Tutorial: Mnesia

1 Introduction

1.1 Credit

This tutorial is adapted (massively copied) from the Mnesia Book on the Erlang docs site.

1.2 About Mnesia

Mnesia is a distributed Database Management System, appropriate for telecommunications applications and other Erlang applications which require continuous operation and soft real-time properties.

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

Due to its focus on telecommunications applications, Mnesia combines many concepts found in traditional databases, such as transactions and queries with concepts 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/LFE, thus almost turning Erlang/LFE into a database programming language.

1.3 Add-on Applications

The following add-ons can be used in conjunction with Mnesia to produce specialized functions which enhance the operational ability of 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/LFE).

1.4 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.