[pbailis]

Good points on both sides. One thing I'll point out is that many clustering, HA, and multi-master replication solutions either rely on a single master (what I think Michael means when he says they're not distributed) or don't provide serializability.

Things get harder when you're distributed. For example, if you cluster with a sharded master-slave configuration, then, to serialize transactions that span shards/partitions, you'll need to do 2 phase commit or similar between masters for writes and, for most read/write transactions, make sure you don't read from slaves. If you cluster via master-master/active-active, then, for serializability, you'll need locking or some other concurrency control across masters for each shard/partition. Both setups are definitely do-able (if not highly available) but require non-trivial engineering.