Merging user-data sections - cloud-init 25.1.2 documentation (original) (raw)

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The ability to merge user-data sections allows a way to specify how cloud-config YAML “dictionaries” provided as user-data are handled when there are multiple YAML files to be merged together (e.g., when performing an #include).

For example merging these two configurations:

#cloud-config (1) runcmd:

bash1
bash2

#cloud-config (2) runcmd:

bash3
bash4

Yields the following merged config:

#cloud-config (merged) runcmd:

bash1
bash2
bash3
bash4

Built-in mergers¶

Cloud-init provides merging for the following built-in types:

Dict
List
String

`Dict`¶

The Dict merger has the following options, which control what is done with values contained within the config.

allow_delete: Existing values not present in the new value can be deleted. Defaults to False.
no_replace: Do not replace an existing value if one is already present. Enabled by default.
replace: Overwrite existing values with new ones.

`List`¶

The List merger has the following options, which control what is done with the values contained within the config.

append: Add new value to the end of the list. Defaults toFalse.
prepend: Add new values to the start of the list. Defaults toFalse.
no_replace: Do not replace an existing value if one is already present. Enabled by default.
replace: Overwrite existing values with new ones.

String¶

The Str merger has the following options, which control what is done with the values contained within the config.

append: Add new value to the end of the string. Defaults to False.

Common options¶

These are the common options for all merge types, which control how recursive merging is done on other types.

recurse_dict: If True, merge the new values of the dictionary. Defaults to True.
recurse_list: If True, merge the new values of the list. Defaults to False.
recurse_array: Alias for recurse_list.
recurse_str: If True, merge the new values of the string. Defaults to False.

Customisation¶

Custom 3rd party mergers can be defined, for more info visitCustom Mergers.

How to activate¶

There are a few ways to activate the merging algorithms, and to customize them for your own usage.

The first way involves the usage of MIME messages in cloud-init to specify multi-part documents (this is one way in which multiple cloud-config can be joined together into a single cloud-config). Two new headers are looked for, both of which can define the way merging is done (the first header to exist “wins”). These new headers (in lookup order) are'Merge-Type' and 'X-Merge-Type'. The value should be a string which will satisfy the new merging format definition (see below for this format).
The second way is to specify the merge type in the body of the cloud-config dictionary. There are two ways to specify this; either as a string, or as a dictionary (see format below). The keys that are looked up for this definition are the following (in order): 'merge_how','merge_type'.

String format¶

The following string format is expected:

classname1(option1,option2)+classname2(option3,option4)....

The class name will be connected to class names used when looking for the class that can be used to merge, and options provided will be given to the class upon construction of that class.

The following example shows the default string that gets used when none is otherwise provided:

Dictionary format¶

A dictionary can be used when it specifies the same information as the string format (i.e., the second option above). For example:

{'merge_how': [{'name': 'list', 'settings': ['append']}, {'name': 'dict', 'settings': ['no_replace', 'recurse_list']}, {'name': 'str', 'settings': ['append']}]}

This would be the dictionary equivalent of the default string format.

Specifying multiple types, and what this does¶

Now you may be asking yourself: “What exactly happens if I specify amerge-type header or dictionary for every cloud-config I provide?”

The answer is that when merging, a stack of 'merging classes' is kept. The first one in the stack is the default merging class. This set of mergers will be used when the first cloud-config is merged with the initial empty cloud-config dictionary. If the cloud-config that was just merged provided a set of merging classes (via the above formats) then those merging classes will be pushed onto the stack. Now if there is a second cloud-config to be merged then the merging classes from the cloud-config before the first will be used (not the default) and so on. In this way a cloud-config can decide how it will merge with a cloud-config dictionary coming after it.

Other uses¶

In addition to being used for merging user-data sections, the default merging algorithm for merging 'conf.d' YAML files (which form an initial YAML config for cloud-init) was also changed to use this mechanism, to take advantage of the full benefits (and customization) here as well. Other places that used the previous merging are also, similarly, now extensible (meta-data merging, for example).

Note, however, that merge algorithms are not used across configuration types. As was the case before merging was implemented, user-data will overwrite'conf.d' configuration without merging.

Example cloud-config¶

A common request is to include multiple runcmd directives in different files and merge all of the commands together. To achieve this, we must modify the default merging to allow for dictionaries to join list values.

The first config:

#cloud-config merge_how:

name: list settings: [append]
name: dict settings: [no_replace, recurse_list]

runcmd:

bash1
bash2

The second config: