MemCached.doc (MediaWiki)

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memcached support for MediaWiki

From ca August 2003, 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.

Requirements

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 give themselves developer access and delete all data from the wiki's database, as well as getting all users' password hashes and e-mail addresses.

********************* W A R N I N G ! ! ! ! ! ***********************

Setup

If you want to start small, just run one memcached on your web server:

 memcached -d -l 127.0.0.1 -p 11000 -m 64 

(to run in daemon mode, accessible only via loopback interface, on port 11000, using up to 64MB of memory)

In your LocalSettings.php file, set:

 $wgUseMemCached = true; 
 $wgMemCachedServers = array( "127.0.0.1:11000" ); 

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:

NaodW29-pre314ba41d77a1ba100000001

PHP client for memcached

As of this writing, MediaWiki includes version 1.0.10 of the PHP memcached client by Ryan Gilfether <hotrodder@rocketmail.com>. You'll find some documentation for it in the 'php-memcached' subdirectory under the present one.

We intend to track updates, but if you want to check for the lastest released version, see http://www.danga.com/memcached/apis.bml

If you don't set $wgUseMemCached, we 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.

Keys used

  • User:
    key: $wgDBname:user:id:$sId
    ex: wikidb:user:id:51
    stores: instance of class User
    set in: User::loadFromSession()
    cleared by: User::saveSettings(), UserTalkUpdate::doUpdate()
  • 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 set to 30 minutes
  • LinkCache:
    key: $wgDBname:lc:title:$title
    ex: wikidb:lc:title:Wikipedia:Welcome,_Newcomers!
    stores: cur_id of page, or 0 if page does not exist
    set in: LinkCache::addLink()
    cleared by: LinkCache::clearBadLink()
    should be cleared on page deletion and rename
  • MediaWiki namespace:
    key: $wgDBname:messages
    ex: wikidb:messages
    stores: an array where the keys are DB keys and the values are messages
    set in: wfMsg(), Article::editUpdates() both call wfLoadAllMessages()
    cleared by: nothing
  • Watchlist:
    key: $wgDBname:watchlist:id:$userID
    ex: wikidb:watchlist:id:4635
    stores: HTML string
    cleared by: nothing, expiry time $wgWLCacheTimeout (1 hour)
    note: emergency optimisation only
  • IP blocks:
    key: $wgDBname:ipblocks
    ex: wikidb:ipblocks
    stores: array of arrays, for the BlockCache class
    cleared by: BlockCache:clear()

... more to come ...

Edit Log

  • 2005-06-13 Transcribed from docs for MediaWiki version 1.4.5