When flushing caches, we want to ensure that even processes which may
have a wrong cache-key-prefix know to fetch the latest data from the
database. This is complicated by the cache-key-prefixes being stored
on disk, and thus checking that every cache delete is not sufficiently
performant.
We store the list of cache-key-prefixes in the cache, itself, with no
prefix. This cache is updated when a new cache-key is written, and is
also allowed to lapse after 24 hours. Updating this global cache
entry on new prefix creation ensures that even a
not-yet-restarted-into deployment will have its caches appropriately
purged if changes are made to the underlying data.
However, this both adds a cache-get, as well as multiplies the size of
all cache clears; for large bulk clears (e.g. for stream renames,
which clear the cache for all message-ids in them) this may prove
untenable.
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