Changes

* The webheads also have a shared ZooKeeper or memcached install, which they use to serialize operations on a per-user basis

* Cassandra is periodically snapshotted into S3 for extra durability.

'''Pros''': Easy management and scalability. Very friendly to write-heavy workloads.

'''Cons''': Unknown and untrusted. Harder to hire expertise. Eventual consistency scares me.

=== Hibernation Cluster ===