complexType <xs:attribute ...>. . .

<xs:element name="contextOne"> xs:complexType xs:sequence <xs:element name="myLocalElement" type="myFirstType"/> <xs:element ref="globalElement"/>

<xs:element name="contextTwo"> xs:complexType xs:sequence <xs:element name="myLocalElement" type="mySecondType"/> <xs:element ref="globalElement"/>

The first example above declares an element whose type, by default, is the·ur-type definition·. The second uses an embedded anonymous complex type definition.

The last two examples illustrate the use of local element declarations. Instances of myLocalElement withincontextOne will be constrained by myFirstType, while those within contextTwo will be constrained bymySecondType.

Note: The possibility that differing attribute declarations and/or content models would apply to elements with the same name in different contexts is an extension beyond the expressive power of a DTD in XML 1.0.

<xs:complexType name="facet"> xs:complexContent <xs:extension base="xs:annotated"> <xs:attribute name="value" use="required"/>

<xs:element name="facet" type="xs:facet" abstract="true"/>

<xs:element name="encoding" substitutionGroup="xs:facet"> xs:complexType xs:complexContent <xs:restriction base="xs:facet"> xs:sequence <xs:element ref="annotation" minOccurs="0"/> <xs:attribute name="value" type="xs:encodings"/>

<xs:element name="period" substitutionGroup="xs:facet"> xs:complexType xs:complexContent <xs:restriction base="xs:facet"> xs:sequence <xs:element ref="annotation" minOccurs="0"/> <xs:attribute name="value" type="xs:duration"/>

<xs:complexType name="datatype"> xs:sequence <xs:element ref="facet" minOccurs="0" maxOccurs="unbounded"/> <xs:attribute name="name" type="xs:NCName" use="optional"/> . . .

An example from a previous version of the schema for datatypes. Thefacet type is defined and the facet element is declared to use it. The facet element is abstract -- it's_only_ defined to stand as the head for a substitution group. Two further elements are declared, each a member of the facet substitution group. Finally a type is defined which refers to facet, thereby allowing either period or encoding (or any other member of the group).

3.3.3 Constraints on XML Representations of Element Declarations

Schema Representation Constraint: Element Declaration Representation OK

In addition to the conditions imposed on element information items by the schema for schemas:all of the following must be true:

1 default and fixed must not both be present.

2 If the item's parent is not , thenall of the following must be true:

2.1 One of ref or name must be present, but not both.

2.2 If ref is present, then all of ,, , ,, nillable, default,fixed, form, block and type must be absent, i.e. only minOccurs, maxOccurs, id are allowed in addition to ref, along with .

3 type and either or are mutually exclusive.

4 The corresponding particle and/or element declarations must satisfy the conditions set out in Constraints on Element Declaration Schema Components (§3.3.6) and Constraints on Particle Schema Components (§3.9.6).

3.3.4 Element Declaration Validation Rules

Validation Rule: Element Locally Valid (Element)

For an element information item to be locally ·valid· with respect to an element declarationall of the following must be true:

1 The declaration must not be ·absent·.

2 Its{abstract} must be false.

3 The appropriate case among the following must be true:

3.1 If {nillable} is false, then there must be no attribute information item among the element information item's [attributes] whose [namespace name] is identical to http://www.w3.org/2001/XMLSchema-instance and whose [local name] is nil.

3.2 If {nillable} is true and there is such an attribute information item and its ·actual value· is true , then all of the following must be true:

3.2.1 The element information item must have no character or element information item[children].

3.2.2 There must be no fixed {value constraint}.

4 If there is an attribute information item among the element information item's [attributes] whose [namespace name] is identical to http://www.w3.org/2001/XMLSchema-instance and whose [local name] is type, thenall of the following must be true:

4.1 The ·normalized value· of that attribute information item must be·valid· with respect to the built-in QName simple type, as defined by String Valid (§3.14.4);

4.2 The ·local name· and ·namespace name· (as defined in QName Interpretation (§3.15.3)), of the ·actual value· of that attribute information item must resolve to a type definition, as defined in QName resolution (Instance) (§3.15.4) -- [Definition:] call this type definition the local type definition;

4.3 The ·local type definition· must be validly derived from the {type definition} given the union of the {disallowed substitutions} and the {type definition}'s {prohibited substitutions}, as defined in Type Derivation OK (Complex) (§3.4.6) (if it is a complex type definition), or given {disallowed substitutions} as defined in Type Derivation OK (Simple) (§3.14.6) (if it is a simple type definition).

[Definition:] The phraseactual type definition occurs below. If the above three clauses are satisfied, this should be understood as referring to the ·local type definition·, otherwise to the {type definition}.

5 The appropriate case among the following must be true:

5.1 If the declaration has a {value constraint}, the item has neither element nor character [children] and clause 3.2 has not applied, then all of the following must be true:

5.2 If the declaration has no {value constraint} or the item has either element or character [children] or clause 3.2 has applied, then all of the following must be true:

5.2.1 The element information item must be ·valid· with respect to the ·actual type definition· as defined by Element Locally Valid (Type) (§3.3.4).

5.2.2 If there is a fixed {value constraint} and clause 3.2 has not applied,all of the following must be true:

5.2.2.1 The element information item must have no element information item [children].

5.2.2.2 The appropriate case among the following must be true:

6 The element information item must be ·valid· with respect to each of the {identity-constraint definitions} as per Identity-constraint Satisfied (§3.11.4).

7 If the element information item is the ·validation root·, it must be ·valid· per Validation Root Valid (ID/IDREF) (§3.3.4).

Validation Rule: Element Locally Valid (Type)

For an element information item to be locally ·valid· with respect to a type definitionall of the following must be true:

1 The type definition must not be ·absent·;

2 It must not have{abstract} with value true.

3 The appropriate case among the following must be true:

3.1 If the type definition is a simple type definition, then all of the following must be true:

3.1.1 The element information item's [attributes] must be empty, excepting those whose [namespace name] is identical to http://www.w3.org/2001/XMLSchema-instance and whose [local name] is one of type, nil, schemaLocation or noNamespaceSchemaLocation.

3.1.2 The element information item must have no element information item [children].

3.1.3 If clause 3.2 of Element Locally Valid (Element) (§3.3.4) did not apply, then the ·normalized value· must be ·valid· with respect to the type definition as defined by String Valid (§3.14.4).

3.2 If the type definition is a complex type definition, then the element information item must be ·valid· with respect to the type definition as per Element Locally Valid (Complex Type) (§3.4.4);

Validation Rule: Validation Root Valid (ID/IDREF)

For an element information item which is the ·validation root· to be ·valid· all of the following must be true:

1 There must be no ID/IDREF binding in the item's[ID/IDREF table] whose [binding] is the empty set.

2 There must be no ID/IDREF binding in the item's [ID/IDREF table] whose [binding] has more than one member.

See ID/IDREF Table (§3.15.5) for the definition of ID/IDREF binding.

Note: The first clause above applies when there is a reference to an undefined ID. The second applies when there is a multiply-defined ID. They are separated out to ensure that distinct error codes (see Outcome Tabulations (normative) (§C)) are associated with these two cases.

Note: Although this rule applies at the ·validation root·, in practice processors, particularly streaming processors, may wish to detect and signal the clause 2 case as it arises.

Note: This reconstruction of [XML 1.0 (Second Edition)]'s ID/IDREFfunctionality is imperfect in that if the ·validation root· is not the document element of an XML document, the results will not necessarily be the same as those a validating parser would give were the document to have a DTD with equivalent declarations.

Validation Rule: Schema-Validity Assessment (Element)

The schema-validity assessment of an element information item depends on its ·validation· and the ·assessment· of its element information item children and associated attribute information items, if any.

So for an element information item's schema-validity to be assessed all of the following must be true:

1 One of the following must be true:

1.1 All of the following must be true:

1.1.1 A non-·absent· element declaration must be known for it, becauseone of the following is true

1.1.1.1 A declaration was stipulated by the processor (see Assessing Schema-Validity (§5.2)).

1.1.1.2 A declaration has been established as its ·context-determined declaration·.

1.1.1.3 All of the following must be true:

1.1.1.3.1 Its ·context-determined declaration· is not skip.

1.1.2 Its ·validity· with respect to that declaration must have been evaluated as per Element Locally Valid (Element) (§3.3.4).

1.1.3 If that evaluation involved the evaluation of Element Locally Valid (Type) (§3.3.4), clause 1 thereof must be satisfied.

1.2 All of the following must be true:

1.2.1 A non-·absent· type definition is known for it because one of the following is true

1.2.1.1 A type definition was stipulated by the processor (see Assessing Schema-Validity (§5.2)).

1.2.1.2 All of the following must be true:

1.2.1.2.1 There is an attribute information item among the element information item's [attributes] whose [namespace name] is identical to http://www.w3.org/2001/XMLSchema-instance and whose [local name] is type.

1.2.1.2.2 The ·normalized value· of that attribute information item is·valid· with respect to the built-in QName simple type, as defined by String Valid (§3.14.4).

1.2.1.2.3 The ·local name· and ·namespace name· (as defined in QName Interpretation (§3.15.3)), of the ·actual value· of that attribute information item resolve to a type definition, as defined in QName resolution (Instance) (§3.15.4) -- [Definition:] call this type definition the local type definition.

1.2.1.2.4 If there is also a processor-stipulated type definition, the ·local type definition· must be validly derived from that type definition given its {prohibited substitutions}, as defined in Type Derivation OK (Complex) (§3.4.6) (if it is a complex type definition), or given the empty set, as defined in Type Derivation OK (Simple) (§3.14.6) (if it is a simple type definition).

1.2.2 The element information item's ·validity· with respect to the ·local type definition· (if present and validly derived) or the processor-stipulated type definition (if no ·local type definition· is present) has been evaluated as per Element Locally Valid (Type) (§3.3.4).

2 The schema-validity of all the element information items among its[children] has been assessed as per Schema-Validity Assessment (Element) (§3.3.4), and the schema-validity of all the attribute information items among its[attributes] has been assessed as per Schema-Validity Assessment (Attribute) (§3.2.4).

[Definition:] If either case of clause 1 above holds, the element information item has been strictly assessed.

If the item cannot be ·strictly assessed·, because neither clause 1.1 nor clause 1.2 above are satisfied,[Definition:] an element information item's schema validity may be laxly assessed if its ·context-determined declaration· is not skip by ·validating· with respect to the ·ur-type definition· as per Element Locally Valid (Type) (§3.3.4).

Note: In general if clause 1.1 above holds clause 1.2 does not, and vice versa. When anxsi:type [attribute] is involved, however, clause 1.2 takes precedence, as is made clear in Element Locally Valid (Element) (§3.3.4).

Note: The {name} and {target namespace} properties are not mentioned above because they are checked during particle ·validation·, as perElement Sequence Locally Valid (Particle) (§3.9.4).

3.3.5 Element Declaration Information Set Contributions

Schema Information Set Contribution: Element Validated by Type

If an element information item is ·valid· with respect to a ·type definition·as per Element Locally Valid (Type) (§3.3.4), in the ·post-schema-validation infoset· the item has a property:

[schema normalized value]

The appropriate case among the following:

1 If clause 3.2 of Element Locally Valid (Element) (§3.3.4) and Element Default Value (§3.3.5) above have_not_ applied and either the ·type definition· is a simple type definition or its {content type} is a simple type definition, then the ·normalized value· of the item as ·validated·.

2 otherwise ·absent·.

Furthermore, the item has one of the following alternative sets of properties:

Either

[type definition]

An·item isomorphic· to the ·type definition· component itself.

[member type definition]

If and only if that type definition is a simple type definition with {variety} union, or a complex type definition whose {content type} is a simple type definition with {variety} union, then an·item isomorphic· to that member of the union's {member type definitions} which actually ·validated· the element item's ·normalized value·.

or

If the ·type definition· is a simple type definition or its {content type} is a simple type definition, and that type definition has {variety} union, then calling[Definition:] that member of the {member type definitions} which actually·validated· the element item's ·normalized value· theactual member type definition, there are three additional properties:

The first (·item isomorphic·) alternative above is provided for applications such as query processors which need access to the full range of details about an item's·assessment·, for example the type hierarchy; the second, for lighter-weight processors for whom representing the significant parts of the type hierarchy as information items might be a significant burden.

Also, if the declaration has a {value constraint}, the item has a property:

Note that if an element is ·laxly assessed·, then the [type definition] and[member type definition] properties, or their alternatives, are based on the ·ur-type definition·.

3.4 Complex Type Definitions

Complex Type Definitions provide for:

• Constraining element information items by providing Attribute Declaration (§2.2.2.3)s governing the appearance and content of[attributes]
• Constraining element information item [children] to be empty, or to conform to a specified element-only or mixed content model, or else constraining the character information item [children] to conform to a specified simple type definition.
• Using the mechanisms of Type Definition Hierarchy (§2.2.1.1) to derive a complex type from another simple or complex type.
• Specifying ·post-schema-validation infoset contributions· for elements.
• Limiting the ability to derive additional types from a given complex type.
• Controlling the permission to substitute, in an instance, elements of a derived type for elements declared in a content model to be of a given complex type.

<xs:complexType name="PurchaseOrderType"> xs:sequence <xs:element name="shipTo" type="USAddress"/> <xs:element name="billTo" type="USAddress"/> <xs:element ref="comment" minOccurs="0"/> <xs:element name="items" type="Items"/> <xs:attribute name="orderDate" type="xs:date"/>

The XML representation of a complex type definition.

3.4.1 The Complex Type Definition Schema Component

A complex type definition schema component has the following properties:

Complex types definitions are identified by their {name} and {target namespace}. Except for anonymous complex type definitions (those with no {name}), since type definitions (i.e. both simple and complex type definitions taken together) must be uniquely identified within an ·XML Schema·, no complex type definition can have the same name as another simple or complex type definition. Complex type {name}s and {target namespace}s are provided for reference from instances (see xsi:type (§2.6.1)), and for use in the XML representation of schema components (specifically in ). See References to schema components across namespaces (§4.2.3) for the use of component identifiers when importing one schema into another.

Note: The {name} of a complex type is not ipso facto the [(local) name] of the element information items ·validated· by that definition. The connection between a name and a type definition is described in Element Declarations (§3.3).

As described in Type Definition Hierarchy (§2.2.1.1), each complex type is derived from a{base type definition} which is itself either a Simple Type Definition (§2.2.1.2) or a Complex Type Definition (§2.2.1.3). {derivation method} specifies the means of derivation as either extension or restriction (see Type Definition Hierarchy (§2.2.1.1)).

A complex type with an empty specification for {final} can be used as a{base type definition} for other types derived by either of extension or restriction; the explicit values extension, and restriction prevent further derivations by extension and restriction respectively. If all values are specified, then [Definition:] the complex type is said to befinal, because no further derivations are possible. Finality is _not_inherited, that is, a type definition derived by restriction from a type definition which is final for extension is not itself, in the absence of any explicit final attribute of its own, final for anything.

Complex types for which {abstract} is true must not be used as the{type definition} for the ·validation· of element information items. It follows that they must not be referenced from anxsi:type (§2.6.1) attribute in an instance document. Abstract complex types can be used as {base type definition}s, or even as the {type definition}s of element declarations, provided in every case a concrete derived type definition is used for ·validation·, either via xsi:type (§2.6.1) or the operation of a substitution group.

{attribute uses} are a set of attribute uses. See Element Locally Valid (Complex Type) (§3.4.4)and Attribute Locally Valid (§3.2.4) for details of attribute ·validation·.

{attribute wildcard}s provide a more flexible specification for ·validation· of attributes not explicitly included in {attribute uses}. Informally, the specific values of {attribute wildcard} are interpreted as follows:

• any: [attributes] can include attributes with any qualified or unqualified name.
• a set whose members are either namespace names or ·absent·: [attributes] can include any attribute(s) from the specified namespace(s). If ·absent· is included in the set, then any unqualified attributes are (also) allowed.
• 'not' and a namespace name: [attributes] cannot include attributes from the specified namespace.
• 'not' and ·absent·: [attributes] cannot include unqualified attributes.

See Element Locally Valid (Complex Type) (§3.4.4) and Wildcard allows Namespace Name (§3.10.4) for formal details of attribute wildcard ·validation·.

{content type} determines the ·validation· of [children] of element information items. Informally:

• A {content type} with the distinguished value empty ·validates· elements with no character or element information item [children].
• A {content type} which is a Simple Type Definition (§2.2.1.2) ·validates·elements with character-only [children].
• An element-only {content type} ·validates· elements with [children] that conform to the supplied ·content model·.
• A mixed {content type} ·validates· elements whose element [children] (i.e. specifically ignoring other [children] such as character information items) conform to the supplied ·content model·.

