A highly scalable, eventually consistent, distributed, structured key-value store.
The Apache Cassandra Project develops a highly scalable second-generation distributed database, bringing together Dynamo's fully distributed design and Bigtable's ColumnFamily-based data model.
Cassandra was open sourced by Facebook in 2008, where it was designed by Avinash Lakshman (one of the authors of Amazon's Dynamo) and Prashant Malik. In a lot of ways you can think of Cassandra as Dynamo 2.0.
The latest release is 0.5.0 (Changes)
Cassandra is in use at Rackspace, Digg, Facebook, Twitter, Cisco, Mahalo, Ooyala, and more companies that have large, active data sets. The largest production cluster has over 100 TB of data in over 150 machines.
Data is automatically replicated to multiple nodes for fault-tolerance. Replication across multiple data centers is supported. Failed nodes can be replaced with no downtime.
Every node in the cluster is identical. There are no network bottlenecks. There are no single points of failure.
Cassandra implements an eventually consistent model and includes sophisticated features such as Hinted Handoff and Read Repair to minimize inconsistency windows.
Allows efficient use for many applications beyond simple key/value.
Read and write throughput both increase linearly as new machines are added, with no downtime or interruption to applications.
Writes and reads offer a tunable ConsistencyLevel, all the way from "writes never fail" to "block for all replicas to be readable," with the quorum level in the middle.
Many of the Cassandra developers and community members hang out in the #cassandra channel on irc.freenode.net.
If you are new to IRC and don't have a client, you can use a web-based client.