Twitter is changing there database from MySQL to a NoSQL database in the form of Apache Software Foundation's Cassandra project. Ryan King, an engineer at Twitter, told the blog MyNoSQL that the MySQL servers are "quickly becoming prohibitively costly to operate.
and We need a system that can grow in a more automated fashion and be highly availible."
While Twitter has looked at other options before choosing Cassandra, like re-architecting MySQL and other NoSQL databases, Cassandra was said to have proved itself as being the more scalable, reliable and easier to manage database.
Cassandra right now is in testing running alongside MySQL. Soon, after a bit more testing, Twitter will completely shut down MySQL and run Cassandra only.