For over a year, Twitter has been working with Apache Cassandra open source distributed database. Due to change in strategy, Twitter said that "we're not working on using Cassandra as a "store for Tweets"….Instead we're going to continue to maintain our existing Mysql-based storage."
"We believe that this isn't the time to make large scale migration to a new technology. We'll focus our Cassandra work on new projects that we wouldn't be able to ship without a large-scale data store."said Twitter.
Also, "Cassandra will be used for large-scale real time analytics products for use both internally and externally."
"Cassandra is an open source Apache project to create a "highly scalable second generation distributed database." It was originally open-sourced by Facebook back in 2008."