Apache Cassandra is a NoSQL, highly available, distributed database based on a row/column structure. The top-level namespace in Cassandra is Keyspace. A Keyspace is the equivalent of a database instance in an SQL relational database. An installation of Cassandra may have several Keyspaces. The top-level data structure for data storage is Column Family (also called a table), which is a set of key-value pairs. A Column Family definition consists of columns with one of the columns being the primary key column and the other columns being the data columns. A Column is the smallest unit of data stored in Cassandra and is associated with a name, a value, and a timestamp. One of the columns in a Column Family is the primary key (or row key). A primary key is identified with PRIMARY KEY in a column family definition.