We have too many different cache objects, configuration variables, methods and factories for various types of cache interfaces.

Entry points

A few of the things we use right now:

• wfGetCache( "hash" ), new HashBagOStuff();.
• wfGetCache( CACHE_ANYTHING ), ObjectCache::newAnything().
• ObjectCache::newAccelerator( fallback ).
• wfGetMainCache(), ObjectCache::getMainClusterInstance().
• ObjectCache::getMainWANInstance().
• ObjectCache::getMainStashInstance().
• ObjectCache::getInstance().
• ObjectCache::getWANInstance().
• more..
• variations on the above due to back-compat aliases.

We should reduce this to a handful only that we use everywhere.

With the multi-datacenter work (T88445) we're also changing which cache developers should use by default. Namely one should now default to WANCache instead of the local-dc cache. It may make sense to also update the notion of "main cache" to start referring to WAN cache to clarify this convention shift.

I propose we standardise on the following four main entry points:

• Server cache or Process cache (e.g. APC, fallback to none, or hash.)
• Local cluster cache (e.g. Memcached).
• WAN cache (e.g. Memcached, with relayed purges and other improvements).
• Main stash (e.g. db-replicated).

With two extra entry points for special cases:

• Identified cache group (e.g. ObjectCache::getGroup( "language-converter" )).
• Identified cache backend (should be unused, but for back-compat, e.g. ObjectCache::getInstance( CACHE_DB );

Reduced interface:

class ObjectCache {
 static function getInstance( string $type ); // Key from $wgObjectCaches

 static function getGroup( string $groupName ); // Key from $wgObjectCacheGroups, defaults to main cache

 static function newAccelerator();

 static function getLocalClusterCache();

 static function getMainCache();

 static function getMainStash();
}

Action items:

• Don't require custom accelerator fallback. Set a default one (empty or hash) and deprecate the fallback parameter.
• Give CACHE_ACCEL sensible default so it doesn't fail on plain installs.
• Reduce ObjectCache entry points to just WAN, LocalServer (APC), LocalCluster (Main) and stash.
• Deprecate wfGetCache(), wfGetMainCache().

Configuration

Some configuration:

• $wgObjectCaches array( .. )
• $wgWANObjectCaches array( .. )
• $wgMainCacheType
• $wgMainWANCache
• $wgMainStash
• $wgMessageCacheType, $wgParserCacheType, $wgSessionCacheType, $wgLanguageConverterCacheType (maybe introduce some kind of cache grouping to allow overriding backends for individual cache groups, defaulting to main cache).

Action items:

• Deprecate cache group config vars ($wgMessageCacheType, $wgParserCacheType etc.) in favour of e.g. $wgObjectCacheGroups.
• Implement ObjectCache::getGroup( $name ) with default to wgMainCacheType.
• Deprecate wfGetMessageCacheStorage(), wfGetParserCacheStorage()

Once we have $wgObjectCacheGroups, it'll be less complicated to introduce new cache groups. Making it easy to move around things in wmf-production and re-use caches without having to add more wg*CacheType variables and ad-hoc instantiation of BagOStuff objects.

See also:

• T186673: Deprecate and remove CACHE_ANYTHING; cache on by default
• T139216: Deprecate and remove $wgObjectCaches[hash]