Cassandra (Apache Cassandra) was developed by Facebook to power the social network's inbox search feature. It was originally created by by Avinash Lakshman and Prashant Malik before it was released as an open source project on Google code in July 2008, an Apache Incubator project in March 2009, and a top-level project in February 2010.
Cassandra's data model is a partitioned in rows. Like other so-called "NoSQL" databases, it did away with the traditional relational model. These rows are then organized into tables; the first component of a table's primary key is the partition key; within a partition, rows are clustered by the remaining columns of the key. Other columns may be indexed separately from the primary key.
Cassandra is the most popular wide column store because of its high-value on performance and its ability to run across a vast cluster of machines to juggle huge amounts of data, with no point of failure.
Facebook's core service abandoned Cassandra when the company replaced their inbox search with Facebook Messaging platform. The technology since then has powered critical web infrastructure at companies like Twitter, Netflix, Apple, and others.