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WIP on fact caching docs.

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Michael DeHaan 2014-08-11 15:14:58 -04:00
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docsite/rst/playbooks_variables.rst
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@ -694,6 +694,47 @@ Here is an example of what that might look like::
In this pattern however, you could also write a fact module as well, and may wish to consider this as an option.
.. _fact_caching:
Fact Caching
````````````
.. versionadded:: 1.8
As shown elsewhere in the docs, it is possible for one server to reference variables about another, like so::
{{ hostvars['asdf.example.com']['ansible_os_family'] }}
With "Fact Caching" disabled, in order to do this, Ansible must have already talked to 'asdf.example.com' in the
current play, or another play up higher in the playbook. This is the default configuration of ansible.
To avoid this, Ansible 1.8 allows the ability to save facts between playbook runs, but this feature must be manually
enabled. Why might this be useful?
Imagine, for instance, a very large infrastructure with thousands of hosts. Fact caching could be configured to run nightly, but
configuration of a small set of servers could run ad-hoc or periodically throughout the day. With fact-caching enabled, it would
not be neccessary to "hit" all servers to reference variables and information about them.
With fact caching enabled, it is possible for machine in one group to reference variables about machines in the other group, despite
the fact that they have not been communicated with in the current execution of /usr/bin/ansible-playbook.
To configure fact caching, enable it in ansible.cfg as follows::
[defaults]
fact_caching = redis
fact_caching_timeout = 86400 # seconds
At the time of writing, Redis is the only supported fact caching engine.
To get redis up and running, perform the equivalent OS commands::
yum install redis
service redis start
pip install redis
Note that the Python redis library should be installed from pip, the version packaged in EPEL is too old for use by Ansible.
In current embodiments, this feature is in beta-level state and the Redis plugin does not support port or password configuration, this is expected to change in the near future.
.. _registered_variables:
Registered Variables