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In recent years there was a high demand for massively distributed databases with high partition tolerance but according to the [[CAP theorem]] it is impossible for a [[Distributed computing|distributed system]] to simultaneously provide [[Consistency model|consistency]], [[availability]] and [[partition tolerance]] guarantees. A distributed system can satisfy any two of these guarantees at the same time, but not all three. For that reason many NoSQL databases are using what is called [[eventual consistency]] to provide both availability and partition tolerance guarantees with a maximum level of data consistency.
The most popular NoSQL systems include: [[MongoDB]], [[Couchbase]], [[Riak]], [[Oracle NoSQL Database]], [[memcached]], [[Redis]], [[CouchDB]], [[Hazelcast]], [[Apache Cassandra]] and [[HBase]].<ref>{{cite web|url=http://db-engines.com/en/ranking |title=DB-Engines Ranking |date=January 2013 |accessdate=22 January 2013}}</ref> Note that all are [[open-source software]] products.
A number of new relational databases continuing use of SQL but aiming for performance comparable to NoSQL are known as [[NewSQL]].
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