{prohibited substitutions} determine whether an element declaration appearing in a ·content model· is prevented from additionally·validating· element items with an xsi:type (§2.6.1) attribute that identifies a complex type definition derived by extension or_restriction_ from this definition, or element items in a substitution group whose type definition is similarly derived: If {prohibited substitutions} is empty, then all such substitutions are allowed, otherwise, the derivation method(s) it names are disallowed.

See Annotations (§3.13) for information on the role of the{annotations} property.

3.4.2 XML Representation of Complex Type Definitions

The XML representation for a complex type definition schema component is a element information item.

The XML representation for complex type definitions with a simple type definition {content type} is significantly different from that of those with other {content type}s, and this is reflected in the presentation below, which displays first the elements involved in the first case, then those for the second. The property mapping is shown once for each case.

<complexType
abstract = boolean : false
block = (#all | List of (extension | restriction))
final = (#all | List of (extension | restriction))
id = ID
mixed = boolean : false
name = NCName
_{any attributes with non-schema namespace . . .}_>
Content: (annotation?, (simpleContent | complexContent | ((group | all | choice | sequence)?, ((attribute | attributeGroup)*, anyAttribute?))))

Whichever alternative for the content of is chosen, the following property mappings apply:

When the alternative is chosen, the following elements are relevant, and the remaining property mappings are as below. Note that either or must be chosen as the content of .

<simpleContent
id = ID
_{any attributes with non-schema namespace . . .}_>
Content: (annotation?, (restriction | extension))

<restriction
**base** = QName
id = ID
_{any attributes with non-schema namespace . . .}_>
Content: (annotation?, (simpleType?, (minExclusive | minInclusive | maxExclusive | maxInclusive | totalDigits | fractionDigits | length | minLength | maxLength | enumeration | whiteSpace | pattern)*)?, ((attribute | attributeGroup)*, anyAttribute?))

<extension
**base** = QName
id = ID
_{any attributes with non-schema namespace . . .}_>
Content: (annotation?, ((attribute | attributeGroup)*, anyAttribute?))

<attributeGroup
id = ID
**ref** = QName
_{any attributes with non-schema namespace . . .}_>
Content: (annotation?)

<anyAttribute
id = ID
namespace = ((##any | ##other) | List of (anyURI | (##targetNamespace | ##local)) ) : ##any
processContents = (lax | skip | strict) : strict
_{any attributes with non-schema namespace . . .}_>
Content: (annotation?)

When the alternative is chosen, the following elements are relevant (as are the and elements, not repeated here), and the additional property mappings are as below. Note that either or must be chosen as the content of , but their content models are different in this case from the case above when they occur as children of.

The property mappings below are also used in the case where the third alternative (neither nor ) is chosen. This case is understood as shorthand for complex content restricting the ·ur-type definition·, and the details of the mappings should be modified as necessary.

<complexContent
id = ID
mixed = boolean
_{any attributes with non-schema namespace . . .}_>
Content: (annotation?, (restriction | extension))

<restriction
**base** = QName
id = ID
_{any attributes with non-schema namespace . . .}_>
Content: (annotation?, (group | all | choice | sequence)?, ((attribute | attributeGroup)*, anyAttribute?))

<extension
**base** = QName
id = ID
_{any attributes with non-schema namespace . . .}_>
Content: (annotation?, ((group | all | choice | sequence)?, ((attribute | attributeGroup)*, anyAttribute?)))

Note: Aside from the simple coherence requirements enforced above, constraining type definitions identified as restrictions to actually _be_restrictions, that is, to ·validate· a subset of the items which are·validated· by their base type definition, is enforced in Constraints on Complex Type Definition Schema Components (§3.4.6).

Note: The only substantive function of the value prohibited for theuse attribute of an is in establishing the correspondence between a complex type defined by restriction and its XML representation. It serves to prevent inheritance of an identically named attribute use from the {base type definition}. Such an does not correspond to any component, and hence there is no interaction with either explicit or inherited wildcards in the operation of Complex Type Definition Validation Rules (§3.4.4) or Constraints on Complex Type Definition Schema Components (§3.4.6).

Careful consideration of the above concrete syntax reveals that a type definition need consist of no more than a name, i.e. that<complexType name="anyThing"/> is allowed.

<xs:complexType name="length1"> xs:simpleContent <xs:extension base="xs:nonNegativeInteger"> <xs:attribute name="unit" type="xs:NMTOKEN"/>

<xs:element name="width" type="length1"/>

25

<xs:complexType name="length2"> xs:complexContent <xs:restriction base="xs:anyType"> xs:sequence <xs:element name="size" type="xs:nonNegativeInteger"/> <xs:element name="unit" type="xs:NMTOKEN"/>

<xs:element name="depth" type="length2"/>

<xs:complexType name="length3"> xs:sequence <xs:element name="size" type="xs:nonNegativeInteger"/> <xs:element name="unit" type="xs:NMTOKEN"/>

Three approaches to defining a type for length: one with character data content constrained by reference to a built-in datatype, and one attribute, the other two using two elements. length3 is the abbreviated alternative tolength2: they correspond to identical type definition components.

<xs:complexType name="personName"> xs:sequence <xs:element name="title" minOccurs="0"/> <xs:element name="forename" minOccurs="0" maxOccurs="unbounded"/> <xs:element name="surname"/>

<xs:complexType name="extendedName"> xs:complexContent <xs:extension base="personName"> xs:sequence <xs:element name="generation" minOccurs="0"/>

<xs:element name="addressee" type="extendedName"/>

A type definition for personal names, and a definition derived by extension which adds a single element; an element declaration referencing the derived definition, and a ·valid· instance thereof.

<xs:complexType name="simpleName"> xs:complexContent <xs:restriction base="personName"> xs:sequence <xs:element name="forename" minOccurs="1" maxOccurs="1"/> <xs:element name="surname"/>

<xs:element name="who" type="simpleName"/>

A simplified type definition derived from the base type from the previous example by restriction, eliminating one optional daughter and fixing another to occur exactly once; an element declared by reference to it, and a ·valid· instance thereof.

<xs:complexType name="paraType" mixed="true"> <xs:choice minOccurs="0" maxOccurs="unbounded"> <xs:element ref="emph"/> <xs:element ref="strong"/> <xs:attribute name="version" type="xs:number"/>

A further illustration of the abbreviated form, with themixed attribute appearing on complexType itself.

3.4.3 Constraints on XML Representations of Complex Type Definitions

Schema Representation Constraint: Complex Type Definition Representation OK

In addition to the conditions imposed on element information items by the schema for schemas, all of the following must be true:

1 If the alternative is chosen, the type definition ·resolved· to by the ·actual value· of the base [attribute] must be a complex type definition;

2 If the alternative is chosen, all of the following must be true:

2.1 The type definition ·resolved· to by the ·actual value· of the base [attribute] must be one of the following:

2.1.1 a complex type definition whose {content type} is a simple type definition;

2.1.2 only if the alternative is also chosen, a complex type definition whose {content type} is mixed and a particle which is·emptiable·, as defined in Particle Emptiable (§3.9.6);

2.1.3 only if the alternative is also chosen, a simple type definition.

2.2 If clause 2.1.2 above is satisfied, then there must be a among the [children] of .

Note: Although not explicitly ruled out either here or in Schema for Schemas (normative) (§A), specifying <xs:complexType . . .mixed='true' when the alternative is chosen has no effect on the corresponding component, and should be avoided. This may be ruled out in a subsequent version of this specification.

3 The corresponding complex type definition component must satisfy the conditions set out in Constraints on Complex Type Definition Schema Components (§3.4.6);

4 If clause 2.2.1 or clause 2.2.2 in the correspondence specification above for {attribute wildcard} is satisfied, the intensional intersection must be expressible, as defined in Attribute Wildcard Intersection (§3.10.6).

3.4.4 Complex Type Definition Validation Rules

Validation Rule: Element Locally Valid (Complex Type)

For an element information item to be locally ·valid· with respect to a complex type definitionall of the following must be true:

1 {abstract} is false.

2 If clause 3.2 of Element Locally Valid (Element) (§3.3.4) did not apply, then the appropriate case among the following must be true:

2.1 If the {content type} is empty, then the element information item has no character or element information item [children].

2.2 If the {content type} is a simple type definition, then the element information item has no element information item [children], and the ·normalized value· of the element information item is ·valid· with respect to that simple type definition as defined by String Valid (§3.14.4).

2.3 If the {content type} is element-only, then the element information item has no character information item [children] other than those whose [character code] is defined as a white spacein [XML 1.0 (Second Edition)].

2.4 If the {content type} is element-only or_mixed_, then the sequence of the element information item's element information item [children], if any, taken in order, is ·valid· with respect to the {content type}'s particle, as defined in Element Sequence Locally Valid (Particle) (§3.9.4).

3 For each attribute information item in the element information item's [attributes] excepting those whose [namespace name] is identical to http://www.w3.org/2001/XMLSchema-instance and whose [local name] is one of type, nil, schemaLocation or noNamespaceSchemaLocation, the appropriate case among the following must be true:

3.1 If there is among the {attribute uses} an attribute use with an {attribute declaration} whose{name} matches the attribute information item's [local name] and whose {target namespace} is identical to the attribute information item's [namespace name] (where an ·absent· {target namespace} is taken to be identical to a [namespace name] with no value), then the attribute information must be ·valid· with respect to that attribute use as per Attribute Locally Valid (Use) (§3.5.4). In this case the {attribute declaration} of that attribute use is the ·context-determined declaration· for the attribute information item with respect to Schema-Validity Assessment (Attribute) (§3.2.4) and Assessment Outcome (Attribute) (§3.2.5).

3.2 otherwise all of the following must be true:

3.2.1 There must be an {attribute wildcard}.

3.2.2 The attribute information item must be ·valid· with respect to it as defined in Item Valid (Wildcard) (§3.10.4).

4 The {attribute declaration} of each attribute use in the {attribute uses} whose{required} is true matches one of the attribute information items in the element information item's [attributes] as per clause 3.1 above.

5 Let [Definition:] the wild IDs be the set of all attribute information item to which clause 3.2 applied and whose ·validation· resulted in a ·context-determined declaration· of_mustFind_ or no ·context-determined declaration· at all, and whose [local name] and [namespace name] resolve (as defined by QName resolution (Instance) (§3.15.4)) to an attribute declaration whose {type definition} is or is derived from ID. Thenall of the following must be true:

5.1 There must be no more than one item in ·wild IDs·.

5.2 If ·wild IDs· is non-empty, there must not be any attribute uses among the {attribute uses} whose {attribute declaration}'s {type definition} is or is derived from ID.

Note: This clause serves to ensure that even via attribute wildcards no element has more than one attribute of type ID, and that even when an element legitimately lacks a declared attribute of type ID, a wildcard-validated attribute must not supply it. That is, if an element has a type whose attribute declarations include one of type ID, it either has that attribute or no attribute of type ID.

Note: When an {attribute wildcard} is present, this does_not_ introduce any ambiguity with respect to how attribute information items for which an attribute use is present amongst the {attribute uses} whose name and target namespace match are ·assessed·. In such cases the attribute use always takes precedence, and the ·assessment· of such items stands or falls entirely on the basis of the attribute use and its {attribute declaration}. This follows from the details of clause 3.

3.4.6 Constraints on Complex Type Definition Schema Components

All complex type definitions (see Complex Type Definitions (§3.4)) must satisfy the following constraints.

Schema Component Constraint: Derivation Valid (Extension)

If the {derivation method} is extension, the appropriate case among the following must be true:

1 If the {base type definition} is a complex type definition, then all of the following must be true:

1.1 The {final} of the {base type definition} must not contain extension.

1.2 Its {attribute uses} must be a subset of the {attribute uses} of the complex type definition itself, that is, for every attribute use in the{attribute uses} of the{base type definition}, there must be an attribute use in the {attribute uses} of the complex type definition itself whose {attribute declaration} has the same {name},{target namespace} and{type definition} as its attribute declaration.

1.3 If it has an {attribute wildcard}, the complex type definition must also have one, and the base type definition's {attribute wildcard}'s {namespace constraint} must be a subset of the complex type definition's {attribute wildcard}'s {namespace constraint}, as defined by Wildcard Subset (§3.10.6).

1.4 One of the following must be true:

1.4.1 The {content type} of the {base type definition} and the{content type} of the complex type definition itself must be the same simple type definition.

1.4.2 The{content type} of both the {base type definition} and the complex type definition itself must be empty.

1.4.3 All of the following must be true:

1.4.3.1 The{content type} of the complex type definition itself must specify a particle.

1.4.3.2 One of the following must be true:

1.4.3.2.1 The{content type} of the {base type definition}must be empty.

1.4.3.2.2 All of the following must be true:

1.4.3.2.2.1 Both{content type}s must be mixed or both must be_element-only_.

1.4.3.2.2.2 The particle of the complex type definition must be a ·valid extension· of the {base type definition}'s particle, as defined in Particle Valid (Extension) (§3.9.6).

1.5 It must in principle be possible to derive the complex type definition in two steps, the first an extension and the second a restriction (possibly vacuous), from that type definition among its ancestors whose {base type definition} is the ·ur-type definition·.

Note: This requirement ensures that nothing removed by a restriction is subsequently added back by an extension. It is trivial to check if the extension in question is the only extension in its derivation, or if there are no restrictions bar the first from the ·ur-type definition·.

Constructing the intermediate type definition to check this constraint is straightforward: simply re-order the derivation to put all the extension steps first, then collapse them into a single extension. If the resulting definition can be the basis for a valid restriction to the desired definition, the constraint is satisfied.

2 If the {base type definition} is a simple type definition, then all of the following must be true:

2.1 The {content type} must be the same simple type definition.

2.2 The {final} of the {base type definition} must not contain extension.

[Definition:] If this constraint Derivation Valid (Extension) (§3.4.6) holds of a complex type definition, it is a valid extension of its {base type definition}.

Schema Component Constraint: Derivation Valid (Restriction, Complex)

If the {derivation method} is restriction all of the following must be true:

1 The {base type definition} must be a complex type definition whose {final} does not contain restriction.

2 For each attribute use (call this R) in the {attribute uses} the appropriate case among the following must be true:

2.1 If there is an attribute use in the{attribute uses} of the {base type definition} (call this B) whose {attribute declaration} has the same {name} and {target namespace}, then all of the following must be true:

2.1.1 one of the following must be true:

2.1.1.1 B's {required} is false.

2.1.1.2 R's {required} is true.

2.1.2 R's {attribute declaration}'s {type definition} must be validly derived from B's {type definition} given the empty set as defined inType Derivation OK (Simple) (§3.14.6).

2.1.3 [Definition:] Let theeffective value constraint of an attribute use be its {value constraint}, if present, otherwise its {attribute declaration}'s {value constraint} . Then one of the following must be true:

2.1.3.1 B's ·effective value constraint· is ·absent· or default.

2.1.3.2 R's ·effective value constraint· is_fixed_ with the same string as B's.

2.2 otherwise the {base type definition} must have an {attribute wildcard} and the {target namespace} of the R's {attribute declaration} must be ·valid· with respect to that wildcard, as defined in Wildcard allows Namespace Name (§3.10.4).

3 For each attribute use in the {attribute uses} of the {base type definition} whose {required} is_true_, there must be an attribute use with an {attribute declaration} with the same {name} and {target namespace} as its {attribute declaration} in the{attribute uses} of the complex type definition itself whose {required} is_true_.

4 If there is an {attribute wildcard}, all of the following must be true:

4.1 The{base type definition} must also have one.

4.2 The complex type definition's {attribute wildcard}'s {namespace constraint} must be a subset of the {base type definition}'s {attribute wildcard}'s {namespace constraint}, as defined by Wildcard Subset (§3.10.6).

4.3 Unless the {base type definition} is the ·ur-type definition·, the complex type definition's {attribute wildcard}'s {process contents} must be identical to or stronger than the {base type definition}'s {attribute wildcard}'s {process contents}, where_strict_ is stronger than lax is stronger than skip.

5 One of the following must be true:

5.1 The {base type definition} must be the ·ur-type definition·.

5.2 All of the following must be true:

5.2.1 The {content type} of the complex type definition must be a simple type definition

5.2.2 One of the following must be true:

5.2.2.1 The {content type} of the {base type definition} must be a simple type definition from which the {content type} is validly derived given the empty set as defined inType Derivation OK (Simple) (§3.14.6).

5.2.2.2 The {base type definition} must be _mixed_and have a particle which is ·emptiable· as defined in Particle Emptiable (§3.9.6).

