Moodle MUC and Redis: Fallback behavior when Redis store fails

1. Documented Fallback Order

In Moodle 4.x the Universal Cache (MUC) is configured with a list of stores. By default the first store is Redis and the second is the file store. When a request for a cached value is made, MUC attempts the primary store first. If the Redis connection cannot be established (e.g. the server is down or the socket times out), MUC logs a warning and automatically switches to the next store in the list. In a standard single‑store setup this means it will read from and write to the file store until Redis becomes reachable again.

2. Runtime Error Surface

The cache API does not raise an exception that propagates to the application code. Instead, the failure is handled internally: the request is served from the secondary store and a warning is written to the cache log. The application continues to run, but performance may degrade because the secondary store is slower and the data may be stale. If a custom store class is used, the behaviour can differ; the store itself may throw, but that is not the default MUC behaviour.

3. Verifying the Active Store

Administrators can check which store is currently in use without purging the cache:

1. Navigate to Site administration > Server > Caching in the Moodle UI. The page lists the active store for each cache region and shows hit/miss statistics.
2. Inspect the configuration file config.php for the $CFG->cache_store array to confirm the order of stores.
3. Enable debugging with $CFG->debug = E_ALL; and $CFG->debugdisplay = 1; to view log messages. When Redis is stopped, look for entries such as Cache store fallback to file in moodledata/var/logs/moodle.log.

These steps allow you to confirm that the fallback mechanism is active and that the system is still serving cached data from the file store.

What to Check If Things Don’t Match

If you notice that the cache is not degrading gracefully, verify:

• That no custom cache store class is overriding the default behaviour.
• That the Redis server’s host/port settings in config.php are correct.