Ecto.Mnesia

Ecto.Adapter for mnesia Erlang term database.

It supports compound mnesia indexes (aka secondary indexes) in database setup. The implementation relies directly on mnesia application. Supports partial Ecto.Query to MatchSpec conversion for mnesia:select (and, join). MatchSpec converion utilities could be found in Ecto.Mnesia.Query.

Configuration Sample

defmodule Sample.Model do
  require Record
    def keys, do: [id_seq:       [:thing],
                   topics:       [:whom,:who,:what],
                   config:       [:key]]

    def meta, do: [id_seq:       [:thing, :id],
                   config:       [:key, :value],
                   topics:       Model.Topics.__schema__(:fields)]
end

where Model.Topics is Ecto.Schema object.

Why Mnesia?

We have production task that needs low read-latency database and our data fits in RAM, so Mnesia is a best choice: it's part of OTP, shares same space as our app does, work fast in RAM and supports transactions (it's critical for fintech projects).

Why do we need adapter? We don't want to lock us to any specific database, since requirements can change. Ecto allows to switch databases by simply modifying config, and we might want to go back to Postres or another DB.

Clustering

We don't recommend to use distributed Mnesia, because it's neither AP, nor CP database. (And there are no such thing as AC DB.) Mnesia requires you to handle network partitions (split brains) manually.

TODO

• Manually prefix names of table (namespaces), because all tables have global scope for all nodes and they can collide.
• Support Ecto-style migrations to create tables. Get rid from Model in configuration.
• Support most of replication and distribution options from configs.
• Refactor Schema module for disambiguation from Mnesia schemas mnesia:create_schema(ListOfNodes).
• Set Mnesia data dir from conf
• Add mnesia:wait_for_tables(TableList, TimeOut) before start
• Support sync transaction type (that waits for all nodes to finish transaction, not only commit them)
• Support Mnesia indexes

Usage in config.exs

config :ecto, :mnesia_meta_schema, Sample.Model
config :ecto, :mnesia_backend,  :ram_copies

Installation

If available in Hex, the package can be installed as:

1. Add ecto_mnesia to your list of dependencies in mix.exs:

```elixir
def deps do
  [{:ecto_mnesia, "~> 0.2.0"}]
end
```

1. Ensure ecto_mnesia is started before your application:

```elixir
def application do
  [applications: [:ecto_mnesia]]
end
```

If published on HexDocs, the docs can be found at https://hexdocs.pm/ecto_mnesia

Thanks

We want to thank meh for his Amnesia package that helped a loot in initial Mnesia investigations. Some pieces of code was copied from hes repo.