Tower as an Ansible Fact Cache

Tower can store and retrieve per-host facts via an Ansible Fact Cache Plugin. This behavior is configurable on a per-job-template basis. When enabled, Tower will serve fact requests for all Hosts in an Inventory related to the Job running. This allows users to use Job Templates with --limit while still having access to the entire Inventory of Host facts. The Tower Ansible Fact Cache supports a global timeout settings that it enforces per-host. The setting is available in the CTiT interface under the Jobs category with the name ANSIBLE_FACT_CACHE_TIMEOUT and is in seconds.

Tower Fact Cache Implementation Details

Tower Injection

In order to understand the behavior of Tower as a fact cache you will need to understand how fact caching is achieved in Tower. Upon a Job invocation with use_fact_cache=True, Tower will inject, into memcached, all ansible_facts associated with each Host in the Inventory associated with the Job. The cache key is of the form inventory_id-host_name so that hosts with the same name in different inventories do not clash. A list of all hosts in the inventory is also injected into memcached with key inventory_id and value [host_name1, host_name2, ..., host_name3].

Ansible plugin usage

The fact cache plugin running in Ansible will connect to the same memcached instance. A get() call to the fact cache interface in Ansible will result in a looked into memcached for the host-specific set of facts. A set() call to the fact cache will result in an update to memcached record along with the modified time.

Tower Cache to DB

When a Job finishes running that has use_fact_cache=True enabled, Tower will go through memcached and get all records for the hosts in the Inventory. Any records with update times newer than the database per-host ansible_facts_modified value will result in the ansible_facts, ansible_facts_modified from memcached being saved to the database.

Caching Behavior

Tower will always inject the host ansible_facts into memcached. The Ansible Tower Fact Cache Plugin will choose to present the cached values to the user or not based on the per-host ansible_facts_modified time and the global ANSIBLE_FACT_CACHE_TIMEOUT.

Tower Fact Logging

New and changed facts will be logged via Tower's logging facility. Specifically, to the system_tracking namespace or logger. The logging payload will include the fields: host_name, inventory_id, and ansible_facts. Where ansible_facts is a dictionary of all ansible facts for host_name in Tower Inventory inventory_id.

Integration Testing

ensure clear_facts set's hosts/<id>/ansible_facts to {}
ensure that gather_facts: False does NOT result in clearing existing facts
ensure that the when a host fact timeout is reached, that the facts are not used from the cache

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