View source for Manual:Architectural modules/Cache

<languages/>
{{Architectural module
| Module = Cache
| Responsibilities = <translate><!--T:13--> Providing caching techniques in order to optimize the performance of MediaWiki</translate>
| Implementation = <translate><!--T:14--> Object Caching and Web Caching</translate>
}}

<translate>
== Responsibilities == <!--T:1-->
</translate>
<translate><!--T:15--> Module cache is responsible for providing caching techniques in order to optimize the performance of MediaWiki.</translate>
<translate><!--T:16--> Both object caching and web caching are supported by MediaWiki software.</translate>
<translate><!--T:17--> Object caching stores data and single objects, that are frequently requested and require resources to be generated.</translate>
<translate><!--T:18--> There are a lot of objects that MediaWiki stores in cache, some of them are presented below:</translate> 

* <translate><!--T:2--> User</translate>
* <translate><!--T:3--> ParserOutput</translate>
* <translate><!--T:4--> Array of data for image (version, size, width, height, metadata etc.)</translate>
* <translate><!--T:5--> Wiki text after xml preprocessing</translate>
* <translate><!--T:6--> Revision text</translate>
* <translate><!--T:7--> Revision differences</translate>
* ...

<translate><!--T:19--> Web caching allows storing of entire HTML pages.</translate>
<translate><!--T:20--> To support web caching MediaWiki provides integration with the Squid proxy server and Varnish accelerator.</translate>
<translate><!--T:21--> It notifies them, when a page should be purged from the cache and regenerated.</translate>

<translate>
== Implementation Information == <!--T:8-->

=== Object Caching === <!--T:9-->
</translate>

<translate><!--T:22--> [[File:Module-Cache-StorageOptions.svg|thumb|300px|Supported options for cache storing]]</translate>
<translate><!--T:23--> Main article: [[<tvar name=1>Special:MyLanguage/Manual:Object cache</tvar>|''Manual:Object cache'']].</translate>

<translate><!--T:24--> MediaWiki supports object caching storage in many places, for example, on a file system, in the database or in an external system like memcached.</translate>
<translate><!--T:25--> The storage mode is set by the global variable <tvar name=1>{{Phpi|1=$wgMainCacheType}}</tvar> in the <tvar name=2>{{Phpi|1=LocalSettings.php}}</tvar>.</translate>
<translate><!--T:26--> Additionally cache type for messages and parser can be specified by setting <tvar name=1>{{Phpi|1=$wgParserCacheType}}</tvar> and <tvar name=2>{{Phpi|1=$wgMessageCacheType}}</tvar>.</translate>
<translate><!--T:27--> The super class for all types of caching is <tvar name=1>{{Phpi|1=BagOStuff}}</tvar>, it provides interface for such methods as getting an item from cache, adding an item to cache, deleting it etc.</translate>
<translate><!--T:28--> All specific implementations of caching inherit from this class, as shown on figure ''Supported options for cache storing''.</translate>
<translate><!--T:29--> Thus, for example, <tvar name=1>{{Phpi|1=SqlBagOStuff}}</tvar> provides object caching using SQL database and <tvar name=2>{{Phpi|1=APCBagOStuff}}</tvar> does so by using PHP's APC accelerator.</translate>

<translate><!--T:30--> <tvar name=1>{{Phpi|1=$wgMemc}}</tvar> is the global variable for accessing the main cache.</translate>
<translate><!--T:31--> It is initialized from <tvar name=1>{{Phpi|1=$wgMainCacheType}}</tvar> by calling <tvar name=2>{{Phpi|1=ObjectCache::getInstance($wgMainCacheType)}}</tvar>.</translate>
<translate><!--T:32--> This initialization returns a specific <tvar name=1>{{Phpi|1=BagOStuff}}</tvar> object, for instance, in case the <tvar name=2>{{Phpi|1=$wgMainCacheType = CACHE_MEMCACHED}}</tvar>, it will return a <tvar name=3>{{Phpi|1=MemcachedPhpBagOStuff}}</tvar>.</translate>
<translate><!--T:33--> Further on whenever necessary every object can be stored by calling <tvar name=1>{{Phpi|1=wgMemc->set()}}</tvar> (providing key and value) and retrieved by calling <tvar name=2>{{Phpi|1=wgMemc->get()}}</tvar> (providing key).</translate>

<translate><!--T:34--> The key generation for saving in cache has some common rules.</translate>
<translate><!--T:35--> It always starts with the name of the database and follows by specification of the type of object that is being cached.</translate>
<translate><!--T:36--> Examples of such keys are shown in the table ''Cache keys''.</translate>
<translate><!--T:37--> Final part of the key varies.</translate>
<translate><!--T:38--> For example, for user object it will be the user's id and for file object it will be the md5 hash of its name.</translate>
<translate><!--T:39--> The generation of key for ParserOutput is explained in the documentation for module Parser.</translate>

