Manual:$wgDBservers

Details
Database load balancer.

This is a two-dimensional array, an array of server info structures.

Fields are: Host name (see ) Default database name (see ) DB user (see ) DB password (see ) "mysql" or "pgsql" (see ) ratio of DB_SLAVE load, must be >=0, the sum of all loads must be >0 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)  bit field:
 * host:
 * dbname:
 * user:
 * password:
 * type:
 * load:
 * groupLoads:
 * flags:
 * DBO_DEFAULT --

turns on DBO_TRX only if  != true (recommended)
 * DBO_DEBUG --

equivalent of 
 * DBO_TRX --

automatically start transactions (see )
 * 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: (<tvar|removed></> 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 <tvar|DBloads></>.

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 <tvar|CookiePrefix></> and <tvar|CachePrefix></> are set explicitly.