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Transition to Ansible Inventory Plugins
Inventory updates have changed from using deprecated inventory scripts, to using dynamically-generated YAML files which are parsed by their respective inventory plugin.
In Ansible 2.8, the inventory plugins which are part of the Ansible core distribution were used. This only applied to a few select sources.
In all other circumstances, inventory imports make use of the inventory plugin from vendored collections. Those collections are downloaded from Ansible Galaxy at the time of packaging building the container image.
Prior Background for Transition
AWX used to maintain logic that parsed .ini inventory file contents, in addition to interpreting the JSON output of scripts, re-calling with the --host option in cases where the _meta.hostvars key was not provided.
Switch to Ansible Inventory
The CLI entry point ansible-inventory was introduced in Ansible 2.4. In Tower 3.2, inventory imports began running this command as an intermediary between the inventory and the import's logic to save content to the database. Using ansible-inventory eliminates the need to maintain source-specific logic, relying on Ansible's code instead. This also enables consistent data structure output from ansible-inventory. There are many valid structures that a script can provide, but the output from ansible-inventory will always be the same, thus the AWX logic to parse the content is simplified. This is why even scripts must be ran through the ansible-inventory CLI.
Removal of Backport
Along with the ansible-inventory switchover, a backported version of ansible-inventory was provided, which supported Ansible versions 2.2 and 2.3.
In AWX 3.0.0 (and Tower 3.5), the backport of ansible-inventory was removed, and support for using custom virtual environments was added. This set the minimum version of Ansible necessary to run any inventory update to 2.4.
Inventory Plugin Versioning
Beginning in Ansible 2.5, inventory sources in Ansible started migrating away from contrib scripts (meaning they lived in the contrib folder) to the inventory plugin model.
In AWX 4.0.0 (and Tower 3.5) inventory source types start to switch over to plugins, provided that sufficient compatibility is in place for the version of Ansible present in the local virtualenv.
To see in which version the plugin transition will happen, see awx/main/models/inventory.py and look for the source name as a subclass of PluginFileInjector, and there should be an initial_version, which is the first version that was deemed (via testing) to have sufficient parity in the content for its inventory plugin returns. For example, openstack will begin using the inventory plugin in Ansible version 2.8. If you run an OpenStack inventory update with Ansible 2.7.x or lower, it will use the script.
At some point, scripts will be removed and the script-related (for credentials and configuration) logic will also be removed.
Management of Collections
Collections are used for inventory imports starting in Ansible 2.9, and each collection has its own versioning independently from Ansible.
Versions for those collections are set in the requirements file requirements/collections_requirements.yml.
The location of vendored collections is set by the file-only setting INVENTORY_COLLECTIONS_ROOT.
For development purposes, this can be changed so that you can test against development versions of those collections.
Instructions for doing this are in tools/collections.
If, for some reason, you need to change the version of a particular collection used in inventory imports,
you can use the ansible-galaxy tool to update the collection inside of the INVENTORY_COLLECTIONS_ROOT.
Note that the logic for building the inventory file is written and tested only for the version pinned in the requirements file.
Changes to Expect in Imports
An effort was made to keep imports working in the exact same way after the switchover. However, the inventory plugins are a fundamental rewrite and many elements of default behavior have changed. These changes also include many backward-incompatible changes. Because of this, what you get via an inventory import will be a superset of what you get from the script but will not match the default behavior you would get from the inventory plugin on the CLI.
Due to the fact that inventory plugins add additional variables, if you downgrade Ansible, you should turn on overwrite and overwrite_vars to get rid of stale variables (and potentially groups) no longer returned by the import.
Changes for Compatibility
Programatically-generated examples of inventory file syntax used in updates (with dummy data) can be found in awx/main/tests/data/inventory/scripts. These demonstrate the inventory file syntax used to restore old behavior from the inventory scripts.
Hostvar Keys and Values
More hostvars will appear if the inventory plugins are used. To maintain backward compatibility, the old names are added back where they have the same meaning as a variable returned by the plugin. New names are not removed.
A small number of hostvars will be lost because of general deprecation needs.
Writing Your Own Inventory File
If you do not want any of this compatibility-related functionality, then you can add an SCM inventory source that points to your own file. You can also apply a credential of a managed_by_tower type to that inventory source that matches the credential you are using, as long as it is not gce or openstack.
All other sources provide secrets via environment variables. These can be re-used without any problems for SCM-based inventory, and your inventory file can be used securely to specify non-sensitive configuration details such as the keyed_groups (to provide) or hostvars (to construct).
Notes on Technical Implementation of Injectors
For an inventory source with a given value of the source field that is of the built-in sources, a credential of the corresponding credential type is required in most cases (ec2 IAM roles are an exception). This privileged credential is obtained by the method get_cloud_credential.
The inputs for this credential constitute one source of data for running inventory updates. The following fields from the InventoryUpdate model are also data sources, including:
source_varssource_regionsinstance_filtersgroup_by
The way this data is applied to the environment (including files and environment vars) is highly dependent on the specific source.
With plugins, the inventory file may reference files that contain secrets from the credential. With scripts, typically an environment variable will reference a filename that contains a ConfigParser format file with parameters for the update, and possibly including fields from the credential.
Caution: Please do not put secrets from the credential into the inventory file for the plugin. Right now there appears to be no need to do this, and by using environment variables to specify secrets, this keeps open the possibility of showing the inventory file contents to the user as a latter enhancement.
Logic for setup for inventory updates using both plugins and scripts live in the inventory injector class, specific to the source type.
Any credentials which are not source-specific will use the generic injection logic which is also used in playbook runs.