The dbm library was a simple database engine, originally written by Ken Thompson and released by AT&T in 1979. The name is a three letter acronym for DataBase Manager, and can also refer to the family of database engines with APIs and features derived from the original dbm.
The hashing scheme used is a form of extendible hashing, so that the hashing scheme expands as new buckets are added to the database, meaning that, when nearly empty, the database starts with one bucket, which is then split when it becomes full. The two resulting child buckets will themselves split when they become full, so the database grows as keys are added.
The dbm library and its derivatives are pre-relational databases – they manage associative arrays, implemented as on-disk hash tables. In practice, they can offer a more practical solution for high-speed storage accessed by key, as they do not require the overhead of connecting and preparing queries. This is balanced by the fact that they can generally only be opened for writing by a single process at a time. An agent daemon can handle requests from multiple processes, but introduces IPC overhead.
The dbm library has had many successors, such as:
- Ndbm: In 1986 Berkeley produced ndbm (standing for New Database Manager). This added support for having multiple databases open concurrently.
- Sdbm: Some versions of Unix excluded ndbm due to licensing issues, so in 1987 Ozan Yigit produced this public-domain clone.
- GDBM (GNU dbm): A Free/Libre version written by Philip A. Nelson for the GNU project. It added support for arbitrary-length data in the database: previously all data had a fixed maximum length. The last version was released on December 25, 2013.
- tdb (trivial database library): developed and used internally within the Samba suite, implements an API inspired by GDBM but also supports multiple writers, released under the LGPL license.
- tdbm: a version of ndbm with atomic transactions, in-memory databases, and other extensions, released under a BSD-style open source license.
- MDBM: Ndbm work-alike hashed database library based on sdbm which is based on Per-Aake Larson's Dynamic Hashing algorithms.