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<img alt="Erlang logo" src="../../../../doc/erlang-logo.png"><br><small><a href="users_guide.html">User's Guide</a><br><a href="index.html">Reference Manual</a><br><a href="release_notes.html">Release Notes</a><br><a href="../pdf/mnesia-4.7.1.pdf">PDF</a><br><a href="../../../../doc/index.html">Top</a></small><p><strong>Mnesia</strong><br><strong>User's Guide</strong><br><small>Version 4.7.1</small></p>
<br><a href="javascript:openAllFlips()">Expand All</a><br><a href="javascript:closeAllFlips()">Contract All</a><p><small><strong>Chapters</strong></small></p>
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<li id="loadscrollpos" title="Introduction" expanded="true">Introduction<ul>
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<li title="About Mnesia"><a href="Mnesia_chap1.html#id57488">About Mnesia</a></li>
<li title="The Mnesia DataBase Management System (DBMS)"><a href="Mnesia_chap1.html#id57481">The Mnesia DataBase Management System (DBMS)</a></li>
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<li id="no" title="Getting Started with Mnesia" expanded="false">Getting Started with Mnesia<ul>
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<li title="Starting Mnesia for the first time"><a href="Mnesia_chap2.html#id61530">Starting Mnesia for the first time</a></li>
<li title="An Introductory Example"><a href="Mnesia_chap2.html#id62101">An Introductory Example</a></li>
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<li id="no" title="Building A Mnesia Database" expanded="false">Building A Mnesia Database<ul>
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<li title="Defining a Schema"><a href="Mnesia_chap3.html#id67814">Defining a Schema</a></li>
<li title="The Data Model"><a href="Mnesia_chap3.html#id68070">The Data Model</a></li>
<li title="Starting Mnesia"><a href="Mnesia_chap3.html#id68125">Starting Mnesia</a></li>
<li title="Creating New Tables"><a href="Mnesia_chap3.html#id72288">Creating New Tables</a></li>
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<li id="no" title="Transactions and Other Access Contexts" expanded="false">Transactions and Other Access Contexts<ul>
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<li title="Transaction Properties"><a href="Mnesia_chap4.html#id72980">Transaction Properties</a></li>
<li title="Locking"><a href="Mnesia_chap4.html#id73193">Locking</a></li>
<li title="Dirty Operations"><a href="Mnesia_chap4.html#id73653">Dirty Operations</a></li>
<li title="Record Names versus Table Names"><a href="Mnesia_chap4.html#id74026">Record Names versus Table Names</a></li>
<li title="Activity Concept and Various Access Contexts"><a href="Mnesia_chap4.html#id74114">Activity Concept and Various Access Contexts</a></li>
<li title="Nested transactions"><a href="Mnesia_chap4.html#id74404">Nested transactions</a></li>
<li title="Pattern Matching"><a href="Mnesia_chap4.html#id74476">Pattern Matching</a></li>
<li title="Iteration"><a href="Mnesia_chap4.html#id74822">Iteration</a></li>
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<li id="no" title="Miscellaneous Mnesia Features" expanded="false">Miscellaneous Mnesia Features<ul>
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<li title="Indexing"><a href="Mnesia_chap5.html#id75164">Indexing</a></li>
<li title="Distribution and Fault Tolerance"><a href="Mnesia_chap5.html#id75284">Distribution and Fault Tolerance</a></li>
<li title="Table Fragmentation"><a href="Mnesia_chap5.html#id75432">Table Fragmentation</a></li>
<li title="Local Content Tables"><a href="Mnesia_chap5.html#id76375">Local Content Tables</a></li>
<li title="Disc-less Nodes"><a href="Mnesia_chap5.html#id76402">Disc-less Nodes</a></li>
<li title="More Schema Management"><a href="Mnesia_chap5.html#id76560">More Schema Management</a></li>
<li title="Mnesia Event Handling"><a href="Mnesia_chap5.html#id76675">Mnesia Event Handling</a></li>
<li title="Debugging Mnesia Applications"><a href="Mnesia_chap5.html#id77250">Debugging Mnesia Applications</a></li>
<li title="Concurrent Processes in Mnesia"><a href="Mnesia_chap5.html#id77395">Concurrent Processes in Mnesia</a></li>
<li title="Prototyping"><a href="Mnesia_chap5.html#id77432">Prototyping</a></li>
<li title="Object Based Programming with Mnesia"><a href="Mnesia_chap5.html#id77546">Object Based Programming with Mnesia</a></li>
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<li id="no" title="Mnesia System Information" expanded="false">Mnesia System Information<ul>
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<li title="Database Configuration Data"><a href="Mnesia_chap7.html#id77779">Database Configuration Data</a></li>
<li title="Core Dumps"><a href="Mnesia_chap7.html#id77817">Core Dumps</a></li>
<li title="Dumping Tables"><a href="Mnesia_chap7.html#id77838">Dumping Tables</a></li>
<li title="Checkpoints"><a href="Mnesia_chap7.html#id77873">Checkpoints</a></li>
<li title="Files"><a href="Mnesia_chap7.html#id78113">Files</a></li>
<li title="Loading of Tables at Start-up"><a href="Mnesia_chap7.html#id78474">Loading of Tables at Start-up</a></li>
<li title="Recovery from Communication Failure"><a href="Mnesia_chap7.html#id78632">Recovery from Communication Failure</a></li>