5.3 All of the following must be true:

5.3.1 The {content type} of the complex type itself must be empty

5.3.2 One of the following must be true:

5.3.2.1 The {content type} of the {base type definition} must also be empty.

5.3.2.2 The {content type} of the {base type definition} must be elementOnly or mixed and have a particle which is ·emptiable· as defined in Particle Emptiable (§3.9.6).

5.4 All of the following must be true:

5.4.1 One of the following must be true:

5.4.1.1 The {content type} of the complex type definition itself must be element-only

5.4.1.2 The {content type} of the complex type definition itself and of the{base type definition} must be mixed

5.4.2 The particle of the complex type definition itself must be a ·valid restriction· of the particle of the {content type} of the {base type definition} as defined in Particle Valid (Restriction) (§3.9.6).

Note: Attempts to derive complex type definitions whose {content type} is element-only by restricting a {base type definition} whose {content type}is empty are not ruled out by this clause. However if the complex type definition itself has a non-pointless particle it will fail to satisfyParticle Valid (Restriction) (§3.9.6). On the other hand some type definitions with pointless element-only content, for example an empty, will satisfy Particle Valid (Restriction) (§3.9.6)with respect to an empty {base type definition}, and so be valid restrictions.

[Definition:] If this constraint Derivation Valid (Restriction, Complex) (§3.4.6) holds of a complex type definition, it is a valid restriction of its {base type definition}.

Note: To restrict a complex type definition with a simple base type definition to empty, use a simple type definition with a fixed value of the empty string: this preserves the type information.

The following constraint defines a relation appealed to elsewhere in this specification.

Note: This constraint is used to check that when someone uses a type in a context where another type was expected (either via xsi:type or substitution groups), that the type used is actually derived from the expected type, and that that derivation does not involve a form of derivation which was ruled out by the expected type.

Note:

The wording of clause

2.1 above appeals to a notion of component identity which is only incompletely defined by this version of this specification. In some cases, the wording of this specification does make clear the rules for component identity. These cases include:

• When they are both top-level components with the same component type, namespace name, and local name;
• When they are necessarily the same type definition (for example, when the two types definitions in question are the type definitions associated with two attribute or element declarations, which are discovered to be the same declaration);
• When they are the same by construction (for example, when an element's type definition defaults to being the same type definition as that of its substitution-group head or when a complex type definition inherits an attribute declaration from its base type definition).

In other cases two conforming implementations may disagree as to whether components are identical.

3.4.7 Built-in Complex Type Definition

There is a complex type definition nearly equivalent to the ·ur-type definition· present in every schema by definition. It has the following properties:

The mixed content specification together with the_lax_ wildcard and attribute specification produce the defining property for the·ur-type definition·, namely that every type definition is (eventually) a restriction of the ·ur-type definition·: its permissions and requirements are (nearly) the least restrictive possible.

Note: This specification does not provide an inventory of built-in complex type definitions for use in user schemas. A preliminary library of complex type definitions is available which includes both mathematical (e.g.rational) and utility (e.g. array) type definitions. In particular, there is a text type definition which is recommended for use as the type definition in element declarations intended for general text content, as it makes sensible provision for various aspects of internationalization. For more details, see the schema document for the type library at its namespace name: http://www.w3.org/2001/03/XMLSchema/TypeLibrary.xsd.

3.5 AttributeUses

An attribute use is a utility component which controls the occurrence and defaulting behavior of attribute declarations. It plays the same role for attribute declarations in complex types that particles play for element declarations.

xs:complexType . . . <xs:attribute ref="xml:lang" use="required"/> <xs:attribute ref="xml:space" default="preserve"/> <xs:attribute name="version" type="xs:number" fixed="1.0"/>

XML representations which all involve attribute uses, illustrating some of the possibilities for controlling occurrence.

3.6 Attribute Group Definitions

A schema can name a group of attribute declarations so that they may be incorporated as a group into complex type definitions.

Attribute group definitions do not participate in ·validation· as such, but the{attribute uses} and {attribute wildcard} of one or more complex type definitions may be constructed in whole or part by reference to an attribute group. Thus, attribute group definitions provide a replacement for some uses of XML'sparameter entity facility. Attribute group definitions are provided primarily for reference from the XML representation of schema components (see and ).

<xs:attributeGroup name="myAttrGroup"> <xs:attribute . . ./> . . .

<xs:complexType name="myelement"> . . . <xs:attributeGroup ref="myAttrGroup"/>

XML representations for attribute group definitions. The effect is as if the attribute declarations in the group were present in the type definition.

3.6.2 XML Representation of Attribute Group Definition Schema Components

The XML representation for an attribute group definition schema component is an element information item. It provides for naming a group of attribute declarations and an attribute wildcard for use by reference in the XML representation of complex type definitions and other attribute group definitions. The correspondences between the properties of the information item and properties of the component it corresponds to are as follows:

<attributeGroup
id = ID
name = NCName
ref = QName
_{any attributes with non-schema namespace . . .}_>
Content: (annotation?, ((attribute | attributeGroup)*, anyAttribute?))

When an appears as a daughter of or , it corresponds to an attribute group definition as below. When it appears as a daughter of or , it does not correspond to any component as such.

The example above illustrates a pattern which recurs in the XML representation of schemas: The same element, in this case attributeGroup, serves both to define and to incorporate by reference. In the first case thename attribute is required, in the second the refattribute is required, and the element must be empty. These two are mutually exclusive, and also conditioned by context: the defining form, with a name, must occur at the top level of a schema, whereas the referring form, with a ref, must occur within a complex type definition or an attribute group definition.

3.6.3 Constraints on XML Representations of Attribute Group Definitions

Schema Representation Constraint: Attribute Group Definition Representation OK

In addition to the conditions imposed on element information items by the schema for schemas,all of the following must be true:

1 The corresponding attribute group definition, if any, must satisfy the conditions set out in Constraints on Attribute Group Definition Schema Components (§3.6.6).

2 If clause 2.2.1 or clause 2.2.2 in the correspondence specification in XML Representation of Complex Type Definitions (§3.4.2) for {attribute wildcard}, as referenced above, is satisfied, the intensional intersection must be expressible, as defined in Attribute Wildcard Intersection (§3.10.6).

3 Circular group reference is disallowed outside . That is, unless this element information item's parent is , then among the[children], if any, there must not be an with ref [attribute] which resolves to the component corresponding to this . Indirect circularity is also ruled out. That is, when QName resolution (Schema Document) (§3.15.3) is applied to a ·QName· arising from any s with aref [attribute] among the [children], it must not be the case that a ·QName· is encountered at any depth which resolves to the component corresponding to this .

3.7 Model Group Definitions

A model group definition associates a name and optional annotations with a Model Group (§2.2.3.1). By reference to the name, the entire model group can be incorporated by reference into a {term}.

Model group definitions are provided primarily for reference from the XML Representation of Complex Type Definitions (§3.4.2) (see and ). Thus, model group definitions provide a replacement for some uses of XML'sparameter entity facility.

<xs:group name="myModelGroup"> xs:sequence <xs:element ref="someThing"/> . . .

<xs:complexType name="trivial"> <xs:group ref="myModelGroup"/> <xs:attribute .../>

<xs:complexType name="moreSo"> xs:choice <xs:element ref="anotherThing"/> <xs:group ref="myModelGroup"/> <xs:attribute .../>

A minimal model group is defined and used by reference, first as the whole content model, then as one alternative in a choice.

3.7.2 XML Representation of Model Group Definition Schema Components

The XML representation for a model group definition schema component is a element information item. It provides for naming a model group for use by reference in the XML representation of complex type definitions and model groups. The correspondences between the properties of the information item and properties of the component it corresponds to are as follows:

<group
id = ID
maxOccurs = (nonNegativeInteger | unbounded) : 1
minOccurs = nonNegativeInteger : 1
name = NCName
ref = QName
_{any attributes with non-schema namespace . . .}_>
Content: (annotation?, (all | choice | sequence)?)

If there is a name [attribute] (in which case the item will have or as parent), then the item corresponds to a model group definition component with properties as follows:

Otherwise, the item will have a ref [attribute], in which case it corresponds to a particle component with properties as follows (unless minOccurs=maxOccurs=0, in which case the item corresponds to no component at all):

The name of this section is slightly misleading, in that the second, un-named, case above (with aref and no name) is not really a named model group at all, but a reference to one. Also note that in the first (named) case above no reference is made to minOccurs ormaxOccurs: this is because the schema for schemas does not allow them on the child of when it is named. This in turn is because the {min occurs} and {max occurs} of the particles which refer to the definition are what count.

Given the constraints on its appearance in content models, an should only occur as the only item in the[children] of a named model group definition or a content model: see Constraints on Model Group Schema Components (§3.8.6).

3.8 Model Groups

When the [children] of element information items are not constrained to be empty or by reference to a simple type definition (Simple Type Definitions (§3.14)), the sequence of element information item [children] content may be specified in more detail with a model group. Because the {term} property of a particle can be a model group, and model groups contain particles, model groups can indirectly contain other model groups; the grammar for content models is therefore recursive.

xs:all <xs:element ref="cats"/> <xs:element ref="dogs"/>

xs:sequence xs:choice <xs:element ref="left"/> <xs:element ref="right"/> <xs:element ref="landmark"/>

XML representations for the three kinds of model group, the third nested inside the second.

3.8.1 The Model Group Schema Component

The model group schema component has the following properties:

specifies a sequential (sequence), disjunctive (choice) or conjunctive (all) interpretation of the {particles}. This in turn determines whether the element information item [children] ·validated· by the model group must:

• (sequence) correspond, in order, to the specified {particles};
• (choice) corresponded to exactly one of the specified {particles};
• (all) contain all and only exactly zero or one of each element specified in {particles}. The elements can occur in any order. In this case, to reduce implementation complexity, {particles} is restricted to contain local and top-level element declarations only, with {min occurs}=0 or1, {max occurs}=1.

When two or more particles contained directly or indirectly in the{particles} of a model group have identically named element declarations as their {term}, the type definitions of those declarations must be the same. By 'indirectly' is meant particles within the {particles}of a group which is itself the {term} of a directly contained particle, and so on recursively.

See Annotations (§3.13) for information on the role of the{annotation} property.

3.8.2 XML Representation of Model Group Schema Components

The XML representation for a model group schema component is either an, a or a element information item. The correspondences between the properties of those information items and properties of the component they correspond to are as follows:

<all
id = ID
maxOccurs = 1 : 1
minOccurs = (0 | 1) : 1
_{any attributes with non-schema namespace . . .}_>
Content: (annotation?, element*)

<choice
id = ID
maxOccurs = (nonNegativeInteger | unbounded) : 1
minOccurs = nonNegativeInteger : 1
_{any attributes with non-schema namespace . . .}_>
Content: (annotation?, (element | group | choice | sequence | any)*)

<sequence
id = ID
maxOccurs = (nonNegativeInteger | unbounded) : 1
minOccurs = nonNegativeInteger : 1
_{any attributes with non-schema namespace . . .}_>
Content: (annotation?, (element | group | choice | sequence | any)*)

Each of the above items corresponds to a particle containing a model group, with properties as follows (unless minOccurs=maxOccurs=0, in which case the item corresponds to no component at all):

3.8.4 Model Group Validation Rules

Validation Rule: Element Sequence Valid

[Definition:] Define apartition of a sequence as a sequence of sub-sequences, some or all of which may be empty, such that concatenating all the sub-sequences yields the original sequence.

For a sequence (possibly empty) of element information items to be locally ·valid· with respect to a model group the appropriate case among the following must be true:

1 If the {compositor} is sequence, then there must be a·partition· of the sequence into n sub-sequences where n is the length of {particles} such that each of the sub-sequences in order is ·valid·with respect to the corresponding particle in the {particles} as defined in Element Sequence Locally Valid (Particle) (§3.9.4).

2 If the {compositor} is choice, then there must be a particle among the {particles} such that the sequence is·valid· with respect to that particle as defined in Element Sequence Locally Valid (Particle) (§3.9.4).

3 If the {compositor} is all, then there must be a·partition· of the sequence into n sub-sequences where n is the length of {particles} such that there is a one-to-one mapping between the sub-sequences and the {particles} where each sub-sequence is ·valid· with respect to the corresponding particle as defined in Element Sequence Locally Valid (Particle) (§3.9.4).

Nothing in the above should be understood as ruling out groups whose{particles} is empty: although no sequence can be ·valid·with respect to such a group whose {compositor} is_choice_, the empty sequence is ·valid· with respect to empty groups whose {compositor} is sequence or all.

Note: The above definition is implicitly non-deterministic, and should not be taken as a recipé for implementations. Note in particular that when {compositor} is all, particles is restricted to a list of local and top-level element declarations (see Constraints on Model Group Schema Components (§3.8.6)). A much simpler implementation is possible than would arise from a literal interpretation of the definition above; informally, the content is ·valid· when each declared element occurs exactly once (or at most once, if {min occurs} is 0), and each is ·valid· with respect to its corresponding declaration. The elements can occur in arbitrary order.

3.8.6 Constraints on Model Group Schema Components

All model groups (see Model Groups (§3.8)) must satisfy the following constraints.

Schema Component Constraint: All Group Limited

When a model group has {compositor} all, thenall of the following must be true:

1 It appears only as the value of one or both of the following properties:

1.1 the {model group} property of a model group definition.

1.2 the{term} property of a particle with {max occurs}=1which is part of a pair which constitutes the {content type} of a complex type definition.

2 The {max occurs} of all the particles in the{particles} of the group must be 0 or 1.

Schema Component Constraint: Element Declarations Consistent

If the {particles} contains, either directly, indirectly (that is, within the {particles} of a contained model group, recursively) or ·implicitly· two or more element declaration particles with the same {name} and{target namespace}, then all their type definitions must be the same top-level definition, that is,all of the following must be true:

1 all their{type definition}s must have a non-·absent· {name}.

2 all their{type definition}s must have the same {name}.

3 all their{type definition}s must have the same {target namespace}.

[Definition:] A list of particles implicitly contains an element declaration if a member of the list contains that element declaration in its ·substitution group·.

Schema Component Constraint: Unique Particle Attribution

A content model must be formed such that during ·validation· of an element information item sequence, the particle component contained directly, indirectly or ·implicitly· therein with which to attempt to ·validate· each item in the sequence in turn can be uniquely determined without examining the content or attributes of that item, and without any information about the items in the remainder of the sequence.

Note: This constraint reconstructs for XML Schema the equivalent constraints of[XML 1.0 (Second Edition)] and SGML. Given the presence of element substitution groups and wildcards, the concise expression of this constraint is difficult, see Analysis of the Unique Particle Attribution Constraint (non-normative) (§H) for further discussion.

Since this constraint is expressed at the component level, it applies to content models whose origins (e.g. via type derivation and references to named model groups) are no longer evident. So particles at different points in the content model are always distinct from one another, even if they originated from the same named model group.

Note: Because locally-scoped element declarations may or may not have a{target namespace}, the scope of declarations is_not_ relevant to enforcing either of the two preceding constraints.

The following constraints define relations appealed to elsewhere in this specification.

Schema Component Constraint: Effective Total Range (all and sequence)

The effective total range of a particle whose {term} is a group whose {compositor} is_all_ or sequence is a pair of minimum and maximum, as follows:

minimum

The product of the particle's {min occurs} and the sum of the {min occurs} of every wildcard or element declaration particle in the group's {particles} and the minimum part of the effective total range of each of the group particles in the group's {particles} (or 0 if there are no {particles}).

maximum

unbounded if the {max occurs} of any wildcard or element declaration particle in the group's {particles} or the maximum part of the effective total range of any of the group particles in the group's{particles} is unbounded, or if any of those is non-zero and the {max occurs} of the particle itself is unbounded, otherwise the product of the particle's {max occurs} and the sum of the {max occurs} of every wildcard or element declaration particle in the group's {particles} and the maximum part of the effective total range of each of the group particles in the group's {particles} (or 0 if there are no {particles}).

Schema Component Constraint: Effective Total Range (choice)

The effective total range of a particle whose {term} is a group whose {compositor} is_choice_ is a pair of minimum and maximum, as follows:

minimum

The product of the particle's {min occurs} and the minimum of the {min occurs} of every wildcard or element declaration particle in the group's {particles} and the minimum part of the effective total range of each of the group particles in the group's {particles} (or 0 if there are no {particles}).

maximum

unbounded if the {max occurs} of any wildcard or element declaration particle in the group's {particles} or the maximum part of the effective total range of any of the group particles in the group's{particles} is unbounded, or if any of those is non-zero and the {max occurs} of the particle itself is unbounded, otherwise the product of the particle's {max occurs} and the maximum of the {max occurs} of every wildcard or element declaration particle in the group's {particles} and the maximum part of the effective total range of each of the group particles in the group's {particles} (or 0 if there are no {particles}).

