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   1  MediaWiki has optional support for memcached, a "high-performance, 
   2  distributed memory object caching system". For general information 
   3  on it, see: http://www.danga.com/memcached/
   4  
   5  Memcached is likely more trouble than a small site will need, but
   6  for a larger site with heavy load, like Wikipedia, it should help
   7  lighten the load on the database servers by caching data and objects
   8  in memory.
   9  
  10  == Installation ==
  11  
  12  Packages are available for Fedora, Debian, Ubuntu and probably other
  13  Linux distributions. If you there's no package available for your 
  14  distribution, you can compile it from source.
  15  
  16  == Compilation ==
  17  
  18  * PHP must be compiled with --enable-sockets
  19  
  20  * libevent: http://www.monkey.org/~provos/libevent/
  21    (as of 2003-08-11, 0.7a is current)
  22  
  23  * optionally, epoll-rt patch for Linux kernel:
  24    http://www.xmailserver.org/linux-patches/nio-improve.html
  25  
  26  * memcached: http://www.danga.com/memcached/download.bml
  27    (as of this writing, 1.1.9 is current)
  28    
  29  Memcached and libevent are under BSD-style licenses.
  30  
  31  The server should run on Linux and other Unix-like systems... you
  32  can run multiple servers on one machine or on multiple machines on
  33  a network; storage can be distributed across multiple servers, and
  34  multiple web servers can use the same cache cluster.
  35  
  36  ********************* W A R N I N G ! ! ! ! ! ***********************
  37  Memcached has no security or authentication. Please ensure that your
  38  server is appropriately firewalled, and that the port(s) used for
  39  memcached servers are not publicly accessible. Otherwise, anyone on
  40  the internet can put data into and read data from your cache.
  41  
  42  An attacker familiar with MediaWiki internals could use this to steal
  43  passwords and email addresses, or to make themselves a sysop and 
  44  install malicious javascript on the site. There may be other types 
  45  of vulnerability, no audit has been done -- so be safe and keep it 
  46  behind a firewall.
  47  ********************* W A R N I N G ! ! ! ! ! ***********************
  48  
  49  == Setup ==
  50  
  51  If you installed memcached using a distro, the daemon should be started
  52  automatically using /etc/init.d/memcached.
  53  
  54  To start the daemon manually, use something like:
  55  
  56    memcached -d -l 127.0.0.1 -p 11211 -m 64
  57  
  58  (to run in daemon mode, accessible only via loopback interface,
  59  on port 11211, using up to 64MB of memory)
  60  
  61  In your LocalSettings.php file, set:
  62  
  63      $wgMainCacheType = CACHE_MEMCACHED;
  64      $wgMemCachedServers = array( "127.0.0.1:11211" );
  65  
  66  The wiki should then use memcached to cache various data. To use
  67  multiple servers (physically separate boxes or multiple caches
  68  on one machine on a large-memory x86 box), just add more items
  69  to the array. To increase the weight of a server (say, because
  70  it has twice the memory of the others and you want to spread
  71  usage evenly), make its entry a subarray:
  72  
  73    $wgMemCachedServers = array(
  74      "127.0.0.1:11211", # one gig on this box
  75      array("192.168.0.1:11211", 2 ) # two gigs on the other box
  76    );
  77  
  78  == PHP client for memcached ==
  79  
  80  MediaWiki uses a fork of Ryan T. Dean's pure-PHP memcached client.
  81  It also supports the PECL PHP extension for memcached.
  82  
  83  MediaWiki uses three object for object caching:
  84  * $wgMemc, controlled by $wgMainCacheType
  85  * $parserMemc, controlled by $wgParserCacheType
  86  * $messageMemc, controlled by $wgMessageCacheType
  87  If you set CACHE_NONE to one of the three control variable, (default
  88  value for $wgMainCacheType), MediaWiki still create a MemCacheClient,
  89  but requests to it are no-ops and we always fall through to the
  90  database. If the cache daemon can't be contacted, it should also
  91  disable itself fairly smoothly.
  92  
  93  By default, $wgMemc is used but when it is $parserMemc or $messageMemc
  94  this is mentioned below.
  95  
  96  == Keys used ==
  97  
  98  (incomplete, out of date)
  99  
 100  Date Formatter:
 101      key: $wgDBname:dateformatter
 102      ex: wikidb:dateformatter
 103      stores: a single instance of the DateFormatter class
 104      cleared by: nothing
 105      expiry: one hour
 106  
 107  Difference Engine:
 108      key: $wgDBname:diff:version:{MW_DIFF_VERSION}:oldid:$old:newid:$new
 109      ex: wikidb:diff:version:1.11a:oldid:1:newid:2
 110      stores: body of a difference
 111      cleared by: nothing
 112      expiry: one week
 113  
 114  Interwiki:
 115      key: $wgDBname:interwiki:$prefix
 116      ex: wikidb:interwiki:w
 117      stores: object from the interwiki table of the database
 118      expiry: $wgInterwikiExpiry
 119      cleared by: nothing
 120  
 121  Lag time of the databases:
 122      key: $wgDBname:lag_times
 123      ex: wikidb:lag_times
 124      stores: array mapping the database id to its lag time
 125      expriy: 5 secondes
 126      cleared by: nothing
 127  
 128  Localisation:
 129      key: $wgDBname:localisation:$lang
 130      ex: wikidb:localisation:de
 131      stores: array of localisation settings
 132      set in: Language::loadLocalisation()
 133      expiry: none
 134      cleared by: Language::loadLocalisation()
 135  
 136  Message Cache:
 137      stored in: $messageMemc
 138      key: $wgDBname:messages, $wgDBname:messages-hash, $wgDBname:messages-status
 139      ex: wikidb:messages, wikidb:messages-hash, wikidb:messages-status
 140      stores: an array where the keys are DB keys and the values are messages
 141      set in: wfMessage(), Article::editUpdates() and Title::moveTo()
 142      expriy: $wgMsgCacheExpiry
 143      cleared by: nothing
 144  
 145  Newtalk:
 146      key: $wgDBname:newtalk:ip:$ip
 147      ex: wikidb:newtalk:ip:123.45.67.89
 148      stores: integer, 0 or 1
 149      set in: User::loadFromDatabase()
 150      cleared by: User::saveSettings() # ?
