[pbailis]

> if I understand correctly there's no way to avoid an extra round trip?

With HATs, you only need to contact one replica for every key. This is to goal behind our definition of "high availability" (http://www.bailis.org/blog/hat-not-cap-introducing-highly-av...).

In general, I haven't seen algorithms guaranteeing serializability or atomicity that complete without a round trip to at least one other replica (or a possibly long trip to the master). Intuitively, the impossibility results dictate that this must be the case, otherwise partitioned replicas could safely serve requests. Daniel Abadi has a great post about this latency-consistency trade-off: http://dbmsmusings.blogspot.com/2010/04/problems-with-cap-an...