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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The original AT&T dbm library has been replaced by its many successor implementations.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.
MySQL is an open-source relational database management system (RDBMS). Its name is a combination of "My", the name of co-founder Michael Widenius's daughter My, and "SQL", the acronym for Structured Query Language. A relational database organizes data into one or more data tables in which data may be related to each other; these relations help structure the data. SQL is a language that programmers use to create, modify and extract data from the relational database, as well as control user access to the database. In addition to relational databases and SQL, an RDBMS like MySQL works with an operating system to implement a relational database in a computer's storage system, manages users, allows for network access and facilitates testing database integrity and creation of backups.
Ingres Database is a proprietary SQL relational database management system intended to support large commercial and government applications.
Db2 is a family of data management products, including database servers, developed by IBM. It initially supported the relational model, but was extended to support object–relational features and non-relational structures like JSON and XML. The product name was originally styled as DB2 until 2017, when it changed to its present form.
In computing, Open Database Connectivity (ODBC) is a standard application programming interface (API) for accessing database management systems (DBMS). The designers of ODBC aimed to make it independent of database systems and operating systems. An application written using ODBC can be ported to other platforms, both on the client and server side, with few changes to the data access code.
In computer programming, glob patterns specify sets of filenames with wildcard characters. For example, the Unix Bash shell command mv *.txttextfiles/ moves all files with names ending in .txt from the current directory to the directory textfiles. Here, * is a wildcard and *.txt is a glob pattern. The wildcard * stands for "any string of any length including empty, but excluding the path separator characters ".
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.
The following tables compare general and technical information for a number of relational database management systems. Please see the individual products' articles for further information. Unless otherwise specified in footnotes, comparisons are based on the stable versions without any add-ons, extensions or external programs.
LAMP is an acronym denoting one of the most common software stacks for the web's most popular applications. Its generic software stack model has largely interchangeable components.
cdb, short for "constant database", refers to both a library and data format created by Daniel J. Bernstein. cdb acts as an on-disk associative array, mapping keys to values, and allows multiple values to be stored for a single key. A constant database allows only two operations: creation and reading. Both operations are designed to be very fast and highly reliable. Since the database does not change while it is in use, multiple processes can access a single database without locking. Additionally, since all modifications create a replacement database, it can take advantage of UNIX filesystem semantics to provide a guarantee of reliability.
Sphinx is a fulltext search engine that provides text search functionality to client applications.
An embedded database system is a database management system (DBMS) which is tightly integrated with an application software; it is embedded in the application. It is a broad technology category that includes:
An indexed file is a computer file with an index that allows easy random access to any record given its file key.
Strozzi NoSQL is a shell-based relational database management system initialized and developed by Carlo Strozzi that runs under Unix-like operating systems, or others with compatibility layers. Its file name NoSQL merely reflects the fact that it does not express its queries using Structured Query Language; the NoSQL RDBMS is distinct from the circa-2009 general concept of NoSQL databases, which are typically non-relational, unlike the NoSQL RDBMS. Strozzi NoSQL is released under the GNU GPL.
The following outline is provided as an overview of and topical guide to the Perl programming language:
A key–value database, or key–value store, is a data storage paradigm designed for storing, retrieving, and managing associative arrays, and a data structure more commonly known today as a dictionary or hash table. Dictionaries contain a collection of objects, or records, which in turn have many different fields within them, each containing data. These records are stored and retrieved using a key that uniquely identifies the record, and is used to find the data within the database.
crypt is a POSIX C library function. It is typically used to compute the hash of user account passwords. The function outputs a text string which also encodes the salt, and identifies the hash algorithm used. This output string forms a password record, which is usually stored in a text file.
Autopsy is computer software that makes it simpler to deploy many of the open source programs and plugins used in The Sleuth Kit. The graphical user interface displays the results from the forensic search of the underlying volume, making it easier for investigators to flag pertinent sections of data. The tool is largely maintained by Basis Technology Corp. with the assistance of programmers from the community. The company sells support services and training for using the product.
Lightning Memory-Mapped Database (LMDB) is an embedded transactional database in the form of a key-value store. LMDB is written in C with API bindings for several programming languages. LMDB stores arbitrary key/data pairs as byte arrays, has a range-based search capability, supports multiple data items for a single key and has a special mode for appending records (MDB_APPEND) without checking for consistency. LMDB is not a relational database, it is strictly a key-value store like Berkeley DB and DBM.
The following outline is provided as an overview of and topical guide to MySQL:
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.