Memcached

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 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 64 MiB of memory)

In your LocalSettings.php file, set:

$wgMainCacheType = CACHE_MEMCACHED;
$wgMemCachedServers = [ "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 = [
    "127.0.0.1:11211", # one gig on this box
    [ "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. It also supports the PECL PHP extension for memcached.

MediaWiki uses the ObjectCache class to retrieve instances of BagOStuff by purpose, controlled by the following variables:

• $wgMainCacheType
• $wgParserCacheType
• $wgMessageCacheType

If you set one of these to CACHE_NONE, MediaWiki still creates a BagOStuff object, but calls it to it are no-ops. If the cache daemon can't be contacted, it should also disable itself fairly smoothly.

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
expiry: 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:

See MessageCache.php.

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:

access: ParserCache
backend: $wgParserCacheType
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
expiry: $wgRevisionCacheExpiry

Sessions:

controlled by: $wgSessionsInObjectCache
key: $wgBDname:session:$id
ex: wikidb:session:38d7c5b8d3bfc51egf40c69bc40f8be3
stores: $SESSION, useful when using a multi-sever wiki
expiry: one hour
cleared by: session_destroy()

Sidebar:

access: WANObjectCache
controlled by: $wgEnableSidebarCache
key: $wgDBname:sidebar
ex: wikidb:sidebar
stores: the html output of the sidebar
expiry: $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):

backend: $wgMessageCacheType
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.