Envío

Envío is basically a GenEvent², the modern idiomatic pub-sub implementation of event passing.

In a nutshell, Envío is a set of handy tools to simplify dealing with Elixir Registry. It includes the instance of Registry to be used out of the box, scaffolds for producing publishers and subscribers.

It is built using “convention over configuration” approach, preserving the whole low-level control over the registry entries.

Just add the application :envio into your list of extra applications and the default Envio.Registry will be started and managed automagically.

def application do
  [
    mod: {MyApplication, []},
    extra_applications: ~w|envio ...|a
  ]
end

Creating a publisher

To create a publisher just use Envio.Publisher in the module that should publish messages to the subscribers. Once use Envio.Publisher is used, broadcast/2 function becomes available. If the optional channel: argument is passed to use Envio.Publisher, this channel is considered the default one and broadcast/1 function appears to publish directly to the default channel.

defmodule MyPub do
  use Envio.Publisher, channel: :main

  def publish(channel, what), do: broadcast(channel, what)
  def publish(what), do: broadcast(what)
end

Creating a subscriber

▶ :dispatch

Simply register the handler anywhere in the code:

Envio.register(
  {MySub, :on_envio}, # the function of arity 2 must exist
  dispatch: %Envio.Channel{source: MyPub, name: :main}
)

As MyPub publishes to the :main channel, MySub.on_envio/2 will be called with a message passed as parameter.

▶ :pub_sub

Use Envio.Subscriber helper to scaffold the registry subscriber. Implement handle_envio/2 for custom message handling. The default implementation collects last 10 messages in it’s state. This amount might be adjusted by changing :envio, :subscriber_queue_size application environment setting.

The implementation below subscribes to :main channel provided by MyPub publisher and prints out each subsequent incoming message to standard output.

defmodule PubSucker do
  use Envio.Subscriber, channels: [{MyPub, :main}]

  def handle_envio(message, state) do
    {:noreply, state} = super(message, state)
    IO.inspect({message, state}, label: "Received")
    {:noreply, state}
  end
end