awx/docs/inventory_plugins.md

# Transition to Ansible Inventory Plugins
Inventory updates change from using scripts which are vendored as executable
python scripts to using dynamically-generated
YAML files which conform to the specifications of the `auto` inventory plugin
which are then parsed by their respective inventory plugin.

The major organizational change is that the inventory plugins are
part of the Ansible core distribution, whereas the same logic used to
be a part of AWX source.

## 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 the case 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 database. Using `ansible-inventory`
eliminates the need to maintain source-specific logic,
relying on Ansible's code instead. This also allows us to
count on a consistent data structure outputted 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.

Along with this switchover, a backported version of
`ansible-inventory` was provided that supported Ansible versions 2.2 and 2.3.

### Removal of Backport

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 switchover
to plugins, provided that sufficient compatibility is in place for
the version of Ansible present in the local virtualenv.

To see what 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 testing deemed to have sufficient parity
in the content 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.

### Sunsetting the scripts

Eventually, it is intended that all source types will have moved to
plugins. For any given source, after the `initial_version` for plugin use
is higher than the lowest supported Ansible version, the script can be
removed and the logic for script credential injection will also be removed.

For example, after AWX no longer supports Ansible 2.7, the script
`awx/plugins/openstack_inventory.py` will be removed.

## 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 has 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.

Because 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.

#### Host names

In many cases, the host names will change. In all cases, accurate host
tracking will still be maintained via the host `instance_id`.
(after: https://github.com/ansible/awx/pull/3362)

## How do I write my 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 that is
not `gce` or `openstack`.

All other sources provide _secrets_ via environment variables, so this
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 (exception being ec2 IAM roles).
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_vars`
 - `source_regions`
 - `instance_filters`
 - `group_by`

The way these data are 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
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.