Question state caching
The current cache is very close indeed to the CAS. Indeed, it is actually calls to the CAS itself which are cached. This gives a dramatic increase in performance when the result already exists.
We recommend using the cache on a production server. Note, the cache needs to be cleared manually, perhaps on an annual update cycle.
Clearing the cache
This can be done through Moodle.
Home > Site administration > Plugins > Question types > STACK
Choose to view the healthcheck page. At the bottom of this is a button to clear the cache.