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In computing, a DBM is a library and file format providing fast, single-keyed access to data. A key-value database from the original Unix, dbm is an early example of a NoSQL system. [1] [2] [3]
The original dbm library and file format 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 dbm library stores arbitrary data by use of a single key (a primary key) in fixed-size buckets and uses hashing techniques to enable fast retrieval of the data by key.
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The original AT&T dbm library has been replaced by its many successor implementations. Notable examples include: [3]
The following databases are dbm-inspired, but they do not directly provide a dbm interface, even though it would be trivial to wrap one:
As of 2001, the ndbm implementation of DBM was standard on Solaris and IRIX, whereas gdbm is ubiquitous on Linux. The Berkeley DB implementations were standard on some free operating systems. [2] [10] After a change of licensing of the Berkeley DB to GNU AGPL in 2013, projects like Debian have moved to LMDB. [11]
A 2018 AFL fuzzing test against many DBM-family databases exposed many problems in implementations when it comes to corrupt or invalid database files. Only freecdb by Daniel J. Bernstein showed no crashes. The authors of gdbm, tdb, and lmdb were prompt to respond. Berkeley DB fell behind due to the sheer amount of other issues; [10] the fixes would be irrelevant to open-source software users due to the licensing change locking them back on an old version. [11]
Berkeley DB (BDB) is an embedded database software library for key/value data, historically significant in open-source software. Berkeley DB is written in C with API bindings for many other programming languages. BDB stores arbitrary key/data pairs as byte arrays and supports multiple data items for a single key. Berkeley DB is not a relational database, although it has database features including database transactions, multiversion concurrency control and write-ahead logging. BDB runs on a wide variety of operating systems, including most Unix-like and Windows systems, and real-time operating systems.
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Free and Open source Software Developers' European Meeting (FOSDEM) is a non-commercial, volunteer-organized European event centered on free and open-source software development. It is aimed at developers and anyone interested in the free and open-source software movement. It aims to enable developers to meet and to promote the awareness and use of free and open-source software.
Windows Services for UNIX (SFU) is a discontinued software package produced by Microsoft which provided a Unix environment on Windows NT and some of its immediate successor operating-systems.
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mod_auth_dbm and mod_rewrite. Both GDBM and Berkeley DB provide NDBM emulations, and Linux distributions ship with one or other of these emulations in place of the 'real' NDBM, which is excluded for licensing reasons. Unfortunately, the various file formats are totally incompatible, and there are subtle differences in behaviour concerning database locking. These issues led a steady stream of Linux users to report problems with DBMs in Apache 1.x.".Pag and contains the bitmap. The second, which has the extension .Dir, contains the data..Pag and .Dir..Db. Unlike DBM and NDBM, both of which use two files, GDBM only uses one file.BTREE) and store information with a record line number (RECNO). The method that you use depends completely on how you want to store and retrieve the information from a database. Berkeley db creates only one file, which has no extension."Note that Ruby used to ship SDBM in the standard distribution up until version 2.7, after which it was made available only as an external library, similarly to the DBM and GDBM libraries, removed from the standard library in Ruby 3.1.