An example pub/sub server in Elixir.

pub_sub.exs

defmodule PubSubServer do
  def start(subscriber_callback \\ nil) do
    spawn(__MODULE__, :run, [[ ], subscriber_callback])
  end

  def subscribe(server, handler) do
    send(server, {:subscribe, self})
    listen(handler)
  end

  def publish(server, message) do
    send(server, {:publish, message})
  end

  def run(subscribers, subscriber_callback) do
    receive do
      {:publish, message} ->
        Enum.each(subscribers, &(send(&1, message)))
        run(subscribers, subscriber_callback)
      {:subscribe, subscriber} ->
        if subscriber_callback, do: subscriber_callback.(subscriber)
        run([subscriber | subscribers], subscriber_callback)
    end
  end

  def listen(handler) do
    receive do
      message ->
        handler.(message)
        listen(handler)
    end
  end
end

main = self
server = PubSubServer.start(fn subscriber ->
  send(main, {:subscriber, subscriber})
end)
listener_count = 10

Stream.repeatedly(fn ->
  spawn(fn ->
    PubSubServer.subscribe(server, fn message ->
      IO.puts "#{inspect self} received:  #{message}"
      send(main, {:written, message})
    end)
  end)
end) |> Enum.take(listener_count)

Stream.repeatedly(fn ->
  receive do
    {:subscriber, _} -> true
  end
end) |> Enum.take(listener_count)

PubSubServer.publish(server, "Hello everyone!")

Stream.repeatedly(fn ->
  receive do
    {:written, _} -> true
  end
end) |> Enum.take(listener_count)