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DevOps / Ansible Interview questions

How does Ansible implement fact caching internally?

Fact caching stores the ansible_facts dictionary gathered for each host in an external cache backend, keyed by hostname, with a configurable time-to-live, so a future run can read cached facts instead of re-running the setup module.

flowchart TD A[Play starts] --> B{Valid cache entry for host exists?} B -- Yes, within TTL --> C[Load ansible_facts from cache backend] B -- No / expired --> D[Run setup module on host] D --> E[Write freshly-gathered facts to cache backend] C --> F[Facts available to play's tasks] E --> F 
# ansible.cfg
[defaults]
fact_caching = redis
fact_caching_connection = localhost:6379:0
fact_caching_timeout = 86400

Supported backends include a simple jsonfile (facts written to local disk as JSON), redis, and other plugin-based options, chosen based on whether facts need to be shared across multiple control nodes (favoring a networked backend like Redis) or just persisted locally between runs (where jsonfile is simpler). On each play, Ansible checks whether a valid, non-expired cache entry exists for a given host; if so, it loads facts from there instead of dispatching setup, and if not (or if the entry expired past fact_caching_timeout), it gathers fresh facts and writes the result back to the cache for next time.

What key does the fact cache use to store a host's facts?
Which backend is favored when facts need to be shared across multiple control nodes?

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