3.9 Particles

As described in Model Groups (§3.8), particles contribute to the definition of content models.

<xs:element ref="egg" minOccurs="12" maxOccurs="12"/>

<xs:group ref="omelette" minOccurs="0"/>

<xs:any maxOccurs="unbounded"/>

XML representations which all involve particles, illustrating some of the possibilities for controlling occurrence.

3.9.1 The Particle Schema Component

The particle schema component has the following properties:

{min occurs}

A non-negative integer.

{max occurs}

Either a non-negative integer or unbounded.

{term}

One of a model group, a wildcard, or an element declaration.

In general, multiple element information item [children], possibly with intervening character [children] if the content type is mixed, can be ·validated· with respect to a single particle. When the {term} is an element declaration or wildcard, {min occurs} determines the minimum number of such element [children] that can occur. The number of such children must be greater than or equal to {min occurs}. If {min occurs} is 0, then occurrence of such children is optional.

Again, when the {term} is an element declaration or wildcard, the number of such element [children] must be less than or equal to any numeric specification of{max occurs}; if {max occurs} is unbounded, then there is no upper bound on the number of such children.

When the {term} is a model group, the permitted occurrence range is determined by a combination of {min occurs} and {max occurs} and the occurrence ranges of the {term}'s {particles}.

3.9.4 Particle Validation Rules

Validation Rule: Element Sequence Locally Valid (Particle)

For a sequence (possibly empty) of element information items to be locally ·valid·with respect to a particle the appropriate case among the following must be true:

1 If the {term} is a wildcard, then all of the following must be true:

1.1 The length of the sequence must be greater than or equal to the {min occurs}.

1.2 If {max occurs} is a number, the length of the sequence must be less than or equal to the {max occurs}.

1.3 Each element information item in the sequence must be ·valid· with respect to the wildcard as defined by Item Valid (Wildcard) (§3.10.4).

2 If the {term} is an element declaration, then all of the following must be true:

2.1 The length of the sequence must be greater than or equal to the {min occurs}.

2.2 If {max occurs} is a number, the length of the sequence must be less than or equal to the {max occurs}.

2.3 For each element information item in the sequenceone of the following must be true:

2.3.1 The element declaration is local (i.e. its{scope} must not be global), its {abstract} is false, the element information item's [namespace name] is identical to the element declaration's {target namespace} (where an ·absent· {target namespace} is taken to be identical to a [namespace name] with no value) and the element information item's [local name] matches the element declaration's {name}.

In this case the element declaration is the ·context-determined declaration· for the element information item with respect to Schema-Validity Assessment (Element) (§3.3.4) and Assessment Outcome (Element) (§3.3.5).

2.3.2 The element declaration is top-level (i.e. its{scope} is global), {abstract} is false, the element information item's [namespace name] is identical to the element declaration's {target namespace} (where an ·absent· {target namespace} is taken to be identical to a [namespace name] with no value) and the element information item's [local name] matches the element declaration's {name}.

In this case the element declaration is the ·context-determined declaration· for the element information item with respect to Schema-Validity Assessment (Element) (§3.3.4) and Assessment Outcome (Element) (§3.3.5).

2.3.3 The element declaration is top-level (i.e. its{scope} is global), its {disallowed substitutions} does not contain substitution, the [local ] and [namespace name] of the element information item resolve to an element declaration, as defined in QName resolution (Instance) (§3.15.4) -- [Definition:] call this declaration the substituting declaration and the ·substituting declaration· together with the particle's element declaration's {disallowed substitutions} is validly substitutable for the particle's element declaration as defined in Substitution Group OK (Transitive) (§3.3.6).

In this case the ·substituting declaration· is the ·context-determined declaration· for the element information item with respect to Schema-Validity Assessment (Element) (§3.3.4) and Assessment Outcome (Element) (§3.3.5).

3 If the {term} is a model group, then all of the following must be true:

3.1 There is a ·partition· of the sequence into n sub-sequences such that n is greater than or equal to {min occurs}.

3.2 If {max occurs} is a number, nmust be less than or equal to {max occurs}.

3.3 Each sub-sequence in the ·partition· is ·valid· with respect to that model group as defined in Element Sequence Valid (§3.8.4).

Note: Clauses clause 1 and clause 2.3.3 do not interact: an element information item validatable by a declaration with a substitution group head in a different namespace is not validatable by a wildcard which accepts the head's namespace but not its own.

3.9.6 Constraints on Particle Schema Components

All particles (see Particles (§3.9)) must satisfy the following constraints.

The following constraints define relations appealed to elsewhere in this specification.

Schema Component Constraint: Particle Valid (Extension)

[Definition:] For a particle (call it E, for extension) to be a valid extension of another particle (call it B, for base)one of the following must be true:

1 They are the same particle.

2 E's {min occurs}={max occurs}=1 and its {term} is a sequence group whose {particles}' first member is a particle all of whose properties, recursively, are identical to those of B, with the exception of {annotation} properties.

The approach to defining a type by restricting another type definition set out here is designed to ensure that types defined in this way are guaranteed to be a subset of the type they restrict. This is accomplished by requiring a clear mapping between the components of the base type definition and the restricting type definition. Permissible mappings are set out below via a set of recursive definitions, bottoming out in the obvious cases, e.g. where an (restricted) element declaration corresponds to another (base) element declaration with the same name and type but the same or wider range of occurrence.

Note: The structural correspondence approach to guaranteeing the subset relation set out here is necessarily verbose, but has the advantage of being checkable in a straightforward way. The working group solicits feedback on how difficult this is in practice, and on whether other approaches are found to be viable.

Schema Component Constraint: Particle Valid (Restriction)

[Definition:] For a particle (call it R, for restriction) to be a valid restriction of another particle (call it B, for base)one of the following must be true:

1 They are the same particle.

2 depending on the kind of particle, per the table below, with the qualifications that all of the following must be true:

2.1 Any top-level element declaration particle (in R orB) which is the{substitution group affiliation} of one or more other element declarations and whose ·substitution group·contains at least one element declaration other than itself is treated as if it were a choice group whose {min occurs} and {max occurs} are those of the particle, and whose {particles} consists of one particle with {min occurs} and {max occurs} of 1 for each of the declarations in its ·substitution group·.

2.2 Any pointless occurrences of , or are ignored, where pointlessness is understood as follows:

One of the following must be true:

2.2.1 {particles} is empty.

2.2.2 All of the following must be true:

2.2.2.1 The particle within which this appears has {max occurs} and {min occurs} of1.

2.2.2.2 One of the following must be true:

2.2.2.2.1 The 's {particles}has only one member.

2.2.2.2.2 The particle within which this appears is itself among the {particles} of a .

One of the following must be true:

2.2.1 {particles} is empty.

2.2.2 {particles} has only one member.

One of the following must be true:

2.2.1 {particles} is empty and the particle within which this appears has {min occurs} of 0.

2.2.2 All of the following must be true:

2.2.2.1 The particle within which this appears has {max occurs} and {min occurs} of1.

2.2.2.2 One of the following must be true:

2.2.2.2.1 The 's {particles}has only one member.

2.2.2.2.2 The particle within which this appears is itself among the {particles} of a .

Schema Component Constraint: Particle Restriction OK (Elt:Elt -- NameAndTypeOK)

For an element declaration particle to be a ·valid restriction· of another element declaration particleall of the following must be true:

1 The declarations' {name}s and {target namespace}s are the same.

2 R's occurrence range is a valid restriction of B's occurrence range as defined by Occurrence Range OK (§3.9.6).

3 One of the following must be true:

3.1 Both B's declaration's {scope} andR's declaration's {scope} are global.

3.2 All of the following must be true:

3.2.1 Either B's {nillable} is true or R's {nillable} is false.

3.2.2 either B's declaration's {value constraint} is absent, or is not fixed, or R's declaration's {value constraint} is_fixed_ with the same value.

3.2.3 R's declaration's {identity-constraint definitions} is a subset of B's declaration's {identity-constraint definitions}, if any.

3.2.4 R's declaration's {disallowed substitutions} is a superset of B's declaration's {disallowed substitutions}.

3.2.5 R's {type definition} is validly derived given {extension, list, union} from B's {type definition} as defined byType Derivation OK (Complex) (§3.4.6) or Type Derivation OK (Simple) (§3.14.6), as appropriate.

Note: The above constraint on {type definition} means that in deriving a type by restriction, any contained type definitions must themselves be explicitly derived by restriction from the corresponding type definitions in the base definition, or be one of the member types of a corresponding union..

Schema Component Constraint: Particle Derivation OK (All:All,Sequence:Sequence -- Recurse)

For an all or sequence group particle to be a ·valid restriction· of another group particle with the same {compositor} all of the following must be true:

1 R's occurrence range is a valid restriction of B's occurrence range as defined by Occurrence Range OK (§3.9.6).

2 There is a complete ·order-preserving· functional mapping from the particles in the{particles} of R to the particles in the {particles} of B such thatall of the following must be true:

2.1 Each particle in the {particles} of R is a·valid restriction· of the particle in the {particles} of B it maps to as defined by Particle Valid (Restriction) (§3.9.6).

2.2 All particles in the {particles} of B which are not mapped to by any particle in the {particles} of Rare ·emptiable· as defined by Particle Emptiable (§3.9.6).

Note: Although the ·validation· semantics of an all group does not depend on the order of its particles, derived all groups are required to match the order of their base in order to simplify checking that the derivation is OK.

[Definition:] A complete functional mapping isorder-preserving if each particle r in the domain R maps to a particle b in the range B which follows (not necessarily immediately) the particle in the rangeB mapped to by the predecessor of r, if any, where "predecessor" and "follows" are defined with respect to the order of the lists which constitute R and B.

Schema Component Constraint: Particle Derivation OK (Choice:Choice -- RecurseLax)

For a choice group particle to be a ·valid restriction· of another choice group particle all of the following must be true:

1 R's occurrence range is a valid restriction of B's occurrence range as defined by Occurrence Range OK (§3.9.6);

2 There is a complete ·order-preserving· functional mapping from the particles in the{particles} of R to the particles in the {particles} of B such that each particle in the {particles} of R is a·valid restriction· of the particle in the {particles} of B it maps to as defined by Particle Valid (Restriction) (§3.9.6).

Note: Although the ·validation· semantics of a choice group does not depend on the order of its particles, derived choice groups are required to match the order of their base in order to simplify checking that the derivation is OK.

Schema Component Constraint: Particle Derivation OK (Sequence:All -- RecurseUnordered)

For a sequence group particle to be a ·valid restriction· of an all group particleall of the following must be true:

1 R's occurrence range is a valid restriction of B's occurrence range as defined by Occurrence Range OK (§3.9.6).

2 There is a complete functional mapping from the particles in the{particles} of R to the particles in the {particles} of B such thatall of the following must be true:

2.1 No particle in the {particles} of B is mapped to by more than one of the particles in the{particles} of R;

2.2 Each particle in the {particles} of R is a·valid restriction· of the particle in the {particles} of B it maps to as defined by Particle Valid (Restriction) (§3.9.6);

2.3 All particles in the {particles} of B which are not mapped to by any particle in the {particles} of Rare ·emptiable· as defined by Particle Emptiable (§3.9.6).

Note: Although this clause allows reordering, because of the limits on the contents of all groups the checking process can still be deterministic.

Schema Component Constraint: Particle Derivation OK (Sequence:Choice -- MapAndSum)

For a sequence group particle to be a ·valid restriction· of a choice group particleall of the following must be true:

1 There is a complete functional mapping from the particles in the{particles} of R to the particles in the {particles} of B such that each particle in the {particles} of R is a·valid restriction· of the particle in the {particles} of B it maps to as defined by Particle Valid (Restriction) (§3.9.6).

2 The pair consisting of the product of the {min occurs} of R and the length of its {particles} and unbounded if {max occurs} is unbounded otherwise the product of the {max occurs} of R and the length of its {particles} is a valid restriction of B's occurrence range as defined by Occurrence Range OK (§3.9.6).

Note: This clause is in principle more restrictive than absolutely necessary, but in practice will cover all the likely cases, and is much easier to specify than the fully general version.

Note: This case allows the "unfolding" of iterated disjunctions into sequences. It may be particularly useful when the disjunction is an implicit one arising from the use of substitution groups.

3.10 Wildcards

In order to exploit the full potential for extensibility offered by XML plus namespaces, more provision is needed than DTDs allow for targeted flexibility in content models and attribute declarations. A wildcard provides for ·validation· of attribute and element information items dependent on their namespace name, but independently of their local name.

<xs:any processContents="skip"/>

<xs:any namespace="##other" processContents="lax"/>

<xs:any namespace="" title="undefined" rel="noopener noreferrer">http://www.w3.org/1999/XSL/Transform"/>

<xs:any namespace="##targetNamespace"/>

<xs:anyAttribute namespace="" title="undefined" rel="noopener noreferrer">http://www.w3.org/XML/1998/namespace"/>

XML representations of the four basic types of wildcard, plus one attribute wildcard.

3.10.1 The Wildcard Schema Component

The wildcard schema component has the following properties:

{namespace constraint} provides for ·validation· of attribute and element items that:

1. (any) have any namespace or are not namespace-qualified;
2. (not and a namespace name) are namespace-qualified with a namespace other than the specified namespace name;
3. (not and ·absent·) are namespace-qualified;
4. (a set whose members are either namespace names or ·absent·) have any of the specified namespaces and/or, if ·absent· is included in the set, are unqualified.

{process contents} controls the impact on ·assessment·of the information items allowed by wildcards, as follows:

strict

There must be a top-level declaration for the item available, or the item must have an xsi:type, and the item must be ·valid· as appropriate.

skip

No constraints at all: the item must simply be well-formed XML.

lax

If the item has a uniquely determined declaration available, it must be ·valid· with respect to that definition, that is, ·validate·if you can, don't worry if you can't.

See Annotations (§3.13) for information on the role of the{annotation} property.

3.10.2 XML Representation of Wildcard Schema Components

The XML representation for a wildcard schema component is an or element information item. The correspondences between the properties of an information item and properties of the components it corresponds to are as follows (see and for the correspondences for ):

<any
id = ID
maxOccurs = (nonNegativeInteger | unbounded) : 1
minOccurs = nonNegativeInteger : 1
namespace = ((##any | ##other) | List of (anyURI | (##targetNamespace | ##local)) ) : ##any
processContents = (lax | skip | strict) : strict
_{any attributes with non-schema namespace . . .}_>
Content: (annotation?)

A particle containing a wildcard, with properties as follows (unless minOccurs=maxOccurs=0, in which case the item corresponds to no component at all):

Wildcards are subject to the same ambiguity constraints (Unique Particle Attribution (§3.8.6)) as other content model particles: If an instance element could match either an explicit particle and a wildcard, or one of two wildcards, within the content model of a type, that model is in error.

3.10.6 Constraints on Wildcard Schema Components

All wildcards (see Wildcards (§3.10)) must satisfy the following constraint.

The following constraints define a relation appealed to elsewhere in this specification.

Schema Component Constraint: Attribute Wildcard Union

For a wildcard's {namespace constraint} value to be the intensional union of two other such values (call them O1 and O2): the appropriate case among the following must be true:

1 If O1 and O2 are the same value, then that value must be the value.

2 If either O1 or O2 is any, then any must be the value.

3 If both O1 and O2 are sets of (namespace names or ·absent·), then the union of those sets must be the value.

4 If the two are negations of different values (namespace names or ·absent·), then a pair of not and ·absent· must be the value.

5 If either O1 or O2 is a pair of _not_and a namespace name and the other is a set of (namespace names or ·absent·) (call this set S), then The appropriate case among the following must be true:

5.1 If the set S includes both the negated namespace name and ·absent·, then any must be the value.

5.2 If the set S includes the negated namespace name but not ·absent·, then a pair of not and ·absent· must be the value.

5.3 If the set S includes ·absent·but not the negated namespace name, then the union is not expressible.

5.4 If the set S does not include either the negated namespace name or ·absent·, then whichever of O1 or O2 is a pair of _not_and a namespace name must be the value.

6 If either O1 or O2 is a pair of _not_and ·absent· and the other is a set of (namespace names or ·absent·) (again, call this set S), then The appropriate case among the following must be true:

6.1 If the set S includes ·absent·, then any must be the value.

6.2 If the set S does not include ·absent·, then a pair of not and ·absent· must be the value.

In the case where there are more than two values, the intensional union is determined by identifying the intensional union of two of the values as above, then the intensional union of that value with the third (providing the first union was expressible), and so on as required.