{| style="border-collapse: collapse; width: 750px; text-align: left; border: 1px solid #aaa" cellpadding="3" align="center"
! colspan="2" style="text-align: center; background-color: #f0f0f0; border-bottom: 1px solid #bbb;" | <translate><!--T:40--> Cache keys</translate>
|-
| style="border-bottom: 1px solid #bbb;" | <translate><!--T:41--> '''Object'''</translate>
| style="border-bottom: 1px solid #bbb;" | <translate><!--T:42--> '''Key'''</translate>
|-
| style="border-bottom: 1px solid #bbb;" | <translate><!--T:43--> ParserOutput</translate> 
| style="border-bottom: 1px solid #bbb;" | buildings_en:pcache:idhash:2-0!*!*!*!*!*!*
|-
| style="border-bottom: 1px solid #bbb;" | <translate><!--T:44--> User</translate> 
| style="border-bottom: 1px solid #bbb;" | buildings_en:user:id:1
|-
| style="border-bottom: 1px solid #bbb;" | <translate><!--T:45--> File</translate>
| style="border-bottom: 1px solid #bbb;" | buildings_en:file:f9f25d227db94434eb6c42fc6016ed83
|-
| style="border-bottom: 1px solid #bbb;" | <translate><!--T:46--> Wiki text preprocessed in XML</translate> 
| style="border-bottom: 1px solid #bbb;" | buildings_en:preprocess-xml:7630d77fb556ce2262f9c890a07e869c:0
|-
| <translate><!--T:47--> Revisions differences</translate> 
| buildings_en:diff:version:1.11a:oldid:139:newid:140
|}

<translate><!--T:48--> Apart from storing objects and data through 'central' caching set up by <tvar name=1>{{Phpi|1=$wgMainCacheType}}</tvar> variable, MediaWiki implements itself additional types of caching.</translate>
<translate><!--T:49--> For example <tvar name=1>{{Phpi|1=LinkCache}}</tvar>, <tvar name=2>{{Phpi|1=HTMLFileCache}}</tvar> or <tvar name=3>{{Phpi|1=LocalizationCache}}</tvar>.</translate>

<translate><!--T:50--> <tvar name=1>{{Phpi|1=LinkCache}}</tvar> is used for caching of title objects and their ids, that are linked from a particular source.</translate>
<translate><!--T:51--> For example, an article would have links to other articles, used templates, categories, its talk page etc.</translate>
<translate><!--T:52--> All these links would be stored in a <tvar name=1>{{Phpi|1=LinkCache}}</tvar> object and retrieved from there when needed.</translate>

<translate><!--T:53--> [[File:Module-Cache-CacheTypes.svg|thumb|400px|Types of cache in MediaWiki]]</translate>
<translate><!--T:54--> <tvar name=1>{{Phpi|1=HTMLFileCache}}</tvar> implements caching of entire HTML pages on the file system.</translate>
<translate><!--T:55--> If <tvar name=1>{{Phpi|1=$wgUseFileCache}}</tvar> is set to true, pages can be cached on the server and served to users when requested.</translate>

<translate><!--T:56--> <tvar name=1>{{Phpi|1=LocalizationCache}}</tvar> represents an object with cached contents of localization files.</translate>
<translate><!--T:57--> MediaWiki uses so called messages for any user-facing part of the interface.</translate>
<translate><!--T:58--> For instance, a message 'privacy' would have value 'Privacy policy' for English and 'Politica de proteccion de datos' for Spanish language.</translate>
<translate><!--T:59--> <tvar name=1>{{Phpi|1=LocalizationCache}}</tvar> holds this kind of messages for loaded languages, and this way they can be easily accessed when producing the customized HTML page.</translate> 

<translate>
=== Web Caching === <!--T:10-->
</translate>

<translate><!--T:60--> To provide integration with [[<tvar name=1>Special:MyLanguage/Squid</tvar>|Squid]] or [[<tvar name=2>Special:MyLanguage/Manual:Varnish caching</tvar>|Varnish]] MediaWiki implements the purging notification.</translate>
<translate><!--T:61--> The class responsible for purging Squid URLs given a title (or titles) is <tvar name=1>{{Phpi|1=CdnCacheUpdate|lang=}}</tvar>.</translate>
<translate><!--T:62--> When an HTML page cache needs to be invalidated, an instance of <tvar name=1>{{Phpi|1=CdnCacheUpdate|lang=}}</tvar> is created and method <tvar name=2>{{Phpi|1=doUpdate()}}</tvar> is called on it.</translate>

<translate>
<!--T:11-->
The overview of different types of caching in the Cache module can be seen on figure ''Types of cache in MediaWiki''.

=== Parser cache === <!--T:12-->
</translate>
<translate><!--T:63--> See ''[[<tvar name=1>Special:MyLanguage/Manual:Parser cache</tvar>|Manual:Parser cache]]''.</translate>{{Architectural modules}}

[[Category:Architectural modules of MediaWiki{{#translation:}}]]
[[Category:Cache{{#translation:}}]]