The Nosql databases currently being used can be grouped into four broad categories:

• Key-value data stores: Data is stored as key-value pairs. Values are retrieved by keys.Redis,Dynomite,and Voldemort are examples of such databases.

• Column-based databases: These databases organize the data in tables,similar to anRDBMS,however,they store the content by columns instead of rows. They are goodfor data warehousing applications. Examples of column-based databases are Hbase,Cassandra,Hypertable,and so on.

• Document-based databases: Data is stored and organized as a collection ofdocuments. The documents are flexible; each document can have any number offields. Apache CouchDB and MongoDB are prominent document databases.

• Graph-based data-stores: These databases apply the computer science graph theoryfor storing and retrieving data. They focus on interconnectivity of different parts of data. Units of data are visualized as nodes and relationships among them aredefined by edges connecting the nodes. Neo4j is an example of such a database.