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<chapter>
  <header>
    <copyright>
      <year>1997</year><year>2016</year>
      <holder>Ericsson AB. All Rights Reserved.</holder>
    </copyright>
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      Licensed under the Apache License, Version 2.0 (the "License");
      you may not use this file except in compliance with the License.
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    <title>Mnesia</title>
    <prepared>Claes Wikstr&ouml;m, Hans Nilsson and H&aring;kan Mattsson</prepared>
    <responsible>Bjarne D&auml;cker</responsible>
    <docno></docno>
    <approved>Bjarne D&auml;cker</approved>
    <checked>Bjarne D&auml;cker</checked>
    <date></date>
    <rev>C</rev>
    <file>Mnesia_overview.xml</file>
  </header>

    <p>The management of data in telecommunications system has many
      aspects, thereof some, but not all, are addressed by traditional
      commercial Database Management Systems (DBMSs). In particular the
      high level of fault tolerance that is required in many nonstop
      systems, combined with requirements on the DBMS to run in the same
      address space as the application, have led us to implement a new
      DBMS, called Mnesia.</p>
    <p>Mnesia is implemented in, and tightly connected to Erlang.
      It provides the functionality that is necessary for the
      implementation of fault tolerant telecommunications systems.</p>
    <p>Mnesia is a multiuser distributed DBMS specially made for
      industrial telecommunications applications written in Erlang,
      which is also the intended target language.
      Mnesia tries to address all the data
      management issues required for typical telecommunications systems.
      It has a number of features that are not normally found in traditional
      databases.</p>
    <p>In telecommunications applications, there are different needs
      from the features provided by traditional DBMSs. The applications now
      implemented in Erlang need a mixture of a broad range
      of features, which generally are not satisfied by traditional DBMSs.
      Mnesia is designed with requirements like the following in
      mind:</p>
    <list type="ordered">
      <item>Fast real-time key/value lookup
      </item>
      <item>Complicated non-real-time queries mainly for
        operation and maintenance
      </item>
      <item>Distributed data because of distributed applications
      </item>
      <item>High fault tolerance
      </item>
      <item>Dynamic reconfiguration
      </item>
      <item>Complex objects
      </item>
    </list>
    <p>Mnesia is designed with the typical data management problems
      of telecommunications applications in mind. This sets Mnesia
      apart from most other DBMS. Hence Mnesia
      combines many concepts found in traditional databases such as
      transactions and queries with concepts found in data management
      systems for telecommunications applications, for example:</p>
      <list type="bulleted">
        <item>Fast real-time operations
        </item>
        <item>Configurable degree of fault tolerance (by replication)
        </item>
        <item>The ability to reconfigure the system without stopping or
          suspending it.
        </item>
      </list>
    <p>Mnesia is also interesting because of its tight coupling to
      Erlang, thus almost turning Erlang into a database programming
      language. This has many benefits, the foremost is that
      the impedance mismatch between the data format used by the DBMS
      and the data format used by the programming language, which is used
      to manipulate the data, completely disappears.</p>

  <section>
    <title>Mnesia Database Management System (DBMS)</title>
    <section>
      <title>Features</title>
      <p>Mnesia contains the following features that combine to
        produce a fault-tolerant, distributed DBMS written in Erlang:
        </p>
      <list type="bulleted">
        <item>Database schema can be dynamically reconfigured at runtime.
        </item>
        <item>Tables can be declared to have properties such as location,
          replication, and persistence.
        </item>
        <item>Tables can be moved or replicated to several nodes to improve
          fault tolerance. The rest of the system can still access the tables
          to read, write, and delete records.
        </item>
        <item>Table locations are transparent to the programmer.
          Programs address table names and the system itself keeps track of
          table locations.
        </item>
        <item>Database transactions can be distributed, and many
          functions can be called within one transaction.
        </item>
        <item>Several transactions can run concurrently, and their execution
          is fully synchronized by the DBMS. Mnesia ensures that no
          two processes manipulate data simultaneously.
        </item>
        <item>Transactions can be assigned the property of being executed on
          all nodes in the system, or on none. Transactions can also be
          bypassed in favor of running "dirty operations", which reduce
          overheads and run fast.
        </item>
      </list>
      <p>Details of these features are described in the following sections.</p>
    </section>

    <section>
      <title>Add-On Application</title>
      <p>Query List Comprehension (QLC) can be used with Mnesia
        to produce specialized functions that enhance the operational
        ability of Mnesia. QLC has its own documentation as part
        of the OTP documentation set. The main features of QLC
        when used with Mnesia are as follows:</p>
      <list type="bulleted">
        <item>QLC can optimize the query compiler for the Mnesia
          DBMS, essentially making the DBMS more efficient.
        </item>
        <item>QLC can be used as a database programming
          language for Mnesia. It includes a notation called "list
          comprehensions" and can be used to make complex database
          queries over a set of tables.
        </item>
      </list>
      <p>For information about QLC, see the
        <seeerl marker="stdlib:qlc">qlc</seeerl> manual page
        in STDLIB.</p>
    </section>

    <section>
      <title>When to Use Mnesia</title>
        <p>Use Mnesia with the following types of applications:</p>
        <list type="bulleted">
          <item>Applications that need to replicate data.
          </item>
          <item>Applications that perform complicated searches on data.
          </item>
          <item>Applications that need to use atomic transactions to
            update several records simultaneously.
          </item>
          <item>Applications that use soft real-time characteristics.
          </item>
        </list>
        <p>Mnesia is not as appropriate with the
          following types of applications:</p>
        <list type="bulleted">
          <item>Programs that process plain text or binary data files.
          </item>
          <item>Applications that merely need a look-up dictionary that
            can be stored to disc. Those applications  use the standard
            library module <c>dets</c>, which is a disc-based version
            of the module <c>ets</c>. For information about <c>dets</c>,
            see the <seeerl marker="stdlib:dets">dets</seeerl>
            manual page in STDLIB.
          </item>
          <item>Applications that need disc logging facilities.
            Those applications can
            use the module <c>disk_log</c> by preference. For
            information about <c>disk_log</c>, see the
            <seeerl marker="kernel:disk_log">disk_log</seeerl>
            manual page in Kernel.
          </item>
          <item>Hard real-time systems.
          </item>
        </list>
    </section>
  </section>
</chapter>
