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No SQL Databases

Recentely there has been lot of discussion about No SQL databases. Some see it as alternate to SQL database which means writing no SQL to query the database Hence No SQL. Another camp is opinion it is 'Not Only SQL' which basically means you can query the database in more than one way.

These databases are also know as Document Databases as the data is stored as Documents (serialised as JSON objects) . There are three main leading open source Document databases

MongoDB - http://www.mongodb.org/

RavenDB - http://ravendb.net/

CouchDB - http://couchdb.apache.org/

Update: fourth contender, Amazon Simple DB : http://aws.amazon.com/simpledb/
RavenDB and CouchDB stores data as JSON objects while MongoDB uses a twist and stored data as Binary JSON (BSON) objects. Mostly the data is queries either through RestFUL API's using HTTP or through TCP/IP.

These databases also support Indexes and Transactions. For transaction critical applications like banking or realtime system, you better stick with Relational databases. The Document databases are emerging as a serious alternative to less critical system for e.g. say session management, event or error logging.

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