<?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 Wikström, Hans Nilsson and Håkan Mattsson</prepared>
<responsible>Bjarne Däcker</responsible>
<docno></docno>
<approved>Bjarne Däcker</approved>
<checked>Bjarne Däcker</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>
