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merge_vars: An action plugin for Ansible

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An Ansible plugin to merge all variables in context with a certain suffix (lists or dicts only) and create a new variable that contains the result of this merge. This is an Ansible action plugin, which is basically an Ansible module that runs on the machine running Ansible rather than on the host that Ansible is provisioning.

Installation

By default, Ansible will look for action plugins in an action_plugins folder adjacent to the running playbook. For more information on this, or to change the location where ansible looks for action plugins, see the Ansible docs.

Ansible action plugins are usually paired with modules (which run on the hosts being provisioned), and Ansible will automatically run an action plugin when you call of a module of the same name in a task. Since we want to be able to call this action plugin by its name (merge_vars) in our tasks, we need an empty file called merge_vars in the place where ansible checks for custom modules; by default, this is a library directory adjacent to the running playbook.

So, from the root of your Ansible playbook repository:

  1. Create the action_plugins directory if it's not created yet:

    mkdir -p action_plugins

  2. Copy the merge_vars.py file to your action_plugins directory.

  3. Create the library directory if it's not created yet:

    mkdir -p library

  4. Create an empty merge_vars.py file in your library directory:

    touch library/merge_vars

Usage

The variables that you want to merge must be suffixed with __to_merge. They can be defined anywhere in the inventory, or by any other means; as long as they're in the context for the running play, they'll be merged.

Merging dicts

Let's say we've got a group someenvironment in group_vars with a file users.yml, with these contents:

users__someenvironment_users__to_merge:
  user1: bob
  user2: henry

and a group somedatacenter in groups_vars with a file users.yml, with these contents:

users__somedatacenter_users__to_merge:
  user3: sally
  user4: jane

and we're running a play against hosts that are in both of those groups. Then this task:

name: Merge user vars
merge_vars:
  suffix_to_merge: users__to_merge
  merged_var_name: merged_users
  expected_type: 'dict'

will set a merged_users var (fact) available to all subsequent tasks that looks like this (if it were to be declared in raw yaml):

merged_users:
  user1: bob
  user2: henry
  user3: sally
  user4: jane

Note that the order in which the dicts get merged is non-deterministic. So this setup:

users__someenvironment_users__to_merge:
  user1: bob
  user2: jekyll
users__somedatacenter_users__to_merge:
  user2: hyde
  user3: sally
name: Merge user vars
merge_vars:
  suffix_to_merge: users__to_merge
  merged_var_name: merged_users
  expected_type: 'dict'

could set a merged_users var that looks like this (if it were to be declared in raw yaml:

merged_users:
  user1: bob
  user2: jekyll
  user3: sally

OR this:

merged_users:
  user1: bob
  user2: hyde
  user3: sally

With great power comes great responsibility...

Merging lists

Let's say we've got a someenvironment group with an open_ports.yml file that looks like this:

open_ports__someenvironment_open_ports__to_merge:
  - 1
  - 2
  - 3

and a somedatacenter group with an open_ports.yml` file that looks like this:

open_ports__somedatacenter_open_ports__to_merge:
  - 3
  - 4
  - 5

Then this task:

name: Merge open ports
merge_vars:
  suffix_to_merge: open_ports__to_merge
  merged_var_name: merged_ports
  expected_type: 'list'

will set a merged_ports fact that looks like this:

merged_ports:
  - 1
  - 2
  - 3
  - 4
  - 5

OR this; remember, the order in which variables get merged is non-deterministic:

merged_ports:
  - 3
  - 4
  - 5
  - 1
  - 2

Notice that 3 only appears once in the merged result. By default, this merge_vars plugin will de-dupe the resulting merged value. If you don't want to de-dupe the merged value, you have to declare the dedup argument:

name: Merge open ports
merge_vars:
  suffix_to_merge: open_ports__to_merge
  merged_var_name: merged_ports
  dedup: false
  expected_type: 'list'

which will set this fact (or some permutation thereof):

merged_ports:
  - 1
  - 2
  - 3
  - 3
  - 4
  - 5

A note about dedup:

  • It has no effect when the merged vars are dictionaries.

Verbosity

Running ansible-playbook with -v will cause this plugin to output the order in which the keys are being merged:

PLAY [Example of merging lists] ************************************************

TASK [Merge port vars] *********************************************************
Merging vars in this order: [u'group2_ports__to_merge', u'group3_ports__to_merge', u'group1_ports__to_merge']
ok: [localhost] => {"ansible_facts": {"merged_ports": [22, 1111, 443, 2222, 80]}, "changed": false}

TASK [debug] *******************************************************************
ok: [localhost] => {
    "merged_ports": [
        22,
        1111,
        443,
        2222,
        80
    ]
}

PLAY RECAP *********************************************************************
localhost                  : ok=6    changed=0    unreachable=0    failed=0

Example Playbooks

There are some example playbooks in the examples folder in this repository. Prerequisites:

  1. You have python 2.7 installed and a python2.7 executable is on your path.
  2. You have the virtualenv tool installed and a virtualenv executable on your path.

To run all of the example playbooks (from the root of this repository):

make examples

This will create a virtualenv, install the relevant dependencies into it, and run each example playbook using the ansible-playbook executable in the virtualenv.

To run a single example, assuming you have your virtualenv and dependencies that the above make command generates:

env/bin/ansible-playbook examples/merge_lists_playbook.yml

Contributing

Please note that this project is released with a Contributor Code of Conduct. By participating in this project you agree to abide by its terms.

These are the only prerequisites to working on this project locally:

  1. You have python 2.7 installed and a python2.7 executable is on your path.
  2. You have virtualenv installed and a virtualenv executable on your path.

If you have any ideas about things to add or improve, or find any bugs to fix, we're all ears! Just a few guidelines:

  1. Please write or update tests (either example-based tests, property-based tests, or both) for any code that you add, change, or remove.

    To run the example-based and property-based tests, run make test from the root of this repository.

  2. Please make sure that make test-all exits zero. This runs a code linter, all of the tests, and all of the examples.

  3. If the linting seems too annoying, it probably is! Feel free to do what you need to do in the .pylintrc at the root of this repository to maintain sanity. Add it to your PR, and we'll most likely take it.

Happy merging!