<li title="Recovery of Transactions"><a href="Mnesia_chap7.html#id78755">Recovery of Transactions</a></li>
<li title="Backup, Fallback, and Disaster Recovery"><a href="Mnesia_chap7.html#id78876">Backup, Fallback, and Disaster Recovery</a></li>
</ul>
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<li id="no" title="Combining Mnesia with SNMP" expanded="false">Combining Mnesia with SNMP<ul>
<li><a href="Mnesia_chap8.html">
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<li title="Combining Mnesia and SNMP "><a href="Mnesia_chap8.html#id79691">Combining Mnesia and SNMP </a></li>
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<li id="no" title="Appendix A: Mnesia Error Messages" expanded="false">Appendix A: Mnesia Error Messages<ul>
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<li title="Errors in Mnesia"><a href="Mnesia_App_A.html#id79834">Errors in Mnesia</a></li>
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<li id="no" title="Appendix B: The Backup Call Back Interface" expanded="false">Appendix B: The Backup Call Back Interface<ul>
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</a></li>
<li title="mnesia_backup callback behavior"><a href="Mnesia_App_B.html#id80051">mnesia_backup callback behavior</a></li>
</ul>
</li>
<li id="no" title="Appendix C: The Activity Access Call Back Interface" expanded="false">Appendix C: The Activity Access Call Back Interface<ul>
<li><a href="Mnesia_App_C.html">
Top of chapter
</a></li>
<li title="mnesia_access callback behavior"><a href="Mnesia_App_C.html#id80172">mnesia_access callback behavior</a></li>
</ul>
</li>
<li id="no" title="Appendix D: The Fragmented Table Hashing Call Back Interface" expanded="false">Appendix D: The Fragmented Table Hashing Call Back Interface<ul>
<li><a href="Mnesia_App_D.html">
Top of chapter
</a></li>
<li title="mnesia_frag_hash callback behavior"><a href="Mnesia_App_D.html#id80315">mnesia_frag_hash callback behavior</a></li>
</ul>
</li>
</ul>
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<h1>1 Introduction</h1>
<p>This book describes the Mnesia DataBase Management
System (DBMS). <strong>Mnesia</strong> is a distributed Database Management
System, appropriate for telecommunications applications and other
Erlang applications which require continuous operation and soft
real-time properties. It is one section of the Open Telecom Platform
(OTP), which is a control system platform for building
telecommunications applications.</p>
<h3><a name="id57488">1.1
About Mnesia</a></h3>
<p>The management of data in telecommunications system has many
aspects whereof some, but not all, are addressed by traditional
commercial DBMSs (Data Base Management Systems). In particular the
very high level of fault tolerance which 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 brand new
DBMS. called Mnesia. Mnesia is implemented in, and very tightly
connected to, the programming language Erlang and it provides the
functionality that is necessary for the implementation of fault
tolerant telecommunications systems. Mnesia is a multiuser Distributed
DBMS specially made for industrial telecommunications applications
written in the symbolic programming language Erlang, which is also
the intended target language. Mnesia tries to address all of the data
management issues required for typical telecommunications systems and
it has a number of features that are not normally found in traditional
databases. <br>
In telecommunications applications there are different needs
from the features provided by traditional DBMSs. The applications now
implemented in the Erlang language 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>
<ul>
<li>Fast real-time key/value lookup</li>
<li>Complicated non real-time queries mainly for
operation and maintenance</li>
<li>Distributed data due to distributed
applications</li>
<li>High fault tolerance</li>
<li>Dynamic re-configuration</li>
<li>Complex objects</li>
</ul>
<p>What
sets Mnesia apart from most other DBMSs is that it is designed with
the typical data management problems of telecommunications applications
in mind. Hence Mnesia combines many concepts found in traditional
databases, such as transactions and queries with concepts found in data
management systems for telecommunications applications, such as very
fast real-time operations, configurable degree of fault tolerance (by
means of replication) and the ability to reconfigure the system without
stopping or suspending it. Mnesia is also interesting due to its tight
coupling to the programming language Erlang, thus almost turning Erlang
into a database programming language. This has many benefits, the
foremost is that
the impedance mismatch between data format used by the
DBMS and data format used by the programming language, which is used
to manipulate the data, completely disappears. <br>
</p>
<h3><a name="id57481">1.2
The Mnesia DataBase Management System (DBMS)</a></h3>
<p></p>
<h4>Features</h4>
<p>Mnesia contains the following features which combine to produce a fault-tolerant,
distributed database management system written in Erlang:
</p>
<ul>
<li>Database schema can be dynamically reconfigured at runtime.