 151      expiry: 30 minutes
 152  
 153  Parser Cache:
 154      stored in: $parserMemc
 155      controlled by: $wgEnableParserCache
 156      key: $wgDBname:pcache:idhash:$pageid-$renderkey!$hash
 157          $pageid: id of the page
 158          $renderkey: 1 if action=render, 0 otherwise
 159          $hash: hash of user options applied to the page, see ParserOptions::optionsHash()
 160      ex: wikidb:pcache:idhash:1-0!1!0!!en!2
 161      stores: ParserOutput object
 162      modified by: WikiPage::doEditUpdates() or PoolWorkArticleView::doWork()
 163      expiry: $wgParserCacheExpireTime or less if it contains short lived functions
 164  
 165      key: $wgDBname:pcache:idoptions:$pageid
 166      stores: CacheTime object with an additional list of used options for the hash,
 167      serves as ParserCache pointer.
 168      modified by: ParserCache::save()
 169      expiry: The same as the ParserCache entry it points to.
 170  
 171  Ping limiter:
 172      controlled by: $wgRateLimits
 173      key: $wgDBname:limiter:action:$action:ip:$ip,
 174          $wgDBname:limiter:action:$action:user:$id,
 175          mediawiki:limiter:action:$action:ip:$ip and
 176          mediawiki:limiter:action:$action:subnet:$sub
 177      ex: wikidb:limiter:action:edit:ip:123.45.67.89,
 178          wikidb:limiter:action:edit:user:1012
 179          mediawiki:limiter:action:edit:ip:123.45.67.89 and
 180          mediawiki:limiter:action:$action:subnet:123.45.67
 181      stores: number of action made by user/ip/subnet
 182      cleared by: nothing
 183      expiry: expiry set for the action and group in $wgRateLimits
 184  
 185  
 186  Proxy Check: (deprecated)
 187      key: $wgDBname:proxy:ip:$ip
 188      ex: wikidb:proxy:ip:123.45.67.89
 189      stores: 1 if the ip is a proxy
 190      cleared by: nothing
 191      expiry: $wgProxyMemcExpiry
 192  
 193  Revision text:
 194      key: $wgDBname:revisiontext:textid:$id
 195      ex: wikidb:revisiontext:textid:1012
 196      stores: text of a revision
 197      cleared by: nothing
 198      expriry: $wgRevisionCacheExpiry
 199  
 200  Sessions:
 201      controlled by: $wgSessionsInObjectCache
 202      key: $wgBDname:session:$id
 203      ex: wikidb:session:38d7c5b8d3bfc51egf40c69bc40f8be3
 204      stores: $SESSION, useful when using a multi-sever wiki
 205      expriy: one hour
 206      cleared by: session_destroy()
 207  
 208  Sidebar:
 209      stored in: $parserMemc
 210      controlled by: $wgEnableSidebarCache
 211      key: $wgDBname:sidebar
 212      ex: wikidb:sidebar
 213      stores: the html output of the sidebar
 214      expriy: $wgSidebarCacheExpiry
 215      cleared by: MessageCache::replace()
 216  
 217  Special:Allpages:
 218      key: $wgDBname:allpages:ns:$ns
 219      ex: wikidb:allpages:ns:0
 220      stores: array of pages in a namespace
 221      expiry: one hour
 222      cleared by: nothing
 223  
 224  Special:Recentchanges (feed):
 225      stored in: $messageMemc
 226      key: $wgDBname:rcfeed:$format:$limit:$hideminor:$target and
 227          rcfeed:$format:timestamp
 228      ex: wikidb:rcfeed:rss:50:: and rcfeed:rss:timestamp
 229      stores: xml output of feed
 230      expiry: one day
 231      clear by: maintenance/rebuildrecentchanges.php script, or
 232      calling Special:Recentchanges?action=purge&feed=rss,
 233      Special:Recentchanges?action=purge&feed=atom,
 234      but note need $wgGroupPermissions[...]['purge'] permission.
 235  
 236  Statistics:
 237      controlled by: $wgStatsMethod
 238      key: $wgDBname:stats:$key
 239      ex: wikibd:stats:request_with_session
 240      stores: counter for statistics (see maintenance/showCacheStats.php script)
 241      expiry: none (?)
 242      cleared by: maintenance/clearCacheStats.php script
 243  
 244  User:
 245      key: $wgDBname:user:id:$sId
 246      ex: wikidb:user:id:51
 247      stores: instance of class User
 248      set in: User::saveToCache()
 249      cleared by: User::saveSettings(), User::clearSharedCache()
 250      
 251  ... more to come ...