Changes

Another promising storage option is Cassandra. It provides a rich-enough data model and automatic cluster management, at the cost of eventual consistency and the vague fear that it will try to do something "clever" when you really don't want it to. To get strong consistency back, we'd use a locking layer such as Zookeeper or memcached.

Basic principles:

* 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.

== Hibernation Cluster ==