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+<?xml version="1.0" encoding="utf-8" ?>
+<!DOCTYPE chapter SYSTEM "chapter.dtd">
+
+<chapter>
+ <header>
+ <copyright>
+ <year>1997</year><year>2016</year>
+ <holder>Ericsson AB. All Rights Reserved.</holder>
+ </copyright>
+ <legalnotice>
+ Licensed under the Apache License, Version 2.0 (the "License");
+ you may not use this file except in compliance with the License.
+ You may obtain a copy of the License at
+
+ http://www.apache.org/licenses/LICENSE-2.0
+
+ Unless required by applicable law or agreed to in writing, software
+ distributed under the License is distributed on an "AS IS" BASIS,
+ WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ See the License for the specific language governing permissions and
+ limitations under the License.
+
+ </legalnotice>
+
+ <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 <c>Mnesia</c>.</p>
+ <p><c>Mnesia</c> 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><c>Mnesia</c> is a multiuser distributed DBMS specially made for
+ industrial telecommunications applications written in Erlang,
+ which is also the intended target language.
+ <c>Mnesia</c> 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.
+ <c>Mnesia</c> 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><c>Mnesia</c> is designed with the typical data management problems
+ of telecommunications applications in mind. This sets <c>Mnesia</c>
+ apart from most other DBMS. Hence <c>Mnesia</c>
+ 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><c>Mnesia</c> 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><c>Mnesia</c> 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. <c>Mnesia</c> 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 <c>Mnesia</c>
+ to produce specialized functions that enhance the operational
+ ability of <c>Mnesia</c>. QLC has its own documentation as part
+ of the OTP documentation set. The main features of QLC
+ when used with <c>Mnesia</c> are as follows:</p>
+ <list type="bulleted">
+ <item>QLC can optimize the query compiler for the <c>Mnesia</c>
+ DBMS, essentially making the DBMS more efficient.
+ </item>
+ <item>QLC can be used as a database programming
+ language for <c>Mnesia</c>. 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
+ <seealso marker="stdlib:qlc">qlc</seealso> manual page
+ in <c>STDLIB</c>.</p>
+ </section>
+
+ <section>
+ <title>When to Use Mnesia</title>
+ <p>Use <c>Mnesia</c> 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><c>Mnesia</c> 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 <seealso marker="stdlib:dets">dets</seealso>
+ manual page in <c>STDLIB</c>.
+ </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
+ <seealso marker="kernel:disk_log">disk_log</seealso>
+ manual page in <c>Kernel</c>.
+ </item>
+ <item>Hard real-time systems.
+ </item>
+ </list>
+ </section>
+ </section>
+</chapter>