Support distributed dispatch consistency

[slashdotdash]

Allow strongly consistent command dispatch to work when Commanded is run on multiple nodes, but not formed as a cluster.

Phoenix's pub/sub library is optionally used since it supports broadcasting messages and tracking presence using PG2 or Redis. The Redis adapter can be used when running multiple nodes that are not connected together.

Usage

The new Commanded.PubSub behaviour defines basic pub/sub and process tracking functions. The default implementation uses Elixir's local-only Registry module.

# `config/config.exs`
config :commanded, pubsub: :local

To use Phoenix pub/sub you must add it to your project's depedencies in mix.exs:

defp deps do
  [
    {:phoenix_pubsub, "~> 1.0"}
  ]
end

Then configure the Phoenix pub/sub adapter you'd like to use:

# `config/config.exs`
config :commanded, pubsub: [
  phoenix_pubsub: [
    adapter: Phoenix.PubSub.PG2,
    pool_size: 1
  ]
]

The available options are:

• Phoenix.PubSub.PG2 - uses Distributed Elixir, directly exchanging notifications between servers
• Phoenix.PubSub.Redis - uses Redis to exchange data between servers