Cassandra had been underway long before Oracle bought Sun and mySQL. Facebook first launched it in 2008 to power their inbox search feature. It was released on Google Code in 2008 and became an Apache Incubator project last year.
Apache describes it simply as "a highly scalable, eventually consistent, distributed, structured key-value store, not a database." It rides as an interface between a relational database and a high-traffic Web presence. The latest release is 0.5.1,
Cassandra itself is not a database. But it does allow someone to use a variety of open source databases and deliver a consistent, high volume write function to a Web page. The project is not ruling out working with a variety of proprietary databases, including Oracle.
While this does not mean a direct replacement for mySQL, Cassandra does perform the goal of mySQL opponents in liberating them from dependence on the underlying mySQL code during implementations.