Manual:$wgExternalServers

Details
An array of external MySQL servers.

$wgExternalServers collects all the usable clusters with all usable nodes of a cluster.

The top-level array's keys denote a cluster's name (The below example defines only one cluster. It has the name demoCluster).

The value to those keys are again arrays. They hold the specifications of the individual nodes.

The first node is consider the master. All writes to the database are performed through this master node.

Zero or more slave nodes may follow. (In the example below, you find two slave nodes).

Each node may have it's own host, user, password, dbname, and type, as shown in the example. The load parameter allows to specify how much of the load should pass through this note.

e.g.

(See External Storage for a discussion of this example, and how to turn this into a usable cluster configuration.)