Cassandra

Cassandra is a distributed, decentralized, fault tolerant, eventually consistent, linearly scalable, and column-oriented data store. This means that Cassandra is made to easily deploy over a cluster of machines located at geographically different places. There is no central master server, so no single point of failure, no bottleneck, data is replicated, and a faulty node can be replaced without any downtime. It's eventually consistent. It is linearly scalable, which means that with more nodes, the requests served per second per node will not go down. Also, the total throughput of the system will increase with each node being added. And finally, it's column oriented, much like a map (or better, a map of sorted maps) or a table with flexible columns where each column is essentially a key-value pair. So, you can add columns as you go, and each row can have a different set of columns (key-value pairs). It does not provide any relational integrity. It is up to the application...