If the only tool I have is a hammer then everything to me looks like a nail. And if I can't find any nails then I start to invent them.
The above saying can be easily applied to the usage of relational databases as the one and only way to persist data. We have been told that relational databases are the only type of data stores that makes sense in a business application. Everything else was considered as unreliable.
Lately, more and more alternative ways of storing data have emerged and have been used quite successfully by many well known and respectable companies on the market. A few of them are Google's Big Table or document databases such as Mongo DB, Couch DB, and Raven DB.
Companies that are using these products have requirements that just cannot be fulfilled by the traditional RDBMS. Thus, these companies and other individuals started to look out for some alternative ways to efficiently store and retrieve data. They were willing to trade in...