pub_sub.ex

defmodule PubSub do
  @name __MODULE__
  use GenServer

  # API

  def start_link(default \\ %{}) do
   GenServer.start_link(__MODULE__, default, name: @name)
  end

  def subscribe(pid, topic) do
    GenServer.call(@name, {:subscribe, pid, topic})
  end

  def publish(topic, event) do
    GenServer.cast(@name, {:publish, topic, event})
  end

  # Callbacks

  def handle_call({:subscribe, pid, topic}, _from, state) do
    listeners = Map.get(state, topic, [])

    if Enum.member?(listeners, pid) do
      {:reply, :noop, state}
    else
      new_state = Map.update(state, topic, [pid], &([ pid | &1]))
      {:reply, :ok, new_state}
    end
  end

  def handle_cast({:publish, topic, event}, state) do
    Map.get(state, topic) |> Enum.each(&(send &1, event))
    {:noreply, state}
  end
end

defmodule LoopListener do
  def new do
    spawn &loop/0
  end

  defp loop do
    receive do
      event -> IO.puts "#{inspect self}: #{event}"
    end

    loop
  end
end

process_1 = LoopListener.new
process_2 = LoopListener.new

PubSub.start_link

PubSub.subscribe(process_1, :general)
PubSub.subscribe(process_2, :general)
PubSub.subscribe(process_2, :software)

PubSub.publish(:general, "Hello everyone!")
PubSub.publish(:software, "Node is Web Scale.")

#=> Hello everyone!
#=> Hello everyone!
#=> Node is Web Scale.