Manual:$wgVirtualDomainsMapping

Details
Mapping of virtual domain to external cluster db.

If no entry is set, the code assumes local database. For example, for routing queries of virtual domain 'vdomain' to 'wikishared' database in 'extension1' cluster. The config should be like this:

If the database needs to be the local domain, just set the 'db' to.

If you want to get another db in the main cluster, just omit 'cluster'. For example: