Currently, each Replica Set has exactly one master at any time. Reads may be performed by slaves, but writes always go to the master.

It may be advantageous to allow this master-slave preference to be controlled per-DB. For example:

• DB: "global"; replicaSet: no-preferred master
• DB: "usa"; replicaSet: prefer USA-located replica members to be master
• DB: "asia"; replicaSet: prefer Asia-located replica members to be master

This would allow a single known-working cluster to be used for multiple region-specific purposes without needing to create and maintain region-specific clusters.