MediaWiki has optional support for memcached, a "high-performance, distributed memory object caching system". For general information on it, see: http://www.danga.com/memcached/ Memcached is likely more trouble than a small site will need, but for a larger site with heavy load, like Wikipedia, it should help lighten the load on the database servers by caching data and objects in memory. == Installation == Packages are available for Fedora, Debian, Ubuntu and probably other Linux distributions. If you there's no package available for your distribution, you can compile it from source. == Compilation == * PHP must be compiled with --enable-sockets * libevent: http://www.monkey.org/~provos/libevent/ (as of 2003-08-11, 0.7a is current) * optionally, epoll-rt patch for Linux kernel: http://www.xmailserver.org/linux-patches/nio-improve.html * memcached: http://www.danga.com/memcached/download.bml (as of this writing, 1.1.9 is current) Memcached and libevent are under BSD-style licenses. The server should run on Linux and other Unix-like systems... you can run multiple servers on one machine or on multiple machines on a network; storage can be distributed across multiple servers, and multiple web servers can use the same cache cluster. ********************* W A R N I N G ! ! ! ! ! *********************** Memcached has no security or authentication. Please ensure that your server is appropriately firewalled, and that the port(s) used for memcached servers are not publicly accessible. Otherwise, anyone on the internet can put data into and read data from your cache. An attacker familiar with MediaWiki internals could use this to steal passwords and email addresses, or to make themselves a sysop and install malicious javascript on the site. There may be other types of vulnerability, no audit has been done -- so be safe and keep it behind a firewall. ********************* W A R N I N G ! ! ! ! ! *********************** == Setup == If you installed memcached using a distro, the daemon should be started automatically using /etc/init.d/memcached. To start the daemon manually, use something like: memcached -d -l 127.0.0.1 -p 11211 -m 64 (to run in daemon mode, accessible only via loopback interface, on port 11211, using up to 64MB of memory) In your LocalSettings.php file, set: $wgMainCacheType = CACHE_MEMCACHED; $wgMemCachedServers = array( "127.0.0.1:11211" ); The wiki should then use memcached to cache various data. To use multiple servers (physically separate boxes or multiple caches on one machine on a large-memory x86 box), just add more items to the array. To increase the weight of a server (say, because it has twice the memory of the others and you want to spread usage evenly), make its entry a subarray: $wgMemCachedServers = array( "127.0.0.1:11211", # one gig on this box array("192.168.0.1:11211", 2 ) # two gigs on the other box ); == PHP client for memcached == MediaWiki uses a fork of Ryan T. Dean's pure-PHP memcached client. The newer PECL module is not yet supported. MediaWiki uses three object for object caching: * $wgMemc, controlled by $wgMainCacheType * $parserMemc, controlled by $wgParserCacheType * $messageMemc, controlled by $wgMessageCacheType If you set CACHE_NONE to one of the three control variable, (default value for $wgMainCacheType), MediaWiki still create a MemCacheClient, but requests to it are no-ops and we always fall through to the database. If the cache daemon can't be contacted, it should also disable itself fairly smoothly. By default, $wgMemc is used but when it is $parserMemc or $messageMemc this is mentionned below. == Keys used == (incomplete, out of date) Date Formatter: key: $wgDBname:dateformatter ex: wikidb:dateformatter stores: a single instance of the DateFormatter class cleared by: nothing expiry: one hour Difference Engine: key: $wgDBname:diff:version:{MW_DIFF_VERSION}:oldid:$old:newid:$new ex: wikidb:diff:version:1.11a:oldid:1:newid:2 stores: body of a difference cleared by: nothing expiry: one week Interwiki: key: $wgDBname:interwiki:$prefix ex: wikidb:interwiki:w stores: object from the interwiki table of the database expiry: $wgInterwikiExpiry cleared by: nothing Lag time of the databases: key: $wgDBname:lag_times ex: wikidb:lag_times stores: array mapping the database id to its lag time expriy: 5 secondes cleared by: nothing Localisation: key: $wgDBname:localisation:$lang ex: wikidb:localisation:de stores: array of localisation settings set in: Language::loadLocalisation() expiry: none cleared by: Language::loadLocalisation() Message Cache: stored in: $messageMemc key: $wgDBname:messages, $wgDBname:messages-hash, $wgDBname:messages-status ex: wikidb:messages, wikidb:messages-hash, wikidb:messages-status stores: an array where the keys are DB keys and the values are messages set in: wfMessage(), Article::editUpdates() and Title::moveTo() expriy: $wgMsgCacheExpiry cleared by: nothing Newtalk: key: $wgDBname:newtalk:ip:$ip ex: wikidb:newtalk:ip:123.45.67.89 stores: integer, 0 or 1 set in: User::loadFromDatabase() cleared by: User::saveSettings() # ? expiry: 30 minutes Parser Cache: stored in: $parserMemc controlled by: $wgEnableParserCache key: $wgDBname:pcache:idhash:$pageid-$renderkey!$hash $pageid: id of the page $renderkey: 1 if action=render, 0 otherwise $hash: hash of user options applied to the page, see ParserOptions::optionsHash() ex: wikidb:pcache:idhash:1-0!1!0!!en!2 stores: ParserOutput object modified by: WikiPage::doEditUpdates() or PoolWorkArticleView::doWork() expiry: $wgParserCacheExpireTime or less if it contains short lived functions key: $wgDBname:pcache:idoptions:$pageid stores: CacheTime object with an additional list of used options for the hash, serves as ParserCache pointer. modified by: ParserCache::save() expiry: The same as the ParserCache entry it points to. Ping limiter: controlled by: $wgRateLimits key: $wgDBname:limiter:action:$action:ip:$ip, $wgDBname:limiter:action:$action:user:$id, mediawiki:limiter:action:$action:ip:$ip and mediawiki:limiter:action:$action:subnet:$sub ex: wikidb:limiter:action:edit:ip:123.45.67.89, wikidb:limiter:action:edit:user:1012 mediawiki:limiter:action:edit:ip:123.45.67.89 and mediawiki:limiter:action:$action:subnet:123.45.67 stores: number of action made by user/ip/subnet cleared by: nothing expiry: expiry set for the action and group in $wgRateLimits Proxy Check: (deprecated) key: $wgDBname:proxy:ip:$ip ex: wikidb:proxy:ip:123.45.67.89 stores: 1 if the ip is a proxy cleared by: nothing expiry: $wgProxyMemcExpiry Revision text: key: $wgDBname:revisiontext:textid:$id ex: wikidb:revisiontext:textid:1012 stores: text of a revision cleared by: nothing expriry: $wgRevisionCacheExpiry Sessions: controlled by: $wgSessionsInObjectCache key: $wgBDname:session:$id ex: wikidb:session:38d7c5b8d3bfc51egf40c69bc40f8be3 stores: $SESSION, useful when using a multi-sever wiki expriy: one hour cleared by: session_destroy() Sidebar: stored in: $parserMemc controlled by: $wgEnableSidebarCache key: $wgDBname:sidebar ex: wikidb:sidebar stores: the html output of the sidebar expriy: $wgSidebarCacheExpiry cleared by: MessageCache::replace() Special:Allpages: key: $wgDBname:allpages:ns:$ns ex: wikidb:allpages:ns:0 stores: array of pages in a namespace expiry: one hour cleared by: nothing Special:Recentchanges (feed): stored in: $messageMemc key: $wgDBname:rcfeed:$format:$limit:$hideminor:$target and rcfeed:$format:timestamp ex: wikidb:rcfeed:rss:50:: and rcfeed:rss:timestamp stores: xml output of feed expiry: one day clear by: maintenance/rebuildrecentchanges.php script, or calling Special:Recentchanges?action=purge&feed=rss, Special:Recentchanges?action=purge&feed=atom, but note need $wgGroupPermissions[...]['purge'] permission. Statistics: controlled by: $wgStatsMethod key: $wgDBname:stats:$key ex: wikibd:stats:request_with_session stores: counter for statistics (see maintenance/showCacheStats.php script) expiry: none (?) cleared by: maintenance/clearCacheStats.php script User: key: $wgDBname:user:id:$sId ex: wikidb:user:id:51 stores: instance of class User set in: User::saveToCache() cleared by: User::saveSettings(), User::clearSharedCache() ... more to come ...