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diff --git a/lib/mnesia/doc/src/Mnesia_overview.xml b/lib/mnesia/doc/src/Mnesia_overview.xml new file mode 100644 index 0000000000..d2d597b85d --- /dev/null +++ b/lib/mnesia/doc/src/Mnesia_overview.xml @@ -0,0 +1,193 @@ +<?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> |