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The next generation of post-relational databases in the 2000s became known as NoSQL databases, including fast key-value stores and document-oriented databases. XML databases are a type of structured document-oriented database that allows querying based on XML document attributes. NoSQL databases are often very fast, do not require fixed table schemas, avoid join operations by storing denormalized data, and are designed to scale horizontally. 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 system to simultaneously provide 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, memcached, Redis, CouchDB, Hazelcast, Apache Cassandra and HBase.[15] 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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