Manual:$wgDBservers

Details
Database load balancer. This is a two-dimensional array, an array of server info structures. Fields are:
 * host: Host name (see $wgDBserver)
 * dbname: Default database name (see $wgDBname)
 * user: DB user (see $wgDBuser)
 * password: DB password (see $wgDBpassword)
 * type: "mysql" or "pgsql" (see $wgDBtype)
 * load: ratio of DB_SLAVE load, must be >=0, the sum of all loads must be >0
 * groupLoads: array of load ratios, the key is the query group name. A query may belong to several groups, the most specific group defined here is used. (added in 1.4.3)
 * flags: bit field:
 * DBO_DEFAULT -- turns on DBO_TRX only if $wgCommandLineMode != true (recommended)
 * DBO_DEBUG -- equivalent of $wgDebugDumpSql
 * DBO_TRX -- automatically start transactions (see Database transactions)
 * DBO_IGNORE -- ignore errors (not useful in LocalSettings.php)
 * DBO_NOBUFFER -- turn off buffering (not useful in LocalSettings.php)
 * DBO_SSL -- use a secure connection
 * DBO_COMPRESS -- compress communication

Leave at false to use the single-server variables listed above.

The master must be the first entry in the array.

Example: This would configure one master and two slaves, which each slave get an equal amount of read access load.


 * max lag: (optional) Maximum replication lag before a slave will be taken out of rotation
 * max threads: optional) Maximum number of running threads

These and any other user-defined properties will be assigned to the mLBInfo member variable of the Database object.

Early versions
Versions of MediaWiki prior to 1.4.0 used a single-dimensional array, listing the server names, e.g. array("larousse", "pliny"), which was used in conjunction with $wgDBloads. From 1.4.0 onwards, $wgDBloads is deprecated and the above method must be used.

The groupLoads field was added in 1.4.3.

max lag and max threads (and the use of other user-defined properties) was added in 1.6.0

Warnings
If you switch to this, ensure you either keep $wgDBname set, or ensure $wgCookiePrefix and $wgCachePrefix are set explicitly.