Schema Component Constraint: Attribute Wildcard Intersection

For a wildcard's {namespace constraint} value to be the intensional intersection of two other such values (call them O1 and O2): the appropriate case among the following must be true:

1 If O1 and O2 are the same value, then that value must be the value.

2 If either O1 or O2 is any, then the other must be the value.

3 If either O1 or O2 is a pair of _not_and a value (a namespace name or ·absent·) and the other is a set of (namespace names or ·absent·), then that set, minus the negated value if it was in the set, minus ·absent· if it was in the set, must be the value.

4 If both O1 and O2 are sets of (namespace names or ·absent·), then the intersection of those sets must be the value.

5 If the two are negations of different namespace names, then the intersection is not expressible.

6 If the one is a negation of a namespace name and the other is a negation of ·absent·, then the one which is the negation of a namespace name must be the value.

In the case where there are more than two values, the intensional intersection is determined by identifying the intensional intersection of two of the values as above, then the intensional intersection of that value with the third (providing the first intersection was expressible), and so on as required.

3.11 Identity-constraint Definitions

Identity-constraint definition components provide for uniqueness and reference constraints with respect to the contents of multiple elements and attributes.

<xs:key name="fullName"> <xs:selector xpath=".//person"/> <xs:field xpath="forename"/> <xs:field xpath="surname"/>

<xs:keyref name="personRef" refer="fullName"> <xs:selector xpath=".//personPointer"/> <xs:field xpath="@first"/> <xs:field xpath="@last"/>

<xs:unique name="nearlyID"> <xs:selector xpath=".//*"/> <xs:field xpath="@id"/>

XML representations for the three kinds of identity-constraint definitions.

3.11.1 The Identity-constraint Definition Schema Component

The identity-constraint definition schema component has the following properties:

Identity-constraint definitions are identified by their {name} and {target namespace}; Identity-constraint definition identities must be unique within an ·XML Schema·. See References to schema components across namespaces (§4.2.3) for the use of component identifiers when importing one schema into another.

Informally, {identity-constraint category} identifies the Identity-constraint definition as playing one of three roles:

• (unique) the Identity-constraint definition asserts uniqueness, with respect to the content identified by {selector}, of the tuples resulting from evaluation of the {fields} XPath expression(s).
• (key) the Identity-constraint definition asserts uniqueness as for_unique_. key further asserts that all selected content actually has such tuples.
• (keyref) the Identity-constraint definition asserts a correspondence, with respect to the content identified by {selector}, of the tuples resulting from evaluation of the {fields} XPath expression(s), with those of the {referenced key}.

These constraints are specified along side the specification of types for the attributes and elements involved, i.e. something declared as of type integer may also serve as a key. Each constraint declaration has a name, which exists in a single symbol space for constraints. The equality and inequality conditions appealed to in checking these constraints apply to the value of the fields selected, so that for example 3.0 and 3would be conflicting keys if they were both number, but non-conflicting if they were both strings, or one was a string and one a number. Values of differing type can only be equal if one type is derived from the other, and the value is in the value space of both.

Overall the augmentations to XML's ID/IDREF mechanism are:

• Functioning as a part of an identity-constraint is in addition to, not instead of, having a type;
• Not just attribute values, but also element content and combinations of values and content can be declared to be unique;
• Identity-constraints are specified to hold within the scope of particular elements;
• (Combinations of) attribute values and/or element content can be declared to be keys, that is, not only unique, but always present and non-nillable;
• The comparison between keyref {fields} and_key_ or unique {fields} is by value equality, not by string equality.

{selector} specifies a restricted XPath ([XPath]) expression relative to instances of the element being declared. This must identify a node set of subordinate elements (i.e. contained within the declared element) to which the constraint applies.

{fields} specifies XPath expressions relative to each element selected by a {selector}. This must identify a single node (element or attribute) whose content or value, which must be of a simple type, is used in the constraint. It is possible to specify an ordered list of {fields}s, to cater to multi-field keys, keyrefs, and uniqueness constraints.

In order to reduce the burden on implementers, in particular implementers of streaming processors, only restricted subsets of XPath expressions are allowed in {selector} and {fields}. The details are given in Constraints on Identity-constraint Definition Schema Components (§3.11.6).

Note: Provision for multi-field keys etc. goes beyond what is supported by xsl:key.

See Annotations (§3.13) for information on the role of the{annotation} property.

3.11.2 XML Representation of Identity-constraint Definition Schema Components

The XML representation for an identity-constraint definition schema component is either a, a or a element information item. The correspondences between the properties of those information items and properties of the component they correspond to are as follows:

<unique
id = ID
**name** = NCName
_{any attributes with non-schema namespace . . .}_>
Content: (annotation?, (selector, field+))

<key
id = ID
**name** = NCName
_{any attributes with non-schema namespace . . .}_>
Content: (annotation?, (selector, field+))

<keyref
id = ID
**name** = NCName
**refer** = QName
_{any attributes with non-schema namespace . . .}_>
Content: (annotation?, (selector, field+))

<selector
id = ID
**xpath** = _a subset of XPath expression, see below_
_{any attributes with non-schema namespace . . .}_>
Content: (annotation?)

<field
id = ID
**xpath** = _a subset of XPath expression, see below_
_{any attributes with non-schema namespace . . .}_>
Content: (annotation?)

<xs:element name="vehicle"> xs:complexType . . . <xs:attribute name="plateNumber" type="xs:integer"/> <xs:attribute name="state" type="twoLetterCode"/>

<xs:element name="state"> xs:complexType xs:sequence <xs:element name="code" type="twoLetterCode"/> <xs:element ref="vehicle" maxOccurs="unbounded"/> <xs:element ref="person" maxOccurs="unbounded"/>

<xs:key name="reg"> <xs:selector xpath=".//vehicle"/> <xs:field xpath="@plateNumber"/>

<xs:element name="root"> xs:complexType xs:sequence . . . <xs:element ref="state" maxOccurs="unbounded"/> . . .

<xs:key name="state"> <xs:selector xpath=".//state"/> <xs:field xpath="code"/>

<xs:keyref name="vehicleState" refer="state"> <xs:selector xpath=".//vehicle"/> <xs:field xpath="@state"/>

<xs:key name="regKey"> <xs:selector xpath=".//vehicle"/> <xs:field xpath="@state"/> <xs:field xpath="@plateNumber"/>

<xs:keyref name="carRef" refer="regKey"> <xs:selector xpath=".//car"/> <xs:field xpath="@regState"/> <xs:field xpath="@regPlate"/>

<xs:element name="person"> xs:complexType xs:sequence . . . <xs:element name="car"> xs:complexType <xs:attribute name="regState" type="twoLetterCode"/> <xs:attribute name="regPlate" type="xs:integer"/>

A state element is defined, which contains a code child and some vehicle and personchildren. A vehicle in turn has a plateNumber attribute, which is an integer, and a state attribute. State'scodes are a key for them within the document. Vehicle'splateNumbers are a key for them within states, andstate andplateNumber is asserted to be a key forvehicle within the document as a whole. Furthermore, a person element has an empty car child, with regState andregPlate attributes, which are then asserted together to refer tovehicles via the carRef constraint. The requirement that a vehicle's state match its containingstate's code is not expressed here.

3.11.4 Identity-constraint Definition Validation Rules

Validation Rule: Identity-constraint Satisfied

For an element information item to be locally ·valid· with respect to an identity-constraintall of the following must be true:

1 The {selector}, with the element information item as the context node, evaluates to a node-set (as defined in[XPath]). [Definition:] Call this the target node set.

2 Each node in the ·target node set· is either the context node oran element node among its descendants.

3 For each node in the ·target node set· all of the {fields}, with that node as the context node, evaluate to either an empty node-set or a node-set with exactly one member, which must have a simple type. [Definition:] Call the sequence of the type-determined values (as defined in [XML Schemas: Datatypes]) of the[schema normalized value] of the element and/or attribute information items in those node-sets in order the key-sequence of the node.

4 [Definition:] Call the subset of the ·target node set· for which all the {fields} evaluate to a node-set with exactly one member which is an element or attribute node with a simple type the qualified node set. The appropriate case among the following must be true:

4.1 If the {identity-constraint category} is unique, then no two members of the ·qualified node set· have ·key-sequences· whose members are pairwise equal, as defined by Equal in [XML Schemas: Datatypes].

4.2 If the {identity-constraint category} is key, then all of the following must be true:

4.2.1 The ·target node set· and the ·qualified node set· are equal, that is, every member of the ·target node set· is also a member of the ·qualified node set· and vice versa.

4.2.2 No two members of the ·qualified node set· have ·key-sequences· whose members are pairwise equal, as defined by Equal in [XML Schemas: Datatypes].

4.2.3 No element member of the ·key-sequence· of any member of the ·qualified node set· was assessed as ·valid· by reference to an element declaration whose {nillable} is true.

4.3 If the {identity-constraint category} is keyref, then for each member of the ·qualified node set· (call this the keyref member), there must be a ·node table· associated with the{referenced key} in the [identity-constraint table]of the element information item (see Identity-constraint Table (§3.11.5), which must be understood as logically prior to this clause of this constraint, below) and there must be an entry in that table whose·key-sequence· is equal to thekeyref member's ·key-sequence· member for member, as defined by Equal in [XML Schemas: Datatypes].

Note: The use of [schema normalized value] in the definition of ·key sequence· above means that_default_ or fixed value constraints may play a part in ·key sequence·s.

Note: Because the validation of keyref (see clause 4.3) depends on finding appropriate entries in a element information item's ·node table·, and ·node tables· are assembled strictly recursively from the node tables of descendants, only element information items within the sub-tree rooted at the element information item being ·validated· can be referenced successfully.

Note: Although this specification defines a ·post-schema-validation infoset·contribution which would enable schema-aware processors to implement clause 4.2.3 above (Element Declaration (§3.3.5)), processors are not required to provide it. This clause can be read as if in the absence of this infoset contribution, the value of the relevant {nillable} property must be available.

3.11.5 Identity-constraint Definition Information Set Contributions

Schema Information Set Contribution: Identity-constraint Table

[Definition:] An eligible identity-constraint of an element information item is one such that clause 4.1 or clause 4.2 of Identity-constraint Satisfied (§3.11.4) is satisfied with respect to that item and that constraint, or such that any of the element information item [children] of that item have an[identity-constraint table] property whose value has an entry for that constraint.

[Definition:] A node table is a set of pairs each consisting of a ·key-sequence· and an element node.

Whenever an element information item has one or more ·eligible identity-constraints·, in the ·post-schema-validation infoset· that element information item has a property as follows:

[identity-constraint table]

oneIdentity-constraint Bindinginformation item for each ·eligible identity-constraint·, with properties as follows:

[definition]

The ·eligible identity-constraint·.

[node table]

A ·node table· with one entry for every·key-sequence· (call it k) and node (call it n) such thatone of the following must be true:

1 There is an entry in one of the ·node tables· associated with the [definition] in anIdentity-constraint Bindinginformation item in at least one of the [identity-constraint table]s of the element information item [children] of the element information item whose ·key-sequence· is k and whose node is n;

2 n appears with·key-sequence· k in the ·qualified node set· for the [definition].

provided no two entries have the same ·key-sequence· but distinct nodes. Potential conflicts are resolved by not including any conflicting entries which would have owed their inclusion to clause 1 above. Note that if all the conflicting entries arose under clause 1 above, this means no entry at all will appear for the offending ·key-sequence·.

Note: The complexity of the above arises from the fact that_keyref_ identity-constraints may be defined on domains distinct from the embedded domain of the identity-constraint they reference, or the domains may be the same but self-embedding at some depth. In either case the ·node table· for the referenced identity-constraint needs to propagate upwards, with conflict resolution.

The Identity-constraint Bindinginformation item, unlike others in this specification, is essentially an internal bookkeeping mechanism. It is introduced to support the definition of Identity-constraint Satisfied (§3.11.4) above. Accordingly, conformant processors may, but are not required to, expose them via[identity-constraint table] properties in the ·post-schema-validation infoset·. In other words, the above constraints may be read as saying ·validation· of identity-constraints proceeds as if such infoset items existed.

3.11.6 Constraints on Identity-constraint Definition Schema Components

All identity-constraint definitions (see Identity-constraint Definitions (§3.11)) must satisfy the following constraint.

Schema Component Constraint: Selector Value OK

All of the following must be true:

1 The {selector} must be a valid XPath expression, as defined in [XPath].

2 One of the following must be true:

2.1 It must conform to the following extended BNF:

2.2 It must be an XPath expression involving the child axis whose abbreviated form is as given above.

For readability, whitespace may be used in selector XPath expressions even though not explicitly allowed by the grammar: whitespace may be freely added within patterns before or after any token.

When tokenizing, the longest possible token is always returned.

Schema Component Constraint: Fields Value OK

All of the following must be true:

2 One of the following must be true:

2.1 It must conform to the extended BNF given above for Selector, with the following modification:

This production differs from the one above in allowing the final step to match an attribute node.

2.2 It must be an XPath expression involving the child and/or attribute axes whose abbreviated form is as given above.

For readability, whitespace may be used in field XPath expressions even though not explicitly allowed by the grammar: whitespace may be freely added within patterns before or after any token.

When tokenizing, the longest possible token is always returned.

3.12 Notation Declarations

Notation declarations reconstruct XML 1.0 NOTATION declarations.

<xs:notation name="jpeg" public="image/jpeg" system="viewer.exe">

The XML representation of a notation declaration.

3.12.2 XML Representation of Notation Declaration Schema Components

The XML representation for a notation declaration schema component is aelement information item. The correspondences between the properties of that information item and properties of the component it corresponds to are as follows:

<xs:notation name="jpeg" public="image/jpeg" system="viewer.exe" />

<xs:element name="picture"> xs:complexType xs:simpleContent <xs:extension base="xs:hexBinary"> <xs:attribute name="pictype"> xs:simpleType <xs:restriction base="xs:NOTATION"> <xs:enumeration value="jpeg"/> <xs:enumeration value="png"/> . . .

...

3.13 Annotations

Annotations provide for human- and machine-targeted annotations of schema components.

<xs:simpleType fn:note="special"> xs:annotation xs:documentationA type for experts only xs:appinfo fn:specialHandlingcheckForPrimes

XML representations of three kinds of annotation.

3.13.1 The Annotation Schema Component

The annotation schema component has the following properties:

{user information} is intended for human consumption,{application information} for automatic processing. In both cases, provision is made for an optional URI reference to supplement the local information, as the value of the source attribute of the respective element information items. ·Validation· does not involve dereferencing these URIs, when present. In the case of {user information}, indication should be given as to the identity of the (human) language used in the contents, using the xml:lang attribute.

{attributes} ensures that when schema authors take advantage of the provision for adding attributes from namespaces other than the XML Schema namespace to schema documents, they are available within the components corresponding to the element items where such attributes appear.

Annotations do not participate in ·validation· as such. Provided an annotation itself satisfies all relevant ·Schema Component Constraints· it cannot affect the ·validation· of element information items.

3.13.2 XML Representation of Annotation Schema Components

Annotation of schemas and schema components, with material for human or computer consumption, is provided for by allowing application information and human information at the beginning of most major schema elements, and anywhere at the top level of schemas. The XML representation for an annotation schema component is anelement information item. The correspondences between the properties of that information item and properties of the component it corresponds to are as follows:

<annotation
id = ID
_{any attributes with non-schema namespace . . .}_>
Content: (appinfo | documentation)*

<appinfo
source = anyURI
{any attributes with non-schema namespace . . .}_>
Content: ({any}_)*

<documentation
source = anyURI
xml:lang = language
{any attributes with non-schema namespace . . .}_>
Content: ({any}_)*

The annotation component corresponding to the element in the example above will have one element item in each of its {user information} and {application information} and one attribute item in its {attributes}.

3.14 Simple Type Definitions

Note: This section consists of a combination of non-normative versions of normative material from [XML Schemas: Datatypes], for local cross-reference purposes, and normative material relating to the interface between schema components defined in this specification and the simple type definition component.

Simple type definitions provide for constraining character information item [children] of element and attribute information items.

<xs:simpleType name="fahrenheitWaterTemp"> <xs:restriction base="xs:number"> <xs:fractionDigits value="2"/> <xs:minExclusive value="0.00"/> <xs:maxExclusive value="100.00"/>

The XML representation of a simple type definition.

3.14.1 (non-normative) The Simple Type Definition Schema Component

The simple type definition schema component has the following properties:

Simple types are identified by their {name} and {target namespace}. Except for anonymous simple types (those with no {name}), since type definitions (i.e. both simple and complex type definitions taken together) must be uniquely identified within an ·XML Schema·, no simple type definition can have the same name as another simple or complex type definition. Simple type {name}s and {target namespace}s are provided for reference from instances (see xsi:type (§2.6.1)), and for use in the XML representation of schema components (specifically in and ). See References to schema components across namespaces (§4.2.3) for the use of component identifiers when importing one schema into another.

