CloudFormation template Conditions syntax - AWS CloudFormation (original) (raw)

The optional Conditions section contains statements that define the circumstances under which entities are created or configured. For example, you can create a condition and then associate it with a resource or output so that CloudFormation only creates the resource or output if the condition is true. Similarly, you can associate the condition with a property so that CloudFormation only sets the property to a specific value if the condition is true. If the condition is false, CloudFormation sets the property to a different value that you specify.

You might use conditions when you want to reuse a template that can create resources in different contexts, such as a test environment versus a production environment. In your template, you can add an EnvironmentType input parameter, which accepts eitherprod or test as inputs. For the production environment, you might include Amazon EC2 instances with certain capabilities; however, for the test environment, you want to use reduced capabilities to save money. With conditions, you can define which resources are created and how they're configured for each environment type.

Conditions are evaluated based on predefined pseudo parameters or input parameter values that you specify when you create or update a stack. Within each condition, you can reference another condition, a parameter value, or a mapping. After you define all your conditions, you can associate them with resources and resource properties in the Resources and Outputs sections of a template.

At stack creation or stack update, AWS CloudFormation evaluates all the conditions in your template before creating any resources. Resources that are associated with a true condition are created. Resources that are associated with a false condition are ignored. CloudFormation also re-evaluates these conditions at each stack update before updating any resources. Resources that are still associated with a true condition are updated. Resources that are now associated with a false condition are deleted.

Important

During a stack update, you can't update conditions by themselves. You can update conditions only when you include changes that add, modify, or delete resources.

How to use conditions

Depending on the entity you want to conditionally create or configure, you must include statements in the following template sections:

Parameters section

Define the inputs that you want your conditions to evaluate. The conditions evaluate to true or false based on the values of these input parameters. If you want your conditions to evaluate pseudo parameters, you don't need to define the pseudo parameters in this section; pseudo parameters are predefined by CloudFormation. For more information about pseudo parameters, see Pseudo parameters reference.

Conditions section

Define conditions by using the intrinsic condition functions. These conditions determine when CloudFormation creates the associated resources. For examples, see Sample templates.

Resources and Outputs sections

Associate conditions with the resources or outputs that you want to conditionally create. CloudFormation creates entities that are associated with a true condition and ignores entities that are associated with a false condition. Use the Condition key and a condition's logical ID to associate it with a resource or output. To conditionally specify a property, use the Fn::If function. For an example, see Associating a condition.

Syntax

The Conditions section consists of the key name Conditions. Each condition declaration includes a logical ID and intrinsic functions that are evaluated when you create or update a stack. The following pseudo template outlines theConditions section:

JSON

"Conditions" : {
  "ConditionLogicalID" : {Intrinsic function}
}

YAML

Conditions:
  ConditionLogicalID:
    Intrinsic function

Condition intrinsic functions

You can use the following intrinsic functions to define conditions:

For the syntax and information about each condition function, see Condition functions. For the syntax and information aboutFn::ForEach, see Fn::ForEach.

Note

Fn::If is only supported in the metadata attribute, update policy attribute, and property values in the Resources section and Outputs sections of a template.

Examples

Simple condition

The following sample template includes an EnvType input parameter, where you can specify prod to create a stack for production ortest to create a stack for testing. For a production environment, CloudFormation creates an Amazon EC2 instance and attaches a volume to the instance. For a test environment, CloudFormation creates only the Amazon EC2 instance.

The CreateProdResources condition evaluates to true if the EnvType parameter is equal to prod. In the sample template, the NewVolume and MountPoint resources are associated with the CreateProdResources condition. Therefore, the resources are created only if the EnvType parameter is equal toprod.

