You can't do multi-master with MySQL anyways, which until very recently has been the only "engine" RDS supports. Even if you could do multi-master, replication is still single threaded. You have to come up with your own sharding scheme. This is a limitation of MySQL not RDS.
You can't do multi-master with MySQL? News to me - we've been using circular replication between two servers, each a master and slave, for quite some time now.
Not possible with RDS, unfortunately, but works fine on two EC2 instances.
You can do it, but it won't scale your write load. In most cases it actually makes it worse due to the having to apply all the writes taken on the other master node in serial.
There is a difference between multi-master and circular replication. To me, Multi-master is that I can write to both masters at the same time, which implies there is a way to resolve conflicts. Databases like Cassandra (timestamps) and Riak (vector clocks) have this, MySQL does not. If you write to the same record on both masters bad shit happens and its very hard to sort out.
You can write to both masters at the same time in a MySQL multi-master circular replication setup. It's done via auto_increment_increment and auto_increment_offset configuration settings in my.cnf - each server generates autoincrement keys that are unique to that server.
This still doesn't protect you from UPDATE statements. I suppose you could pull it off if you could be absolutely certain that you were only CREATEing. You still have the problem that replication is single threaded, so this doesn't scale your writes beyond one thread.