Note: The {name} of a simple type is not ipso facto the [(local) name] of the element or attribute information items ·validated· by that definition. The connection between a name and a type definition is described in Element Declarations (§3.3) and Attribute Declarations (§3.2).

A simple type definition with an empty specification for {final} can be used as the{base type definition} for other types derived by either of extension or restriction, or as the {item type definition} in the definition of a list, or in the {member type definitions} of a union; the explicit values extension, restriction,list and union prevent further derivations by extension (to yield a complex type) and restriction (to yield a simple type) and use in constructing lists and unions respectively.

{variety} determines whether the simple type corresponds to an atomic, list or union type as defined by [XML Schemas: Datatypes].

As described in Type Definition Hierarchy (§2.2.1.1), every simple type definition is a ·restriction· of some other simple type (the {base type definition}), which is the ·simple ur-type definition· if and only if the type definition in question is one of the built-in primitive datatypes, or a list or union type definition which is not itself derived by restriction from a list or union respectively. Each_atomic_ type is ultimately a restriction of exactly one such built-in primitive datatype, which is its {primitive type definition}.

{facets} for each simple type definition are selected from those defined in[XML Schemas: Datatypes]. For atomic definitions, these are restricted to those appropriate for the corresponding {primitive type definition}. Therefore, the value space and lexical space (i.e. what is ·validated· by any atomic simple type) is determined by the pair ({primitive type definition}, {facets}).

As specified in [XML Schemas: Datatypes], list simple type definitions ·validate· space separated tokens, each of which conforms to a specified simple type definition, the {item type definition}. The item type specified must not itself be a list type, and must be one of the types identified in [XML Schemas: Datatypes] as a suitable item type for a list simple type. In this case the {facets}apply to the list itself, and are restricted to those appropriate for lists.

A union simple type definition ·validates· strings which satisfy at least one of its {member type definitions}. As in the case of_list_, the {facets}apply to the union itself, and are restricted to those appropriate for unions.

The ·simple ur-type definition·must not be named as the ·base type definition· of any user-defined atomic simple type definitions: as it has no constraining facets, this would be incoherent.

See Annotations (§3.13) for information on the role of the{annotation} property.

3.14.2 (non-normative) XML Representation of Simple Type Definition Schema Components

<simpleType
final = (#all | List of (list | union | restriction))
id = ID
name = NCName
_{any attributes with non-schema namespace . . .}_>
Content: (annotation?, (restriction | list | union))

<restriction
base = QName
id = ID
_{any attributes with non-schema namespace . . .}_>
Content: (annotation?, (simpleType?, (minExclusive | minInclusive | maxExclusive | maxInclusive | totalDigits | fractionDigits | length | minLength | maxLength | enumeration | whiteSpace | pattern)*))

<list
id = ID
itemType = QName
_{any attributes with non-schema namespace . . .}_>
Content: (annotation?, simpleType?)

<union
id = ID
memberTypes = List of QName
_{any attributes with non-schema namespace . . .}_>
Content: (annotation?, simpleType*)

If the {variety} is atomic, the following additional property mappings also apply:

If the {variety} is list, the following additional property mappings also apply:

If the {variety} is union, the following additional property mappings also apply:

3.14.6 Constraints on Simple Type Definition Schema Components

All simple type definitions other than the ·simple ur-type definition· and the built-in primitive datatype definitions (see Simple Type Definitions (§3.14)) must satisfy both the following constraints.

Schema Component Constraint: Derivation Valid (Restriction, Simple)

The appropriate case among the following must be true:

1 If the {variety} is atomic, then all of the following must be true:

1.1 The {base type definition} must be an atomic simple type definition or a built-in primitive datatype.

1.2 The {final} of the {base type definition} must not contain restriction.

1.3 For each facet in the {facets} (call this DF) all of the following must be true:

1.3.1 DF must be an allowed constraining facet for the{primitive type definition}, as specified in the appropriate subsection of 3.2 Primitive datatypes.

1.3.2 If there is a facet of the same kind in the{facets} of the {base type definition} (call this BF),then the DF's{value} must be a valid restriction of BF's {value} as defined in [XML Schemas: Datatypes].

2 If the {variety} is list, then all of the following must be true:

2.1 The {item type definition} must have a {variety} of atomic or union (in which case all the {member type definitions} must be atomic).

2.2

2.3 The appropriate case among the following must be true:

2.3.1 If the {base type definition}is the·simple ur-type definition· , then all of the following must be true:

2.3.1.1 The {final} of the {item type definition} must not contain list.

2.3.1.2 The {facets} must only contain the_whiteSpace_ facet component.

2.3.2 otherwise all of the following must be true:

2.3.2.1 The {base type definition} must have a {variety} of list.

2.3.2.2 The {final} of the {base type definition} must not contain restriction.

2.3.2.3 The {item type definition} must be validly derived from the {base type definition}'s {item type definition} given the empty set, as defined in Type Derivation OK (Simple) (§3.14.6).

2.3.2.4 Only length, minLength, maxLength, whiteSpace,pattern and enumeration facet components are allowed among the {facets}.

2.3.2.5 For each facet in the {facets} (call this DF), if there is a facet of the same kind in the{facets} of the {base type definition} (call this BF),then the DF's{value} must be a valid restriction of BF's {value} as defined in [XML Schemas: Datatypes].

The first case above will apply when a list is derived by specifying an item type, the second when derived by restriction from another list.

3 If the {variety} is union, then all of the following must be true:

3.1 The {member type definitions} must all have {variety} of atomic or list.

3.2

3.3 The appropriate case among the following must be true:

3.3.1 If the {base type definition}is the·simple ur-type definition· , then all of the following must be true:

3.3.1.1 All of the {member type definitions} must have a {final} which does not contain union.

3.3.1.2 The {facets} must be empty.

3.3.2 otherwise all of the following must be true:

3.3.2.1 The {base type definition} must have a {variety} of union.

3.3.2.2 The {final} of the {base type definition} must not contain restriction.

3.3.2.3 The {member type definitions}, in order, must be validly derived from the corresponding type definitions in the {base type definition}'s {member type definitions} given the empty set, as defined in Type Derivation OK (Simple) (§3.14.6).

3.3.2.4 Only pattern and enumeration facet components are allowed among the {facets}.

3.3.2.5 For each facet in the {facets} (call this DF), if there is a facet of the same kind in the{facets} of the {base type definition} (call this BF),then the DF's{value} must be a valid restriction of BF's {value} as defined in [XML Schemas: Datatypes].

The first case above will apply when a union is derived by specifying one or more member types, the second when derived by restriction from another union.

[Definition:] If this constraint Derivation Valid (Restriction, Simple) (§3.14.6) holds of a simple type definition, it is a valid restriction of its ·base type definition·.

The following constraint defines relations appealed to elsewhere in this specification.

Note: With respect to clause 1, see the Note on identity at the end of (§3.4.6) above.

3.14.7 Built-in Simple Type Definition

There is a simple type definition nearly equivalent to the ·simple ur-type definition· present in every schema by definition. It has the following properties:

The ·simple ur-type definition· is the root of the simple type definition hierarchy, and as such mediates between the other simple type definitions, which all eventually trace back to it via their {base type definition} properties, and the ·ur-type definition·, which is_its_ {base type definition}. This is why the ·simple ur-type definition· is exempted from the first clause of Simple Type Definition Properties Correct (§3.14.6), which would otherwise bar it because of its derivation from a complex type definition and absence of {variety}.

Simple type definitions for all the built-in primitive datatypes, namely string, boolean, float,double, number, dateTime, duration,time, date, gMonth, gMonthDay, gDay, gYear, gYearMonth, hexBinary, base64Binary, anyURI (see the Primitive Datatypes section of [XML Schemas: Datatypes]) are present by definition in every schema. All are in the XML Schema {target namespace} (namespace name http://www.w3.org/2001/XMLSchema), have an atomic {variety} with an empty{facets} and the ·simple ur-type definition· as their ·base type definition· and themselves as {primitive type definition}.

Similarly, simple type definitions for all the built-in derived datatypes (see the Derived Datatypes section of [XML Schemas: Datatypes]) are present by definition in every schema, with properties as specified in [XML Schemas: Datatypes] and as represented in XML inSchema for Schemas (normative) (§A).

3.15 Schemas as a Whole

A schema consists of a set of schema components.

<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema" targetNamespace="" title="undefined" rel="noopener noreferrer">http://www.example.com/example"> . . .

The XML representation of the skeleton of a schema.

3.15.2 XML Representations of Schemas

A schema is represented in XML by one or more ·schema documents·, that is, one or more element information items. A ·schema document· contains representations for a collection of schema components, e.g. type definitions and element declarations, which have a common {target namespace}. A ·schema document· which has one or more element information items corresponds to a schema with components with more than one {target namespace}, see Import Constraints and Semantics (§4.2.3).

<schema
attributeFormDefault = (qualified | unqualified) : unqualified
blockDefault = (#all | List of (extension | restriction | substitution)) : ''
elementFormDefault = (qualified | unqualified) : unqualified
finalDefault = (#all | List of (extension | restriction | list | union)) : ''
id = ID
targetNamespace = anyURI
version = token
xml:lang = language
_{any attributes with non-schema namespace . . .}_>
Content: ((include | import | redefine | annotation)*, (((simpleType | complexType | group | attributeGroup) | element | attribute | notation), annotation*)*)

Note that none of the attribute information items displayed above correspond directly to properties of schemas. The blockDefault,finalDefault, attributeFormDefault, elementFormDefaultand targetNamespace attributes are appealed to in the sub-sections above, as they provide global information applicable to many representation/component correspondences. The other attributes (id and version) are for user convenience, and this specification defines no semantics for them.

The definition of the schema abstract data model in XML Schema Abstract Data Model (§2.2) makes clear that most components have a {target namespace}. Most components corresponding to representations within a given element information item will have a {target namespace} which corresponds to the targetNamespace attribute.

Since the empty string is not a legal namespace name, supplying an empty string for targetNamespace is incoherent, and is not the same as not specifying it at all. The appropriate form of schema document corresponding to a ·schema· whose components have no{target namespace} is one which has notargetNamespace attribute specified at all.

Note: The XML namespaces Recommendation discusses only instance document syntax for elements and attributes; it therefore provides no direct framework for managing the names of type definitions, attribute group definitions, and so on. Nevertheless, the specification applies the target namespace facility uniformly to all schema components, i.e. not only declarations but also definitions have a {target namespace}.

Although the example schema at the beginning of this section might be a complete XML document, need not be the document element, but can appear within other documents. Indeed there is no requirement that a schema correspond to a (text) document at all: it could correspond to an element information item constructed 'by hand', for instance via a DOM-conformant API.

Aside from and , which do not correspond directly to any schema component at all, each of the element information items which may appear in the content of corresponds to a schema component, and all except are named. The sections below present each such item in turn, setting out the components to which it may correspond.

3.15.2.1 References to Schema Components

Reference to schema components from a schema document is managed in a uniform way, whether the component corresponds to an element information item from the same schema document or is imported (References to schema components across namespaces (§4.2.3)) from an external schema (which may, but need not, correspond to an actual schema document). The form of all such references is a·QName·.

[Definition:] A QName is a name with an optional namespace qualification, as defined in [XML-Namespaces]. When used in connection with the XML representation of schema components or references to them, this refers to the simple type QName as defined in [XML Schemas: Datatypes].

[Definition:] An NCName is a name with no colon, as defined in [XML-Namespaces]. When used in connection with the XML representation of schema components in this specification, this refers to the simple type NCName as defined in [XML Schemas: Datatypes].

In each of the XML representation expositions in the following sections, an attribute is shown as having type QName if and only if it is interpreted as referencing a schema component.

<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema" xmlns:xhtml="http://www.w3.org/1999/xhtml" xmlns="http://www.example.com" targetNamespace="" title="undefined" rel="noopener noreferrer">http://www.example.com"> . . .

<xs:element name="elem1" type="Address"/>

<xs:element name="elem2" type="xhtml:blockquote"/>

<xs:attribute name="attr1" type="xsl:quantity"/> . . .

The first of these is most probably a local reference, i.e. a reference to a type definition corresponding to a element information item located elsewhere in the schema document, the other two refer to type definitions from schemas for other namespaces and assume that their namespaces have been declared for import. See References to schema components across namespaces (§4.2.3) for a discussion of importing.

3.15.2.2 References to Schema Components from Elsewhere

The names of schema components such as type definitions and element declarations are not of type ID: they are not unique within a schema, just within a symbol space. This means that simple fragment identifiers will not always work to reference schema components from outside the context of schema documents.

There is currently no provision in the definition of the interpretation of fragment identifiers for the text/xml MIME type, which is the MIME type for schemas, for referencing schema components as such. However, [XPointer] provides a mechanism which maps well onto the notion of symbol spaces as it is reflected in the XML representation of schema components. A fragment identifier of the form#xpointer(xs:schema/xs:element[@name="person"]) will uniquely identify the representation of a top-level element declaration with name person, and similar fragment identifiers can obviously be constructed for the other global symbol spaces.

Short-form fragment identifiers may also be used in some cases, that is when a DTD or XML Schema is available for the schema in question, and the provision of an id attribute for the representations of all primary and secondary schema components, which is of typeID, has been exploited.

It is a matter for applications to specify whether they interpret document-level references of either of the above varieties as being to the relevant element information item (i.e. without special recognition of the relation of schema documents to schema components) or as being to the corresponding schema component.

3.15.3 Constraints on XML Representations of Schemas

[Definition:] Whenever the word resolve in any form is used in this chapter in connection with a ·QName· in a schema document, the following definition QName resolution (Schema Document) (§3.15.3) should be understood:

Schema Representation Constraint: QName resolution (Schema Document)

For a ·QName·to resolve to a schema component of a specified kindall of the following must be true:

1 That component is a member of the value of the appropriate property of the schema which corresponds to the schema document within which the ·QName·appears, that is the appropriate case among the following must be true:

1.1 If the kind specified is simple or complex type definition, then the property is the {type definitions}.

1.2 If the kind specified is attribute declaration, then the property is the {attribute declarations}.

1.3 If the kind specified is element declaration, then the property is the {element declarations}.

1.4 If the kind specified is attribute group, then the property is the {attribute group definitions}.

1.5 If the kind specified is model group, then the property is the {model group definitions}.

1.6 If the kind specified is notation declaration, then the property is the {notation declarations}.

2 The component's {name} matches the ·local name· of the ·QName·;

3 The component's {target namespace} is identical to the ·namespace name· of the ·QName·;

4 The appropriate case among the following must be true:

4.1 If the ·namespace name· of the ·QName· is ·absent·, then one of the following must be true:

4.1.1 The element information item of the schema document containing the ·QName· has no targetNamespace [attribute].

4.1.2 The element information item of the that schema document contains an element information item with no namespace [attribute].

4.2 otherwise the ·namespace name· of the ·QName· is the same asone of the following:

4.2.1 The ·actual value· of the targetNamespace [attribute] of the element information item of the schema document containing the ·QName·.

4.2.2 The ·actual value· of the namespace [attribute] of some element information item contained in the element information item of that schema document.

.

3.15.4 Validation Rules for Schemas as a Whole

As the discussion above at Schema Component Details (§3) makes clear, at the level of schema components and ·validation·, reference to components by name is normally not involved. In a few cases, however, qualified names appearing in information items being·validated· must be resolved to schema components by such lookup. The following constraint is appealed to in these cases.

Validation Rule: QName resolution (Instance)

A pair of a local name and a namespace name (or ·absent·) resolve to a schema component of a specified kind in the context of ·validation· by appeal to the appropriate property of the schema being used for the ·assessment·. Each such property indexes components by name. The property to use is determined by the kind of component specified, that is, the appropriate case among the following must be true:

1 If the kind specified is simple or complex type definition, then the property is the {type definitions}.

2 If the kind specified is attribute declaration, then the property is the {attribute declarations}.

3 If the kind specified is element declaration, then the property is the {element declarations}.

4 If the kind specified is attribute group, then the property is the {attribute group definitions}.

5 If the kind specified is model group, then the property is the {model group definitions}.

6 If the kind specified is notation declaration, then the property is the {notation declarations}.

The component resolved to is the entry in the table whose {name} matches the local name of the pair and whose {target namespace} is identical to the namespace name of the pair.

3.15.5 Schema Information Set Contributions

Schema Information Set Contribution: Schema Information

Schema components provide a wealth of information about the basis of·assessment·, which may well be of relevance to subsequent processing. Reflecting component structure into a form suitable for inclusion in the ·post-schema-validation infoset· is the way this specification provides for making this information available.

