We have started our journey of deprecating the ObjectCache
class and moving to ObjectCacheFactory
: https://gerrit.wikimedia.org/r/c/mediawiki/core/+/955771. This means we
have stopped using the *$instances* member in ObjectCache. If you are an
extension author or maintainer, please look at the new interface in
ObjectCacheFactory and migrate callers (if you find any).

*Longer version*

Recently, there has been some on-going work on ObjectCache and we realized
code was taking a factory based pattern due to various code paths to
configure, setup and obtain BagOStuff cache instances. With this patch:
https://gerrit.wikimedia.org/r/c/mediawiki/core/+/955771, we created a
proper MediaWiki factory service and deprecated various methods on the
ObjectCache class including the static member `ObjectCache::$instances`
which used to hold references to various instances of the caches
(BagOStuff) - sounds familiar? Yes, we're getting rid of scattered usage of
global state one class at a time :).

We have taken care of places [1][2][3] where this public member was
referenced, so, we are certain there are no consumers of this field
in MW core and extensions that we deploy today in production, and we
encourage extension authors and maintainers to migrate to the new way of
constructing and/or obtaining cache instances via ObjectCacheFactory which
goes through our global services container (with DI capabilities).

You can also have a look at the Phabricator task [4] which explains the
problem that this refactoring solves/improves and the impact it has on
MediaWiki. The patch will ride the train next week (starting March 25th,
2024) and if there are any issues found along the way, please file a task
and add #MediaWiki-lib-BagOStuff.

This work is only step 1 into unifying, centralizing and getting rid of
global state in the logic related to the ObjectCache class and making it
consistent with how we do things with the global services locator today in

External/related links, see:
* https://www.mediawiki.org/wiki/Manual:$wgObjectCaches
* https://www.mediawiki.org/wiki/Object_cache

Thank you!

P.S: I personally want to thank Daniel Kinzler and Timo Tijhof for all the
code review and guidance into making this work materialize and about to hit
production. <3

[1] https://gerrit.wikimedia.org/r/c/mediawiki/core/+/1011159
[4] https://phabricator.wikimedia.org/T358346

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