A database implemented in 20,000 lines of code that does the work of several major products, (distributed/replicated/nested transactions/flexible views based on logic variables) Mnesia is a part of the open source ErlangLanguage system.

It is free and proven in industrial (e.g. telecommunications) settings.  The framework containing Erlang and Mnesia is the Open Telecommunications Platform.

http://www.erlang.se/doc/doc-5.0.1/doc/index.html

Which is free and open source, available for download from the Erlang site:

http://www.erlang.org/download.html

The Mnesia user's manual is at:

http://www.erlang.se/doc/doc-5.0.1/lib/mnesia-3.9.2/doc/html/part_frame.html

Also there is a short (15 page)  overview of Mnesia that's worth a read.

http://www.erlang.se/publications/mnesia_overview.pdf

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This is a cracking little-big database system. I am very impressed with what I have seen so far. It is a distributed database with built-in replication support. I normally need three or four heavyweight products to do all this stuff. One very special thing it does is allow transactions to be wrapped up as a functor and passed to the database. Since transactions are completely specified they cannot hang; the system just retries till any lock problems are solved. A lovely thing that I wish I could use for my work. Erlang though. Tricky sell to a client :). --RichardHenderson.

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After a brief look, I am unable to figure out what kind of database this is (hierarchical, network, relational, etc.) It seems similar to LightweightDirectoryAccessProtocol.

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See also: NimbleDatabase