Cache expiration settings
A cache item can be set as CACHE_PERMANENT, meaning that it will only be removed when explicitly cleared, or it can have an expiration time (a Unix timestamp).
Cache item does not exist
Any cache item can be set to only expire when explicitly cleared, or to expire at a given time.
Expire and remove options
Type of cache clearing to do
We have APIs to expire cached items and also to just remove them. Unfortunately, they're all the same API, cache_clear_all