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Reasons why not to use Relational database?

Plain text files in a filesystem

	Very simple to create and edit

	Easy for users to manipulate with simple tools (i.e. text editors, grep etc)

	Efficient storage of binary documents


 




 



XML or JSON files on disk




	As above, but with a bit more ability to validate the structure.


 




 



Spreadsheet / CSV file




	Very easy model for business users to understand


 




 



Subversion (or similar disk based version control system)




	Very good support for versioning of data


 




 



[Berkley DB](http://www.oracle.com/technology/products/berkeley-db/index.html) (Basically, a disk based hashtable)




	Very simple conceptually (just un-typed key/value)

	Quite fast

	No administration overhead

	Supports transactions I believe


 




 



[Amazon's Simple DB](http://www.amazon.com/SimpleDB-AWS-Service-Pricing/b/ref=sc_fe_l_2?ie=UTF8&node=342335011&no=3435361&me=A36L942TSJ2AJA)




	Much like Berkley DB I believe, but hosted


 




 



[Google's App Engine Datastore](http://code.google.com/appengine/docs/datastore/)




	Hosted and highly scalable

	Per document key-value storage (i.e. flexible data model)


 




 



[CouchDB](http://incubator.apache.org/couchdb/)




	Document focus

	Simple storage of semi-structured / document based data


 




 



Native language collections (stored in memory or serialised on disk)




	Very tight language integration


 




 



Custom (hand-written) storage engine




	Potentially very high performance in required uses cases