mnesia_eleveldb

An eleveldb backend for Mnesia

This permits Erlang/OTP applications to use LevelDB as a backend for mnesia tables.

Prerequisites

• eleveldb (https://github.com/basho/eleveldb)
• Erlang/OTP 21.0 or newer (https://github.com/erlang/otp)

Getting started

Call mnesia_eleveldb:register() immediately after starting mnesia.

Put {leveldb_copies, [node()]} into the table definitions of tables you want to be in LevelDB.

Special features

LevelDB tables support efficient selects on prefix keys.

The backend uses the mnesia_eleveldb_sext module (see https://github.com/uwiger/sext) for mapping between Erlang terms and the binary data stored in the tables. This provides two useful properties:

• The records are stored in the Erlang term order of their keys.
• A prefix of a composite key is ordered just before any key for which it is a prefix. For example, {x, '_'} is a prefix for keys {x, a}, {x, b} and so on.

This means that a prefix key identifies the start of the sequence of entries whose keys match the prefix. The backend uses this to optimize selects on prefix keys.

Caveats

Avoid placing bag tables in LevelDB. Although they work, each write requires additional reads, causing substantial runtime overheads. There are better ways to represent and process bag data (see above about prefix keys).

The mnesia:table_info(T, size) call always returns zero for LevelDB tables. LevelDB itself does not track the number of elements in a table, and although it is possible to make the mnesia_eleveldb backend maintain a size counter, it incurs a high runtime overhead for writes and deletes since it forces them to first do a read to check the existence of the key. If you depend on having an up to date size count at all times, you need to maintain it yourself. If you only need the size occasionally, you may traverse the table to count the elements.