TLDR: Previously, PHPUnit enabled all caches except WANObjectCache. This is now
enabled as well. If you encounter a new test failure this week on an extension
repo that is not covered by the automated "wmf CI gate", then that may be the
reason. This could point to a genuine bug in your code. However, you can choose
to turn of all caching layers in the relevant test cases by calling
$this->setMainCache( CACHE_NONE ).
Hi,
MediaWiki provides broudly four layers of object caching
<https://www.mediawiki.org/wiki/Object_cache>, that are individually
configurable (to optimise large wiki farms like Wikipedia). Each of these
auto-configures themselves or falls back to $wgMainCacheType
<https://doc.wikimedia.org/mediawiki-core/master/php/classMediaWiki_1_1MainConfigSchema.html#a0b3d0535aa2fc03327944280797b1c54>,
thus in practice you don't need to be aware of their differing backends. They
have a simple unified interface: BagOStuff.
In our PHPUnit context, we set each layer to 'hash' (for HashBagOStuff) which
represents an in-memory cache that we automatically reset between each test and
each data set. In addition to the four levels of caching, this therefore also
ensures realistic caching behaviour during your tests for services such as
ParserCache, MessageCache, SessionStore, MainStash, etc.
For example, your extension might have a test that asserts "no database
queries" made during a cache hit, or it might assert that if an edit is made
the same value is returned as before, and then you call your code's purge
method, and assert the updated value.
By default and during local development, (other) CI jobs, beta cluster, and
production - WANObjectCache is explicitly a wrapper for the main cache (e.g.
Memcached at WMF). It is never configured separately from the main cache, nor
is there any known use case (among both third parties, WMF, and local
development) for doing so.
As of this week, configuring WANObjectCache separately from the "main" cache is
no longer supported. This improves defaults for sysadmins and reduces the
number things one has to get right for optimal performance. It also removes an
apparent concept from the code going forward to ease debugging by not having a
level of indirection that doesn't translate to a specific narrative or purpose.
In dong so, we found one place that does configure the WANCache backend, namely
our PHPUnit config. While expected to override it, it overrode it to wrap the
no-op "none" EmptyBagOStuff instead instead of the in-memory "hash"
HashBagOStuff. This quirky setup was surprising a times to developers and
required numerous tests to copy boilerplate around in order to opt-in to to the
"normal" cache behaviour that we expect and follow in other cache levels
already. The boiler plate in question was also atypical in that it wasn't
covered by our standard ServiceWiring and its automatic resets (the main cache
is now recognised an injectable internal service).
Our shared "wmf CI gate" job identified two extensions where this was relied
upon, and I've updated those before making the change
<https://gerrit.wikimedia.org/r/c/mediawiki/core/+/889244> (example 1
<https://gerrit.wikimedia.org/r/c/mediawiki/extensions/GlobalPreferences/+/894115>,
and example 2
<https://gerrit.wikimedia.org/r/c/mediawiki/extensions/Wikibase/+/894228>).
If you encounter a new test failure this week on an extension repo that is not
covered by the CI gate, then this may be the reason. Tests that fail with
caching enabled, may expose a bug in the underlying code (e.g. after a cache it
should probably not use the cache for subsequent reads of the same data, unless
it is expected that it will be stale), or may reflect that the code (or the
test) is missing a call to your "purge" method, or that a service reset is
needed if the test is meant to reflect what a subsequent web request would see,
etc. You can (temporarily or not) choose to turn of all caching layers in the
relevant test cases by calling $this->setMainCache( CACHE_NONE ). Example
1-line patch
<https://gerrit.wikimedia.org/r/c/mediawiki/extensions/GlobalPreferences/+/894115>.
--
Timo Tijhof
Performance Team,
Wikimedia Foundation.
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