</li>
<li>Tables can be declared to have properties such as location,
replication, and persistence.
</li>
<li>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.
</li>
<li>Table locations are transparent to the programmer.
Programs address table names and the system itself keeps track of
table locations.
</li>
<li>Database transactions can be distributed, and a large number of
functions can be called within one transaction.
</li>
<li>Several transactions can run concurrently, and their execution is
fully synchronized by the database management system.
Mnesia ensures that no two processes manipulate data simultaneously.
</li>
<li>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 so called "dirty operations", which reduce
overheads and run very fast.
</li>
</ul>
<p>Details of these features are described in the following sections.</p>
<p></p>
<h4>Add-on Applications</h4>
<p>QLC and Mnesia Session can be used in conjunction with Mnesia to produce
specialized functions which enhance the operational ability of Mnesia.
Both Mnesia Session and QLC have their own documentation as part
of the OTP documentation set. Below are the main features of Mnesia Session
and QLC when used in conjunction with Mnesia:</p>
<ul>
<li>
<strong>QLC</strong> has the ability to optimize the query
compiler for the Mnesia Database Management System, essentially making
the DBMS more efficient.</li>
<li>
<strong>QLC</strong>, 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.</li>
<li>
<strong>Mnesia Session</strong> is an interface for the Mnesia Database
Management System</li>
<li>
<strong>Mnesia Session</strong> enables access to the
Mnesia DBMS from foreign programming languages (i.e. other
languages than Erlang).</li>
</ul>
<p></p>
<h4>When to Use Mnesia</h4>
<p>Use Mnesia with the following types of applications:
</p>
<ul>
<li>Applications that need to replicate data.
</li>
<li>Applications that perform complicated searches on data.
</li>
<li>Applications that need to use atomic transactions to
update several records simultaneously.
</li>
<li>Applications that use soft real-time characteristics.
</li>
</ul>
<p>On the other hand, Mnesia may not be appropriate with the
following types of applications:
</p>
<ul>
<li>Programs that process plain text or binary data files
</li>
<li>Applications that merely need a look-up dictionary
which can be stored to disc can utilize the standard
library module <span class="code">dets</span>, which is a disc based version
of the module <span class="code">ets</span>.
</li>
<li>Applications which need disc logging facilities can
utilize the module <span class="code">disc_log</span> by preference.
</li>
<li>Not suitable for hard real time systems.
</li>
</ul>
<h4>Scope and Purpose</h4>
<p>This manual is included in the OTP document set. It describes
how to build Mnesia database applications, and how to integrate
and utilize the Mnesia database management system with
OTP. Programming constructs are described, and numerous
programming examples are included to illustrate the use of
Mnesia.
</p>
<h4>Prerequisites</h4>
<p>Readers of this manual are assumed to be familiar with system
development principles and database management systems. Readers
are also assumed to be familiar with the Erlang programming
language.</p>
<h4>About This Book</h4>
<p>This book contains the following chapters:
</p>
<ul>
<li>Chapter 2, "Getting Started with Mnesia", introduces
Mnesia with an example database. Examples are shown of how to
start an Erlang session, specify a Mnesia database directory,
initialize a database schema, start Mnesia, and create
tables. Initial prototyping of record definitions is also
discussed.
</li>
<li>Chapter 3, "Building a Mnesia Database", more formally
describes the steps introduced in Chapter 2, namely the Mnesia
functions which define a database schema, start Mnesia, and
create the required tables.
</li>
<li>Chapter 4, "Transactions and other access contexts",
describes the transactions properties which make Mnesia into a
fault tolerant, real-time distributed database management
system. This chapter also describes the concept of locking in
order to ensure consistency in tables, and so called "dirty
operations", or short cuts which bypass the transaction system
to improve speed and reduce overheads.
</li>
<li>Chapter 5, "Miscellaneous Mnesia Features", describes
features which enable the construction of more complex
database applications. These features includes indexing,
checkpoints, distribution and fault tolerance, disc-less
nodes, replication manipulation, local content tables, concurrency,
and object based programming in Mnesia.
</li>
<li>Chapter 6, "Mnesia System Information", describes the
files contained in the Mnesia database directory, database
configuration data, core and table dumps, as well as the
important subject of backup, fall-back, and disaster recovery
principles.
</li>
<li>Chapter 7, "Combining Mnesia with SNMP", is a short
chapter which outlines Mnesia integrated with SNMP.
</li>
<li>Appendix A, "Mnesia Errors Messages", lists Mnesia error
messages and their meanings.
</li>
<li>Appendix B, "The Backup Call Back Interface", is a
program listing of the default implementation of this facility.
</li>
<li>Appendix C, "The Activity Access Call Back Interface",
is a program outlining of one possible implementations of this facility.
</li>
</ul>
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