Open Source Multi-master replication ideas?
I favor Galera clustering (see Percona's PXC or MariaDB).  Put 1 node in each of 3 different datacenters.  (One node could be a trivial 'arbitrator' if you need to cut corners.)

Galera requires the use of InnoDB, but that is reasonably efficient.

If a node goes down, or is separated from the other two nodes due to a network outage, the system hums along, with the restriction that you can't write to the dead/isolated node.  When if comes back up (or is replaced/repaired), the cluster will repair it automagically.

If no node can talk to any other node (which seems extreme), manual intervention will be required.  (Actually, if more that 50% of the nodes are communicating with each other, the "system" is still alive and usable.)

Transactions are synchronous (read: latency), but there in only one round trip per transaction.

NDB Cluster (which is perhaps what you are referring to) is a distinctly different animal.  It uses an "eventual consistency" for dealing with replication.  This is not at all related to the rest of MySQL replication options.

Key:value can be implemented easily on top of InnoDB or NDB cluster.