Manual:Caching

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MediaWiki uses caching in many components and at multiple layers. This page documents the various caches we use inside the MediaWiki PHP application.

General[edit]

The ObjectCache class provides interfaces for two kinds of caches:

  1. A place to store the result of a computation, or data fetched from an external source (for higher access speeds). This is a "cache" in the computer science definition.
  2. A place to store lightweight data not stored anywhere else. Also known as a stash (or "hoard" of objects).

The document covers the first use case in an attempt to help understand which caches exists, and how they are managed. E.g. when we purge a key, how we update the value, and if the key or value can be verified.

A cache key is said to be "verifiable" if the value cannot be stale. For example, a key is verifiable if it contains a hash of the input, and the output can be deterministically recomputed.

A cache value is verifiable if the caller can inspect the value and determine whether it is still up to date. For example, by comparing an identifier that is embedded in the value.

Interfaces[edit]

These are the generic stores used by the various logical purposes described in the Uses section.

Main cache[edit]

Accessed through ObjectCache::getMainWANInstance().

Values in this store are kept centrally in the current cluster (typically using Memcached as backend). While values are not replicated to other clusters, delete and purge events are broadcasted to other DCs main cache for cache invalidation. See WANObjectCache class reference for how to use this.

In short: Invalidate cache by purging, not by setting the new value. Each cluster will compute its own value as needed.

Local server[edit]

Accessed through ObjectCache::getLocalServerInstance().

Values in this store only kept on the local web server only (typically using APC as backend). Not replicated to the other servers or clusters.

This store defaults to a null interface if APC (or other supported PHP extensions) are not installed. It is also set to a null interface for maintenance scripts and other command-line modes. MediaWiki supports APC, APCu, XCache, and WinCache.

Local cluster[edit]

Mostly for internal coordination within the current cluster. Accessed through ObjectCache::getLocalClusterInstance().

Uses[edit]

Interwiki cache[edit]

See also clearInterwikiCache.php.

Localisation cache[edit]

..

Parser cache[edit]

  • Accessed via the ParserCache class.
  • Backend configured by $wgParserCacheType (typically MySQL).
  • Keys are canonical by page ID and populated when a page is parsed.
  • Revision ID is verified on retrieval.

See also Manual:Parser#ParserCache and purgeParserCache.php.

Revision text cache[edit]

  • Accessed via Revision::loadText.
  • Stored in the main cache.
  • Keys are verifiable and values immutable. Cache populated on demand.

Revision meta data cache[edit]

  • Accessed via Revision::newKnownCurrent.
  • Stored in the main cache.
  • Keys are verifiable (by page and revision ID) and values immutable. Cache populated on demand.

MessageBlobStore[edit]

  • Accessed via MessageBlobStore.
  • Stored in the main cache.
  • Keys are verifiable (by module name and hash of message keys). Values are mutable and expire after a week. Cache populated on demand.
  • All keys are purged a LocalisationCache rebuild.
  • When MediaWiki-namespace messages are edited, relevant module keys are purged.

Minification cache[edit]

ResourceLoader caches minification output of JavaScript and CSS.

  • Accessed via ResourceLoader::filter.
  • Stored locally on the server (APC).
  • Keys are verifiable values deterministic (no purge strategy needed). Cache populated on demand.

LESS compilation cache[edit]

ResourceLoader caches the meta data and parser output when compiling LESS files.

File content hasher[edit]

ResourceLoader caches the checksum when hashing a file's content.

See also[edit]