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Explicitly detail the behavior of `use_fact_cache=False`
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Chris Meyers 2017-07-14 13:58:08 -04:00 committed by GitHub
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@ -3,13 +3,13 @@ Tower can store and retrieve per-host facts via an Ansible Fact Cache Plugin. Th
## 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]`.
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. Jobs invoked with `use_fact_cache=False` will not inject `ansible_facts` into memcached. 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.
The Ansible fact cache plugin that ships with Tower will only be enabled on Jobs that have fact cache enabled, `use_fact_cache=True`. 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.
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. Note that the last value of the Ansible fact cache is retained in `ansible_facts`. The globla timeout and/or individual job template `use_fact_cache` setting will not clear the per-host `ansible_facts`.
### 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`.