Erlang Programming

Chapter 13. Introducing Mnesia

Try to picture a cluster of Erlang nodes, distributed over half a dozen computers to which requests are forwarded. Data has to be accessible and up-to-date across the cluster and destructive database operations, even if they are rare, have to be executed in a transaction to avoid inconsistent data as a result of race conditions. You need to be able to add and remove nodes during runtime and provide persistence to ensure a speedy recovery from all possible failure scenarios.

The solution is to merge the efficiency and simplicity of ETS and Dets tables with the Erlang distribution and to add a transaction layer on top. This solution, called Mnesia, is a powerful database that comes as part of the standard Erlang distribution. Mnesia is the brainchild of Claes “Klacke” Wikström^[31] from the days when he was working at Ericsson’s Computer Science Lab. Håkan Mattsson eventually took over and brought Mnesia to the next level, productizing it and adding lots of functionality.

Mnesia can be as easy or as complex as you want it to be. The aim of this chapter is to introduce you to Mnesia and its capabilities without losing you in too many details.

When to Use Mnesia

Mnesia was originally built for integration in distributed, massively concurrent, soft real-time systems with high availability requirements (i.e., telecoms). You want to use Mnesia if your system requires the following:

• Fast key-value lookups of potentially complex data

• Distributed and replicated data ...