Accordingly, [Definition:] by an item isomorphic to a component is meant an information item whose type is equivalent to the component's, with one property per property of the component, with the same name, and value either the same atomic value, or an information item corresponding in the same way to its component value, recursively, as necessary.

Processors must add a property in the ·post-schema-validation infoset·to the element information item at which ·assessment· began, as follows:

[schema information]

A set of namespace schema information information items, one for each namespace name which appears as the{target namespace} of any schema component in the schema used for that assessment, and one for ·absent· if any schema component in the schema had no {target namespace}. Each namespace schema information information item has the following properties and values:

[schema namespace]

A namespace name or ·absent·.

[schema components]

A (possibly empty) set of schema component information items, each one an ·item isomorphic· to a component whose {target namespace} is the sibling [schema namespace]property above, drawn from the schema used for ·assessment·.

[schema documents]

A (possibly empty) set of schema document information items, with properties and values as follows, for each schema document which contributed components to the schema, and whosetargetNamespace matches the sibling [schema namespace] property above (or whosetargetNamespace was ·absent·but that contributed components to that namespace by being d by a schema document with that targetNamespace as per Assembling a schema for a single target namespace from multiple schema definition documents (§4.2.1)):

The {schema components} property is provided for processors which wish to provide a single access point to the components of the schema which was used during ·assessment·. Lightweight processors are free to leave it empty, but if it is provided, it must contain at a minimum all the top-level (i.e. named) components which actually figured in the ·assessment·, either directly or (because an anonymous component which figured is contained within) indirectly.

Schema Information Set Contribution: ID/IDREF Table

In the ·post-schema-validation infoset· a set of ID/IDREF binding information items is associated with the ·validation root· element information item:

[ID/IDREF table]

A (possibly empty) set ofID/IDREF binding information items, as specified below.

[Definition:] Let theeligible item set be the set of consisting of every attribute or element information item for whichall of the following are true

1 its [validation context] is the·validation root·;

2 it was successfully ·validated· with respect to an attribute declaration as per Attribute Locally Valid (§3.2.4) or element declaration as perElement Locally Valid (Element) (§3.3.4) (as appropriate) whose attribute {type definition} or element {type definition} (respectively) is the built-in ID, IDREF or IDREFS simple type definition or a type derived from one of them.

Then there is one ID/IDREF binding in the [ID/IDREF table]for every distinct string which isone of the following:

1 the ·actual value· of a member of the ·eligible item set· whose type definition is or is derived from ID or IDREF;

2 one of the items in the ·actual value· of a member of the ·eligible item set· whose type definition is or is derived from IDREFS.

Each ID/IDREF binding has properties as follows:

[id]

The string identified above.

[binding]

A set consisting of every element information item for whichall of the following are true

1 its [validation context] is the·validation root·;

2 it has an attribute information item in its [attributes] or an element information item in its [children] which was ·validated· by the built-in ID simple type definition or a type derived from it whose[schema normalized value] is the [id] of this ID/IDREF binding.

The net effect of the above is to have one entry for every string used as an id, whether by declaration or by reference, associated with those elements, if any, which actually purport to have that id. See Validation Root Valid (ID/IDREF) (§3.3.4) above for the validation rule which actually checks for errors here.

Note: The ID/IDREF bindinginformation item, unlike most other aspects of this specification, is essentially an internal bookkeeping mechanism. It is introduced to support the definition of Validation Root Valid (ID/IDREF) (§3.3.4) above. Accordingly, conformant processors may, but are not required to, expose it in the ·post-schema-validation infoset·. In other words, the above constraint may be read as saying ·assessment· proceeds as if such an infoset item existed.

4 Schemas and Namespaces: Access and Composition

This chapter defines the mechanisms by which this specification establishes the necessary precondition for ·assessment·, namely access to one or more schemas. This chapter also sets out in detail the relationship between schemas and namespaces, as well as mechanisms for modularization of schemas, including provision for incorporating definitions and declarations from one schema in another, possibly with modifications.

Conformance (§2.4) describes three levels of conformance for schema processors, and Schemas and Schema-validity Assessment (§5) provides a formal definition of·assessment·. This section sets out in detail the 3-layer architecture implied by the three conformance levels. The layers are:

1. The ·assessment· core, relating schema components and instance information items;
2. Schema representation: the connections between XML representations and schema components, including the relationships between namespaces and schema components;
3. XML Schema web-interoperability guidelines: instance->schema and schema->schema connections for the WWW.

Layer 1 specifies the manner in which a schema composed of schema components can be applied to in the ·assessment· of an instance element information item. Layer 2 specifies the use of elements in XML documents as the standard XML representation for schema information in a broad range of computer systems and execution environments. To support interoperation over the World Wide Web in particular, layer 3 provides a set of conventions for schema reference on the Web. Additional details on each of the three layers is provided in the sections below.

4.1 Layer 1: Summary of the Schema-validity Assessment Core

The fundamental purpose of the ·assessment· core is to define ·assessment· for a single element information item and its descendants with respect to a complex type definition. All processors are required to implement this core predicate in a manner which conforms exactly to this specification.

·assessment· is defined with reference to an ·XML Schema· (note not a·schema document·) which consists of (at a minimum) the set of schema components (definitions and declarations) required for that·assessment·. This is not a circular definition, but rather a_post facto_ observation: no element information item can be fully assessed unless all the components required by any aspect of its (potentially recursive) ·assessment· are present in the schema.

As specified above, each schema component is associated directly or indirectly with a target namespace, or explicitly with no namespace. In the case of multi-namespace documents, components for more than one target namespace will co-exist in a schema.

Processors have the option to assemble (and perhaps to optimize or pre-compile) the entire schema prior to the start of an ·assessment· episode, or to gather the schema lazily as individual components are required. In all cases it is required that:

• The processor succeed in locating the ·schema components· transitively required to complete an ·assessment· (note that components derived from ·schema documents· can be integrated with components obtained through other means);
• no definition or declaration changes once it has been established;
• if the processor chooses to acquire declarations and definitions dynamically, that there be no side effects of such dynamic acquisition that would cause the results of ·assessment· to differ from that which would have been obtained from the same schema components acquired in bulk.

Note: the ·assessment· core is defined in terms of schema components at the abstract level, and no mention is made of the schema definition syntax (i.e. ). Although many processors will acquire schemas in this format, others may operate on compiled representations, on a programmatic representation as exposed in some programming language, etc.

The obligation of a schema-aware processor as far as the ·assessment·core is concerned is to implement one or more of the options for ·assessment· given below in Assessing Schema-Validity (§5.2). Neither the choice of element information item for that ·assessment·, nor which of the means of initiating ·assessment· are used, is within the scope of this specification.

Although ·assessment· is defined recursively, it is also intended to be implementable in streaming processors. Such processors may choose to incrementally assemble the schema during processing in response, for example, to encountering new namespaces. The implication of the invariants expressed above is that such incremental assembly must result in an ·assessment·outcome that is the same as would be given if ·assessment· was undertaken again with the final, fully assembled schema.

4.2 Layer 2: Schema Documents, Namespaces and Composition

The sub-sections of Schema Component Details (§3) define an XML representation for type definitions and element declarations and so on, specifying their target namespace and collecting them into schema documents. The two following sections relate to assembling a complete schema for ·assessment· from multiple sources. They should not be understood as a form of text substitution, but rather as providing mechanisms for distributed definition of schema components, with appropriate schema-specific semantics.

Note: The core ·assessment· architecture requires that a complete schema with all the necessary declarations and definitions be available. This may involve resolving both instance->schema and schema->schema references. As observed earlier in Conformance (§2.4), the precise mechanisms for resolving such references are expected to evolve over time. In support of such evolution, this specification observes the design principle that references from one schema document to a schema use mechanisms that directly parallel those used to reference a schema from an instance document.

Note: In the sections below, "schemaLocation" really belongs at layer 3. For convenience, it is documented with the layer 2 mechanisms of import and include, with which it is closely associated.

4.2.1 Assembling a schema for a single target namespace from multiple schema definition documents

Schema components for a single target namespace can be assembled from several ·schema documents·, that is several element information items:

<include
id = ID
**schemaLocation** = anyURI
_{any attributes with non-schema namespace . . .}_>
Content: (annotation?)

A information item may contain any number of elements. Their schemaLocation attributes, consisting of a URI reference, identify other ·schema documents·, that is information items.

The ·XML Schema· corresponding to contains not only the components corresponding to its definition and declaration [children], but also all the components of all the ·XML Schemas· corresponding to any d schema documents. Such included schema documents must either (a) have the sametargetNamespace as the ing schema document, or (b) no targetNamespace at all, in which case the d schema document is converted to the ing schema document's targetNamespace.

Schema Representation Constraint: Inclusion Constraints and Semantics

In addition to the conditions imposed on element information items by the schema for schemas, all of the following must be true:

1 If the ·actual value· of the schemaLocation [attribute]successfully resolvesone of the following must be true:

1.1 It resolves to (a fragment of) a resource which is an XML document (of typeapplication/xml or text/xml with an XML declaration for preference, but this is not required), which in turn corresponds to a element information item in a well-formed information set, which in turn corresponds to a valid schema.

1.2 It resolves to a element information item in a well-formed information set, which in turn corresponds to a valid schema.

In either case call the d item SII, the valid schema I and the ing item's parent item SII’.

2 One of the following must be true:

3 The appropriate case among the following must be true:

3.1 If clause 2.1 or clause 2.2 above is satisfied, then the schema corresponding to SII’ must include not only definitions or declarations corresponding to the appropriate members of its own [children], but also components identical to all the ·schema components· of I.

3.2 If clause 2.3 above is satisfied, then the schema corresponding to thed item's parent must include not only definitions or declarations corresponding to the appropriate members of its own [children], but also components identical to all the ·schema components· of I, except that anywhere the ·absent· target namespace name would have appeared, the ·actual value· of thetargetNamespace [attribute] of SII’ is used. In particular, it replaces ·absent· in the following places:

3.2.1 The {target namespace} of named schema components, both at the top level and (in the case of nested type definitions and nested attribute and element declarations whose code was qualified) nested within definitions;

3.2.2 The {namespace constraint} of a wildcard, whether negated or not;

It is not an error for the ·actual value· of theschemaLocation [attribute] to fail to resolve it all, in which case no corresponding inclusion is performed. It_is_ an error for it to resolve but the rest of clause 1 above to fail to be satisfied. Failure to resolve may well cause less than complete·assessment· outcomes, of course.

As discussed in Missing Sub-components (§5.3), ·QName·s in XML representations may fail to·resolve·, rendering components incomplete and unusable because of missing subcomponents. During schema construction, implementations must retain ·QName· values for such references, in case an appropriately-named component becomes available to discharge the reference by the time it is actually needed. ·Absent· target ·namespace name·s of such as-yet unresolved reference ·QName·s in d components must also be converted if clause 3.2 is satisfied.

Note: The above is carefully worded so that multiple ing of the same schema document will not constitute a violation of clause 2 of Schema Properties Correct (§3.15.6), but applications are allowed, indeed encouraged, to avoid ing the same schema document more than once to forestall the necessity of establishing identity component by component.

4.2.2 Including modified component definitions

In order to provide some support for evolution and versioning, it is possible to incorporate components corresponding to a schema document_with modifications_. The modifications have a pervasive impact, that is, only the redefined components are used, even when referenced from other incorporated components, whether redefined themselves or not.

<redefine
id = ID
**schemaLocation** = anyURI
_{any attributes with non-schema namespace . . .}_>
Content: (annotation | (simpleType | complexType | group | attributeGroup))*

A information item may contain any number of elements. Their schemaLocation attributes, consisting of a URI reference, identify other ·schema documents·, that is information items.

The ·XML Schema· corresponding to contains not only the components corresponding to its definition and declaration [children], but also all the components of all the ·XML Schemas· corresponding to any d schema documents. Such schema documents must either (a) have the sametargetNamespace as the ing schema document, or (b) no targetNamespace at all, in which case the d schema document is converted to the ing schema document's targetNamespace.

The definitions within the element itself are restricted to be redefinitions of components from the d schema document, in terms of themselves. That is,

• Type definitions must use themselves as their base type definition;
• Attribute group definitions and model group definitions must be supersets or subsets of their original definitions, either by including exactly one reference to themselves or by containing only (possibly restricted) components which appear in a corresponding way in their d selves.

Not all the components of the d schema document need be redefined.

This mechanism is intended to provide a declarative and modular approach to schema modification, with functionality no different except in scope from what would be achieved by wholesale text copying and redefinition by editing. In particular redefining a type is not guaranteed to be side-effect free: it may have unexpected impacts on other type definitions which are based on the redefined one, even to the extent that some such definitions become ill-formed.

Note: The pervasive impact of redefinition reinforces the need for implementations to adopt some form of lazy or 'just-in-time' approach to component construction, which is also called for in order to avoid inappropriate dependencies on the order in which definitions and references appear in (collections of) schema documents.

v1.xsd: <xs:complexType name="personName"> xs:sequence <xs:element name="title" minOccurs="0"/> <xs:element name="forename" minOccurs="0" maxOccurs="unbounded"/>

<xs:element name="addressee" type="personName"/>

v2.xsd: <xs:redefine schemaLocation="v1.xsd"> <xs:complexType name="personName"> xs:complexContent <xs:extension base="personName"> xs:sequence <xs:element name="generation" minOccurs="0"/>

<xs:element name="author" type="personName"/>

The schema corresponding to v2.xsd has everything specified by v1.xsd, with the personName type redefined, as well as everything it specifies itself. According to this schema, elements constrained by the personName type may end with a generationelement. This includes not only the author element, but also theaddressee element.

Schema Representation Constraint: Redefinition Constraints and Semantics

In addition to the conditions imposed on element information items by the schema for schemasall of the following must be true:

1 If there are any element information items among the [children]other than then the ·actual value· of theschemaLocation [attribute] must successfully resolve.

2 If the ·actual value· of the schemaLocation [attribute]successfully resolvesone of the following must be true:

2.1 it resolves to (a fragment of) a resource which is an XML document (see clause 1.1), which in turn corresponds to a element information item in a well-formed information set, which in turn corresponds to a valid schema.

2.2 It resolves to a element information item in a well-formed information set, which in turn corresponds to a valid schema.

In either case call the d item SII, the valid schema I and the ing item's parent item SII’.

3 One of the following must be true:

4 The appropriate case among the following must be true:

4.1 If clause 3.1 or clause 3.2 above is satisfied, then the schema corresponding to SII’ must include not only definitions or declarations corresponding to the appropriate members of its own[children], but also components identical to all the ·schema components· of I, with the exception of those explicitly redefined (see Individual Component Redefinition (§4.2.2) below).

4.2 If clause 3.3 above is satisfied, then the schema corresponding to SII’ must include not only definitions or declarations corresponding to the appropriate members of its own [children], but also components identical to all the ·schema components· of I, with the exception of those explicitly redefined (see Individual Component Redefinition (§4.2.2)below), except that anywhere the ·absent· target namespace name would have appeared, the ·actual value· of thetargetNamespace [attribute] of SII’ is used (see clause 3.2 in Inclusion Constraints and Semantics (§4.2.1) for details).

5 Within the [children], each must have a among its [children] and each must have a restriction or extension among its grand-[children] the ·actual value· of whose base [attribute] must be the same as the ·actual value· of its own name attribute plus target namespace;

6 Within the [children], for each the appropriate case among the following must be true:

6.1 If it has a among its contents at some level the ·actual value· of whoseref [attribute] is the same as the ·actual value· of its ownname attribute plus target namespace, then all of the following must be true:

6.1.1 It must have exactly one such group.

6.1.2 The ·actual value· of both that group's minOccurs andmaxOccurs [attribute] must be 1 (or ·absent·).

6.2 If it has no such self-reference, then all of the following must be true:

6.2.1 The ·actual value· of its own name attribute plus target namespace must successfully ·resolve· to a model group definition in I.

6.2.2 The {model group} of the model group definition which corresponds to it per XML Representation of Model Group Definition Schema Components (§3.7.2) must be a ·valid restriction· of the {model group} of that model group definition in I, as defined in Particle Valid (Restriction) (§3.9.6).

7 Within the [children], for each the appropriate case among the following must be true:

7.1 If it has an among its contents the ·actual value· of whoseref [attribute] is the same as the ·actual value· of its ownname attribute plus target namespace, then it must have exactly one such group.

7.2 If it has no such self-reference, then all of the following must be true:

7.2.1 The ·actual value· of its own name attribute plus target namespace must successfully ·resolve· to an attribute group definition in I.

