There are a huge number of relational and non-relational open source database products. In recent years, there have been a series of acquisitions which has led to a mixed-market of open source and commercial versions of the same product. Often the code of both versions is the same with the only real difference being in the technical and infrastructure support.

There is also a trend for "forking" of open-source databases into truly open source and semi- or wholly commercial databases. Nevertheless, the "copy-left" nature of many of the software's original open source licenses has meant that commercial companies have continued to release open source versions.

This chapter covers both relational database servers - such as those using SQL - and the non-relational servers - which are grouped under the title of "NoSQL".

Currently, the greatest amount of development work is concentrating on cloud-database application (covered in the Cloud Computing chapter). Technologies such as Hadoop, Erlang and SaaS are moving data away from local servers and distributing it on the net.

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