Memcached Manual (mediawiki.org) (web Page)
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Manual:Memcached
memcached is a memory-based object store, developed originally to lighten the load on
LiveJournal 's database servers. memcached has been supported in MediaWiki since pre v1.1.
Memcached is likely more trouble than a small site will need (for a single server, consider storing
both code and data in APC - CACHE_ACCEL), 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.
Contents [hide]
1Requirements
2Security
3Setup
4PHP client for memcached
5Troubleshooting
5.1 Loss of session data when saving
6Using memcached in your code
7Old Development Notes
7.1 Expiration model
7.2 Questions & Answers
8See also
Requirements [edit ]
PHP must be compiled with --enable-sockets
libevent
optionally, epoll-rt patch for Linux kernel
memcached
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.
Security [edit ]
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.
Setup [edit ]
If you want to start small, just run one memcached on your web server:
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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; $wgParserCacheType =
CACHE_MEMCACHED; # optional $wgMessageCacheType = CACHE_MEMCACHED; #
optional $wgMemCachedServers = array( "127.0.0.1:11211" );
$wgSessionsInObjectCache = true; # optional $wgSessionCacheType =
CACHE_MEMCACHED; # optional
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 / Power 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 [edit ]
As of this writing (MediaWiki 1.27), MediaWiki uses a fork of Ryan T. Dean's pure-PHP
memcached client. It also supports the PECL PHP extension for memcached.
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.
Troubleshooting [edit ]
Loss of session data when saving [edit ]
If you store session data in memcached, and users see this message intermittently when they try
to save edits:
Sorry! We could not process your edit due to a loss of session data.
You might have been logged out. Please verify that you're still logged in and try again.
If it still does not work, try logging out and logging back in, and check that your browser
allows cookies from this site.
then one or more of your memcached servers might have a misconfigured /etc/hosts file. On
each of your memcached servers, make sure the server's own hostname is mapped to localhost:
127.0.0.1 servername.here localhost localhost.localdomain ...
Otherwise, the server might not be able to connect to its own memcached process.
Using memcached in your code [edit ]
If you're writing an extension that does expensive database queries, it might be useful to cache
the data in memcached. There are a few main ways to get a handle to memcached:
$cache = ObjectCache::getMainWANInstance()
...use this if you want a memory-based shared cache with explicit purge ability in order to
store values derived from persistent sources
$cache = ObjectCache::getLocalClusterInstance()
...use this if you want a memory-based ephemeral store that is not shared among
datacenters
$cache = ObjectCache::getLocalServerInstance()
...use this if you want a memory-based ephemeral cache that is not shared among web
servers
$cache = wfGetCache( CACHE_ANYTHING )
...use this if you want any available cache, which may or may not be per-datacenter, even
an emulated one that uses a SQL database
Note that these may return handles that talk to Redis, APC, MySQL or other stores instead. The
use of the word "memcached" is historically due to the API being defined around the simple
commands that memcached supports and the fact that, to date, memcached is normally the best
general-purpose cache store.
Extensions that have specific needs (like persistence) should define new configuration settings
like $wgMyExtCache or $wgMyExtWANCache . Code using the caches can pass them to
wfGetCache() and ObjectCache::getWANInstance() , respectively.
The following code snippet demonstrates how to cache the results of a database query into
memcached for 15 minutes and query memcached first for the results instead of the database.
class MyExtensionFooBars { public function getPopularTen() { $cache =
ObjectCache::getMainWANInstance(); return $cache->getWithSetCallback( //
The variadic arguments to wfMemcKey() are used to construct the key for
the cache // in memcached. It must be unique to the query you are
saving. The first value is normally // the extension or component name,
and following values tell you what query you are saving. $cache-
>makeKey( 'myextension', 'foobars', 'popular', '10' ), // Cache for 15
minutes $cache::TTL_MINUTE * 15, // Function to generate the value on
cache miss function ( $oldValue, &$ttl, &$setOpts ) { $dbr = wfGetDB(
DB_SLAVE ); // Adjust TTL based on DB replication lag $setOpts =
Database::getCacheSetOptions( $dbr ); $res = $dbr->select( // your
database query goes here // see Database::select for docs ); $data =
array(); foreach ( $res as $row ) { $data[] = array( // Do something
with the data we // just got from the database // For example, if we
looked up // page_id from the page table, // we could do this: 'id' =>
$row->page_id ); } return $data; } ); } }
The abstract BagOStuff and WANObjectCache classes define and document all of the available
functions:
BagOStuff
WANObjectCache
Old Development Notes [edit ]
Broadly speaking, we'd like to be able to dump lots of data in the cache, use it whenever we can,
and automatically expire it when changes are made.
Expiration model [edit ]
explicit expiration times: memcached lets us set an expiration time on an object when we
store it. After the time is up, another request for the object will find that it has expired and
return nothing to us.
pro: last-ditch fallback to let data that could be updated badly eventually fall out of the
cache
con: we have to know ahead of time when it will cease to be invalid. hard to do when
we're dealing with user edits!
delete cached objects when we know we're doing something that will cause them
to be invalid but are not in a position to update them while we're at it
pro: fairly simple; the item will be reloaded from the database and recached when it's next
needed
con: if this will affect a large number of related items (for instance, creating or deleting a
page invalidates the links/brokenlinks tables and rendered HTML cache of pages that link
to that page) we may have to hunt them all down and do a lot of updating
include timestamps on cached objects and do our own expiries based on
dependencies
pro: can expire many objects at once by updating a single node they depend on
con: more things to load; multiple dependencies could be trickier to work with
Questions & Answers [edit ]
Q: The current plan is to deploy six load balanced Apaches, the likeliness that one of them
renders the same page twice should be 1/6 of the current value, right?
A: Memcached is a shared cache between all Apaches, communication is done with TCP.
Q: Squid will cache the majority of content, reducing repetitions drastically. What's the point in
memcached then?
A: The squid only replaces the anonymous cache. Memcached has far wider applicability, both
currently implemented and potentially. -- Tim Starling 02:12, 16 Jan 2004 (UTC)
Q: Does Memcached have anything to do with your browser or your browser's cache?
A: NO!
Q: Can I have multiple clients written in different programming languages access the same
Memcached server?
A: Of course.
Q: Can I search on part of a key or a regular expression on a Memcached server?
A: No, you can only search for an exact key if you need more information on what you could
possibly do you can check out the Memcached protocol:
http://cvs.danga.com/browse.cgi/wcmtools/memcached/doc/protocol.txt (or
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This page was last edited on 4 December 2018, at 01:22.
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details.
https://github.com/memcached/memcached/blob/master/doc/protocol.txt )
Q: Can I have multiple wikis point to the same Memcached server?
A: Yes, as long as each have different wiki-ids ($wgDBname). Certain cache keys are
intentionally shared in such a scenario, such as rate limiting stuff.
See also [edit ]
docs/memcached.txt in the source documentation
$wgMainCacheType
$wgMemCachedServers
Extension:Memcached
Manual:Performance tuning
Memcached on Bluehost
meta:Tugela Cache
"Memcached on 1and1 with MediaWiki"
Categories: Pages with ignored display titles Cache Performance tuning
