Manual:$wgAlternateMaster

Details
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  as class.