JSON

{
    "AWSTemplateFormatVersion": "2010-09-09",
    "Parameters": {
        "EnvType": {
            "Description": "Environment type.",
            "Default": "test",
            "Type": "String",
            "AllowedValues": [
                "prod",
                "test"
            ],
            "ConstraintDescription": "must specify prod or test."
        }
    },
    "Conditions": {
        "CreateProdResources": {
            "Fn::Equals": [
                {
                    "Ref": "EnvType"
                },
                "prod"
            ]
        }
    },
    "Resources": {
        "EC2Instance": {
            "Type": "AWS::EC2::Instance",
            "Properties": {
                "ImageId": "ami-0ff8a91507f77f867"
            }
        },
        "MountPoint": {
            "Type": "AWS::EC2::VolumeAttachment",
            "Condition": "CreateProdResources",
            "Properties": {
                "InstanceId": {
                    "Ref": "EC2Instance"
                },
                "VolumeId": {
                    "Ref": "NewVolume"
                },
                "Device": "/dev/sdh"
            }
        },
        "NewVolume": {
            "Type": "AWS::EC2::Volume",
            "Condition": "CreateProdResources",
            "Properties": {
                "Size": 100,
                "AvailabilityZone": {
                    "Fn::GetAtt": [
                        "EC2Instance",
                        "AvailabilityZone"
                    ]
                }
            }
        }
    }
}

YAML

AWSTemplateFormatVersion: 2010-09-09
Parameters:
  EnvType:
    Description: Environment type.
    Default: test
    Type: String
    AllowedValues:
      - prod
      - test
    ConstraintDescription: must specify prod or test.
Conditions:
  CreateProdResources: !Equals 
    - !Ref EnvType
    - prod
Resources:
  EC2Instance:
    Type: 'AWS::EC2::Instance'
    Properties:
      ImageId: ami-0ff8a91507f77f867
  MountPoint:
    Type: 'AWS::EC2::VolumeAttachment'
    Condition: CreateProdResources
    Properties:
      InstanceId: !Ref EC2Instance
      VolumeId: !Ref NewVolume
      Device: /dev/sdh
  NewVolume:
    Type: 'AWS::EC2::Volume'
    Condition: CreateProdResources
    Properties:
      Size: 100
      AvailabilityZone: !GetAtt 
        - EC2Instance
        - AvailabilityZone

Nested condition

The following sample template references a condition within another condition. You can create a stack that creates an s3 bucket. For a stack deployed in a production environment, CloudFormation creates a policy for the S3 bucket.

JSON
{
    "Parameters": {
        "EnvType": {
            "Type": "String",
            "AllowedValues": [
                "prod",
                "test"
            ]
        },
        "BucketName": {
            "Default": "",
            "Type": "String"
        }
    },
    "Conditions": {
        "IsProduction": {
            "Fn::Equals": [
                {
                    "Ref": "EnvType"
                },
                "prod"
            ]
        },
        "CreateBucket": {
            "Fn::Not": [
                {
                    "Fn::Equals": [
                        {
                            "Ref": "BucketName"
                        },
                        ""
                    ]
                }
            ]
        },
        "CreateBucketPolicy": {
            "Fn::And": [
                {
                    "Condition": "IsProduction"
                },
                {
                    "Condition": "CreateBucket"
                }
            ]
        }
    },
    "Resources": {
        "Bucket": {
            "Type": "AWS::S3::Bucket",
            "Condition": "CreateBucket"
        },
        "Policy": {
            "Type": "AWS::S3::BucketPolicy",
            "Condition": "CreateBucketPolicy",
            "Properties": {
                "Bucket": {
                    "Ref": "Bucket"
                },
                "PolicyDocument": "..."
            }
        }
    }
}
YAML
Parameters:
  EnvType:
    Type: String
    AllowedValues:
      - prod
      - test
  BucketName:
    Default: ''
    Type: String
Conditions:
  IsProduction: !Equals 
    - !Ref EnvType
    - prod
  CreateBucket: !Not 
    - !Equals 
      - !Ref BucketName
      - ''
  CreateBucketPolicy: !And 
    - !Condition IsProduction
    - !Condition CreateBucket
Resources:
  Bucket:
    Type: 'AWS::S3::Bucket'
    Condition: CreateBucket
  Policy:
    Type: 'AWS::S3::BucketPolicy'
    Condition: CreateBucketPolicy
    Properties:
      Bucket: !Ref BucketName
      PolicyDocument: ...