7.2.2 The {attribute uses} and {attribute wildcard} of the attribute group definition which corresponds to it per XML Representation of Attribute Group Definition Schema Components (§3.6.2) must be ·valid restrictions· of the {attribute uses} and{attribute wildcard} of that attribute group definition in I, as defined in clause 2, clause 3 and clause 4 of Derivation Valid (Restriction, Complex) (§3.4.6) (where references to the base type definition are understood as references to the attribute group definition in I).

Note: An attribute group restrictively redefined per clause 7.2 corresponds to an attribute group whose {attribute uses} consist all and only of those attribute uses corresponding to s explicitly present among the [children] of the ing . No inheritance from the d attribute group occurs. Its {attribute wildcard} is similarly based purely on an explicit , if present.

Schema Representation Constraint: Individual Component Redefinition

Corresponding to each non- member of the[children] of a there are one or two schema components in the ing schema:

1 The and [children] information items each correspond to two components:

1.1 One component which corresponds to the top-level definition item with the same name in the d schema document, as defined in Schema Component Details (§3), except that its {name} is ·absent·;

1.2 One component which corresponds to the information item itself, as defined in Schema Component Details (§3), except that its {base type definition} is the component defined in 1.1 above.

This pairing ensures the coherence constraints on type definitions are respected, while at the same time achieving the desired effect, namely that references to names of redefined components in both the ing andd schema documents resolve to the redefined component as specified in 1.2 above.

2 The and [children] each correspond to a single component, as defined in Schema Component Details (§3), except that if and when a self-reference based on a ref [attribute] whose ·actual value· is the same as the item's name plus target namespace is resolved, a component which corresponds to the top-level definition item of that name and the appropriate kind inI is used.

In all cases there must be a top-level definition item of the appropriate name and kind in the d schema document.

Note: The above is carefully worded so that multiple equivalent ing of the same schema document will not constitute a violation of clause 2 of Schema Properties Correct (§3.15.6), but applications are allowed, indeed encouraged, to avoid ing the same schema document in the same way more than once to forestall the necessity of establishing identity component by component (although this will have to be done for the individual redefinitions themselves).

4.2.3 References to schema components across namespaces

As described in XML Schema Abstract Data Model (§2.2), every top-level schema component is associated with a target namespace (or, explicitly, with none). This section sets out the exact mechanism and syntax in the XML form of schema definition by which a reference to a foreign component is made, that is, a component with a different target namespace from that of the referring component.

Two things are required: not only a means of addressing such foreign components but also a signal to schema-aware processors that a schema document contains such references:

<import
id = ID
namespace = anyURI
schemaLocation = anyURI
_{any attributes with non-schema namespace . . .}_>
Content: (annotation?)

The element information item identifies namespaces used in external references, i.e. those whose·QName· identifies them as coming from a different namespace (or none) than the enclosing schema document'stargetNamespace. The ·actual value· of its namespace [attribute] indicates that the containing schema document may contain qualified references to schema components in that namespace (via one or more prefixes declared with namespace declarations in the normal way). If that attribute is absent, then the import allows unqualified reference to components with no target namespace. Note that components to be imported need not be in the form of a·schema document·; the processor is free to access or construct components using means of its own choosing.

The·actual value· of the schemaLocation, if present, gives a hint as to where a serialization of a ·schema document· with declarations and definitions for that namespace (or none) may be found. When noschemaLocation [attribute] is present, the schema author is leaving the identification of that schema to the instance, application or user, via the mechanisms described below in Layer 3: Schema Document Access and Web-interoperability (§4.3). When a schemaLocation is present, it must contain a single URI reference which the schema author warrants will resolve to a serialization of a ·schema document· containing the component(s) in theed namespace referred to elsewhere in the containing schema document.

Note: Since both the namespace and schemaLocation [attribute] are optional, a bare <import/> information item is allowed. This simply allows unqualified reference to foreign components with no target namespace without giving any hints as to where to find them.

The same namespace may be used both for real work, and in the course of defining schema components in terms of foreign components:

. . .

The treatment of references as ·QNames· implies that since (with the exception of the schema for schemas) the target namespace and the XML Schema namespace differ, without massive redeclaration of the default namespace_either_ internal references to the names being defined in a schema document_or_ the schema declaration and definition elements themselves must be explicitly qualified. This example takes the first option -- most other examples in this specification have taken the second.

Schema Representation Constraint: Import Constraints and Semantics

In addition to the conditions imposed on element information items by the schema for schemasall of the following must be true:

1 The appropriate case among the following must be true:

2 If the application schema reference strategy using the ·actual value·s of the schemaLocation and namespace [attributes], provides a referent, as defined by Schema Document Location Strategy (§4.3.2), one of the following must be true:

2.1 The referent is (a fragment of) a resource which is an XML document (see clause 1.1), which in turn corresponds to a element information item in a well-formed information set, which in turn corresponds to a valid schema.

2.2 The referent is a element information item in a well-formed information set, which in turn corresponds to a valid schema.

In either case call the item SII and the valid schema I.

3 The appropriate case among the following must be true:

It is not an error for the application schema reference strategy to fail. It_is_ an error for it to resolve but the rest of clause 2 above to fail to be satisfied. Failure to find a referent may well cause less than complete·assessment· outcomes, of course.

The ·schema components· (that is{type definitions}, {attribute declarations}, {element declarations}, {attribute group definitions}, {model group definitions}, {notation declarations}) of a schema corresponding to a element information item with one or more element information items must include not only definitions or declarations corresponding to the appropriate members of its [children], but also, for each of those element information items for which clause 2 above is satisfied, a set of ·schema components· identical to all the ·schema components· of I.

Note: The above is carefully worded so that multiple ing of the same schema document will not constitute a violation of clause 2 of Schema Properties Correct (§3.15.6), but applications are allowed, indeed encouraged, to avoid ing the same schema document more than once to forestall the necessity of establishing identity component by component. Given that the schemaLocation [attribute] is only a hint, it is open to applications to ignore all but the first for a given namespace, regardless of the ·actual value· ofschemaLocation, but such a strategy risks missing useful information when new schemaLocations are offered.

4.3 Layer 3: Schema Document Access and Web-interoperability

Layers 1 and 2 provide a framework for ·assessment· and XML definition of schemas in a broad variety of environments. Over time, a range of standards and conventions may well evolve to support interoperability of XML Schema implementations on the World Wide Web. Layer 3 defines the minimum level of function required of all conformant processors operating on the Web: it is intended that, over time, future standards (e.g. XML Packages) for interoperability on the Web and in other environments can be introduced without the need to republish this specification.

4.3.1 Standards for representation of schemas and retrieval of schema documents on the Web

For interoperability, serialized ·schema documents·, like all other Web resources, may be identified by URI and retrieved using the standard mechanisms of the Web (e.g. http, https, etc.) Such documents on the Web must be part of XML documents (see clause 1.1), and are represented in the standard XML schema definition form described by layer 2 (that is as element information items).

Note: there will often be times when a schema document will be a complete XML 1.0 document whose document element is . There will be other occasions in which items will be contained in other documents, perhaps referenced using fragment and/or XPointer notation.

Note: The variations among server software and web site administration policies make it difficult to recommend any particular approach to retrieval requests intended to retrieve serialized ·schema documents·. An Accept header of application/xml, text/xml; q=0.9, */* is perhaps a reasonable starting point.

4.3.2 How schema definitions are located on the Web

As described in Layer 1: Summary of the Schema-validity Assessment Core (§4.1), processors are responsible for providing the schema components (definitions and declarations) needed for ·assessment·. This section introduces a set of normative conventions to facilitate interoperability for instance and schema documents retrieved and processed from the Web.

Note: As discussed above in Layer 2: Schema Documents, Namespaces and Composition (§4.2), other non-Web mechanisms for delivering schemas for ·assessment· may exist, but are outside the scope of this specification.

Processors on the Web are free to undertake ·assessment· against arbitrary schemas in any of the ways set out in Assessing Schema-Validity (§5.2). However, it is useful to have a common convention for determining the schema to use. Accordingly, general-purpose schema-aware processors (i.e. those not specialized to one or a fixed set of pre-determined schemas) undertaking ·assessment· of a document on the web must behave as follows:

• unless directed otherwise by the user, ·assessment· is undertaken on the document element information item of the specified document;
• unless directed otherwise by the user, the processor is required to construct a schema corresponding to a schema document whose targetNamespace is identical to the namespace name, if any, of the element information item on which ·assessment· is undertaken.

The composition of the complete schema for use in ·assessment· is discussed in Layer 2: Schema Documents, Namespaces and Composition (§4.2) above. The means used to locate appropriate schema document(s) are processor and application dependent, subject to the following requirements:

1. Schemas are represented on the Web in the form specified above in Standards for representation of schemas and retrieval of schema documents on the Web (§4.3.1);
2. The author of a document uses namespace declarations to indicate the intended interpretation of names appearing therein; there may or may not be a schema retrievable via the namespace name. Accordingly whether a processor's default behavior is or is not to attempt such dereferencing, it must always provide for user-directed overriding of that default.
   Note: Experience suggests that it is not in all cases safe or desirable from a performance point of view to dereference namespace names as a matter of course. User community and/or consumer/provider agreements may establish circumstances in which such dereference is a sensible default strategy: this specification allows but does not require particular communities to establish and implement such conventions. Users are always free to supply namespace names as schema location information when dereferencing is desired: see below.
3. On the other hand, in case a document author (human or not) created a document with a particular schema in view, and warrants that some or all of the document conforms to that schema, theschemaLocation and noNamespaceSchemaLocation [attributes] (in the XML Schema instance namespace, that is, http://www.w3.org/2001/XMLSchema-instance) (hereafterxsi:schemaLocation andxsi:noNamespaceSchemaLocation) are provided. The first records the author's warrant with pairs of URI references (one for the namespace name, and one for a hint as to the location of a schema document defining names for that namespace name). The second similarly provides a URI reference as a hint as to the location of a schema document with no targetNamespace [attribute].
   Unless directed otherwise, for example by the invoking application or by command line option, processors should attempt to dereference each schema document location URI in the·actual value· of suchxsi:schemaLocation and xsi:noNamespaceSchemaLocation [attributes], see details below.
4. xsi:schemaLocation andxsi:noNamespaceSchemaLocation [attributes] can occur on any element. However, it is an error if such an attribute occurs_after_ the first appearance of an element or attribute information item within an element information item initially ·validated· whose [namespace name] it addresses. According to the rules ofLayer 1: Summary of the Schema-validity Assessment Core (§4.1), the corresponding schema may be lazily assembled, but is otherwise stable throughout ·assessment·. Although schema location attributes can occur on any element, and can be processed incrementally as discovered, their effect is essentially global to the ·assessment·. Definitions and declarations remain in effect beyond the scope of the element on which the binding is declared.

Multiple schema bindings can be declared using a single attribute. For example consider a stylesheet:

The namespace names used in schemaLocation can, but need not be identical to those actually qualifying the element within whose start tag it is found or its other attributes. For example, as above, all schema location information can be declared on the document element of a document, if desired, regardless of where the namespaces are actually used.

Improved or alternative conventions for Web interoperability can be standardized in the future without reopening this specification. For example, the W3C is currently considering initiatives to standardize the packaging of resources relating to particular documents and/or namespaces: this would be an addition to the mechanisms described here for layer 3. This architecture also facilitates innovation at layer 2: for example, it would be possible in the future to define an additional standard for the representation of schema components which allowed e.g. type definitions to be specified piece by piece, rather than all at once.

5 Schemas and Schema-validity Assessment

The architecture of schema-aware processing allows for a rich characterization of XML documents: schema validity is not a binary predicate.

This specification distinguishes between errors in schema construction and structure, on the one hand, and schema validation outcomes, on the other.

5.1 Errors in Schema Construction and Structure

Before ·assessment· can be attempted, a schema is required. Special-purpose applications are free to determine a schema for use in ·assessment· by whatever means are appropriate, but general purpose processors should implement the strategy set out in Schema Document Location Strategy (§4.3.2), starting with the namespaces declared in the document whose ·assessment· is being undertaken, and the ·actual value·s of thexsi:schemaLocation and xsi:noNamespaceSchemaLocation [attributes] thereof, if any, along with any other information about schema identity or schema document location provided by users in application-specific ways, if any.

It is an error if a schema and all the components which are the value of any of its properties, recursively, fail to satisfy all the relevant Constraints on Schemas set out in the last section of each of the subsections of Schema Component Details (§3).

If a schema is derived from one or more schema documents (that is, one or more element information items) based on the correspondence rules set out in Schema Component Details (§3) and Schemas and Namespaces: Access and Composition (§4), two additional conditions hold:

• It is an error if any such schema document would not be fully valid with respect to a schema corresponding to the Schema for Schemas (normative) (§A), that is, following schema-validation with such a schema, the element information items would have a [validation attempted]property with value full or partial and a[validity] property with value valid.
• It is an error if any such schema document is or contains any element information items which violate any of the relevant Schema Representation Constraints set out in Schema Representation Constraints (§C.3).

The three cases described above are the only types of error which this specification defines. With respect to the processes of the checking of schema structure and the construction of schemas corresponding to schema documents, this specification imposes no restrictions on processors after an error is detected. However ·assessment· with respect to schema-like entities which do _not_satisfy all the above conditions is incoherent. Accordingly, conformant processors must not attempt to undertake ·assessment· using such non-schemas.

5.2 Assessing Schema-Validity

With a schema which satisfies the conditions expressed in Errors in Schema Construction and Structure (§5.1) above, the schema-validity of an element information item can be assessed. Three primary approaches to this are possible:

1 The user or application identifies a complex type definition from among the{type definitions} of the schema, and appeals to Schema-Validity Assessment (Element) (§3.3.4) (clause 1.2);

2 The user or application identifies a element declaration from among the{element declarations} of the schema, checks that its {name} and {target namespace} match the [local name] and [namespace name] of the item, and appeals to Schema-Validity Assessment (Element) (§3.3.4) (clause 1.1);

3 The processor starts from Schema-Validity Assessment (Element) (§3.3.4) with no stipulated declaration or definition, and either ·strict· or ·lax· assessment ensues, depending on whether or not the element information and the schema determine either an element declaration (by name) or a type definition (via xsi:type) or not.

The outcome of this effort, in any case, will be manifest in the[validation attempted]and [validity] properties on the element information item and its [attributes] and [children], recursively, as defined by Assessment Outcome (Element) (§3.3.5) and Assessment Outcome (Attribute) (§3.2.5). It is up to applications to decide what constitutes a successful outcome.

Note that every element and attribute information item participating in the ·assessment· will also have a [validation context] property which refers back to the element information item at which ·assessment· began. [Definition:] This item, that is the element information item at which ·assessment· began, is called the validation root.

Note: This specification does not reconstruct the XML 1.0 notion of_root_ in either schemas or instances. Equivalent functionality is provided for at ·assessment·invocation, via clause 2 above.

Note: This specification has nothing normative to say about multiple ·assessment·episodes. It should however be clear from the above that if a processor restarts ·assessment· with respect to a ·post-schema-validation infoset·some ·post-schema-validation infoset· contributions from the previous ·assessment·may be overwritten. Restarting nonetheless may be useful, particularly at a node whose[validation attempted] property is none, in which case there are three obvious cases in which additional useful information may result:

• ·assessment· was not attempted because of a ·validation· failure, but declarations and/or definitions are available for at least some of the [children] or [attributes];
• ·assessment· was not attempted because a named definition or declaration was missing, but after further effort the processor has retrieved it.
• ·assessment· was not attempted because it was _skip_ped, but the processor has at least some declarations and/or definitions available for at least some of the [children] or [attributes].

5.3 Missing Sub-components

At the beginning of Schema Component Details (§3), attention is drawn to the fact that most kinds of schema components have properties which are described therein as having other components, or sets of other components, as values, but that when components are constructed on the basis of their correspondence with element information items in schema documents, such properties usually correspond to QNames, and the·resolution· of such QNames may fail, resulting in one or more values of or containing ·absent· where a component is mandated.

If at any time during ·assessment·, an element or attribute information item is being ·validated· with respect to a component of any kind any of whose properties has or contains such an ·absent· value, the ·validation· is modified, as following:

• In the case of attribute information items, the effect is as if clause 1 of Attribute Locally Valid (§3.2.4) had failed;
• In the case of element information items, the effect is as if clause 1 of Element Locally Valid (Element) (§3.3.4) had failed;
• In the case of element information items, processors may choose to continue ·assessment·: see ·lax assessment·.

Because of the value specification for [validation attempted] in Assessment Outcome (Element) (§3.3.5), if this situation ever arises, the document as a whole cannot show a [validation attempted]of full.