Merging user data sections#

The ability to merge user data sections is a feature that was implemented by popular request. It was identified that there should be 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).

The previous merging algorithm was very simple and would only overwrite (and not append). So, it was decided to create a new and improved way to merge dictionaries (and their contained objects) together in a customisable way, thus allowing users who provide cloud-config user data to determine exactly how their objects will be merged.

For example:

#cloud-config (1)
runcmd:
  - bash1
  - bash2

#cloud-config (2)
runcmd:
  - bash3
  - bash4

The previous way of merging the two objects above would result in a final cloud-config object that contains the following:

#cloud-config (merged)
runcmd:
  - bash3
  - bash4

Typically this is not what users want - instead they would prefer:

#cloud-config (merged)
runcmd:
  - bash1
  - bash2
  - bash3
  - bash4

This change makes it easier to combine the various cloud-config objects you have into a more useful list. In this way, we reduce the duplication necessary to accomplish the same result with the previous method.

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 to False.

  • prepend: Add new values to the start of the list. 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.

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#

Because the above merging algorithm may not always be desired (just as the previous merging algorithm was not always the preferred one), the concept of customised merging was introduced through merge classes.

A merge class is a class definition providing functions that can be used to merge a given type with another given type.

An example of one of these merging classes is the following:

class Merger:
    def __init__(self, merger, opts):
        self._merger = merger
        self._overwrite = 'overwrite' in opts

    # This merging algorithm will attempt to merge with
    # another dictionary, on encountering any other type of object
    # it will not merge with said object, but will instead return
    # the original value
    #
    # On encountering a dictionary, it will create a new dictionary
    # composed of the original and the one to merge with, if 'overwrite'
    # is enabled then keys that exist in the original will be overwritten
    # by keys in the one to merge with (and associated values). Otherwise
    # if not in overwrite mode the 2 conflicting keys themselves will
    # be merged.
    def _on_dict(self, value, merge_with):
        if not isinstance(merge_with, (dict)):
            return value
        merged = dict(value)
        for (k, v) in merge_with.items():
            if k in merged:
                if not self._overwrite:
                    merged[k] = self._merger.merge(merged[k], v)
                else:
                    merged[k] = v
            else:
                merged[k] = v
        return merged

As you can see, there is an _on_dict method here that will be given a source value, and a value to merge with. The result will be the merged object.

This code itself is called by another merging class which “directs” the merging to happen by analysing the object types to merge, and attempting to find a known object that will merge that type. An example of this can be found in the mergers/__init__.py file (see LookupMerger and UnknownMerger).

So, following the typical cloud-init approach of allowing source code to be downloaded and used dynamically, it is possible for users to inject their own merging files to handle specific types of merging as they choose (the basic ones included will handle lists, dicts, and strings). Note how each merge can have options associated with it, which affect how the merging is performed. For example, a dictionary merger can be told to overwrite instead of attempting to merge, or a string merger can be told to append strings instead of discarding other strings to merge with.

How to activate#

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

  1. 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).

  2. 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:

list()+dict()+str()

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 a merge-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 customisation) here as well. Other places that used the previous merging are also, similarly, now extensible (metadata 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:

#cloud-config
merge_how:
 - name: list
   settings: [append]
 - name: dict
   settings: [no_replace, recurse_list]

runcmd:
  - bash3
  - bash4