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Mnesia
User's Guide
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# 1 Introduction

This book describes the Mnesia DataBase Management System (DBMS). Mnesia 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.

### 1.1 About Mnesia

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.
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:

- Fast real-time key/value lookup
- Complicated non real-time queries mainly for operation and maintenance
- Distributed data due to distributed applications
- High fault tolerance
- Dynamic re-configuration
- Complex objects

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.

### 1.2 The Mnesia DataBase Management System (DBMS)

#### Features

Mnesia contains the following features which combine to produce a fault-tolerant, distributed database management system written in Erlang:

- Database schema can be dynamically reconfigured at runtime.
- Tables can be declared to have properties such as location, replication, and persistence.
- 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.
- Table locations are transparent to the programmer. Programs address table names and the system itself keeps track of table locations.
- Database transactions can be distributed, and a large number of functions can be called within one transaction.
- 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.
- 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.

Details of these features are described in the following sections.

#### Add-on Applications

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:

- QLC has the ability to optimize the query compiler for the Mnesia Database Management System, essentially making the DBMS more efficient.
- 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.
- Mnesia Session is an interface for the Mnesia Database Management System
- Mnesia Session enables access to the Mnesia DBMS from foreign programming languages (i.e. other languages than Erlang).

#### When to Use Mnesia

Use Mnesia with the following types of applications:

- Applications that need to replicate data.
- Applications that perform complicated searches on data.
- Applications that need to use atomic transactions to update several records simultaneously.
- Applications that use soft real-time characteristics.

On the other hand, Mnesia may not be appropriate with the following types of applications:

- Programs that process plain text or binary data files
- Applications that merely need a look-up dictionary which can be stored to disc can utilize the standard library module dets, which is a disc based version of the module ets.
- Applications which need disc logging facilities can utilize the module disc_log by preference.
- Not suitable for hard real time systems.

#### Scope and Purpose

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.

#### Prerequisites

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.

#### About This Book

This book contains the following chapters:

- 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.
- 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.
- 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.
- 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.
- 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.
- Chapter 7, "Combining Mnesia with SNMP", is a short chapter which outlines Mnesia integrated with SNMP.
- Appendix A, "Mnesia Errors Messages", lists Mnesia error messages and their meanings.
- Appendix B, "The Backup Call Back Interface", is a program listing of the default implementation of this facility.
- Appendix C, "The Activity Access Call Back Interface", is a program outlining of one possible implementations of this facility.