Mongodb – master-master replication in mongodb

You can start a single node replica set by starting the mongod with the replSet argument/config option, that is all that is required. It takes a name argument, which will then function as the name of the replica set in question. Once you have done that, you simply run rs.initiate() from the shell when connected to the node (this is only needed once).

This is described in detail here:

http://docs.mongodb.org/manual/tutorial/convert-standalone-to-replica-set/

To stay with just a single node, simply do not follow the "Expanding the set" instructions.

MongoDB does not currently support a multi-master setup such as you describe.

If you were looking to provide localized regional writes via MongoDB, your best bet is to setup a sharded collection, using shard tag ranges. You would have 1 (or more) shard per region, with the primary residing within the region it manages and remote secondaries. Region would be the first member in your shard key and you would then peg a region's range to the shard that owns that region.

Reads for that region's data performed from a different region could then use a read preference of "nearest", which would result in a secondary read to the local secondary.