In the recent couple of years there has been steady development in areas of non-relational data storage, where in the data does not fit into a more orderly schema instead the data tends to have flexible schema. These center around the recent developments in social media and social networking where in enormous amounts of data is being stored and retrieved. Since the vendors/companies in social networking space did not find traditional RDBMS to suit their needs, the emergence of NoSQL systems took place. NoSQL means "Not only SQL". These systems (NoSQL) have flexible schema,Massive scalability (since lot of requests can be handled in parallel) and higher performance and availability. There are different implementations of NoSQL Systems, they are:
1. MapReduce FrameWork.
2. Key-Value Stores
Example Systems: Google BigTable,Amazon Dynamo,Cassandra.
3. Document Stores.
Example Systems: CouchDB,MongoDB,SimpleDB.
4. Graph Database Systems.
Example Systems: Neo4j,FlockDB,Pregel.
The MapReduce Framework is kind of analogous to OLAP in the Business Intelligence world, the Key-Value Stores can be kind of analogous to the OLTP world. Document Stores use a Key,Document pair to persist documents and also enables faster retrieval of documents, one of the popular systems which implements Document Store is MongoDB.
One of the popular implementation of NoSQL system is the MapReduce Framework which was originally developed in Google, then followed by the Open Source implementation called Hadoop. This Map Reduce has No Data model, the data is stored in files on the Google File System (GFS) or the Hadoop file System called HDFS. There are specific functions in this Framework such as Map(),Reduce(),Reader(),Writer() and Combiner(). Microsoft has plans to work with Hadoop in some form, the NoSQL has been gaining traction in the last couple of years and this space is bound to see a lot of changes in the coming years. I would to thank Prof. Jennifer Widom (Stanford University, Computer Science) for providing information/material on NoSQL Systems as part of a online course in database systems.