Manual:$wgAlternateMaster

LoadBalancer settings: $wgAlternateMaster
A map of database name to connection index.
Introduced in version:1.10.0
Removed in version:1.13.0
Allowed values:(array)
Default value:array()

Details edit

A map of database name (key) to connection index (value), describing the master database server for each database. The connection index is an index into the $wgDBservers array. If a database is not listed here, the default load balancing settings will be used.

To use this feature in 1.13 and above, you have to customize $wgLBFactoryConf and use LBFactory_Multi as class.