Cassandra provides the BigTable data model with a peer-to-peer architecture. It can do row-level writes, reads, and range queries. Yet, the BigTable model is not enough to fully emulate the Google App Engine Datastore API, as it is based on Megastore, which provides the added benefit of transactions on partitioned data. For this AppScale uses ZooKeeper, the open source implementation of Google’s Chubby. ZooKeeper provides a locking API using a variant of the Paxos algorithm.