PubSub server

pubsub.ex

defmodule Pubsub do
  use Application

  def start(_type, _options) do
    import Supervisor.Spec

    IO.puts("Initializing Pubsub app on #{inspect self}")

    children = [
      worker(Pubsub.Server, [], [name: :server_supervisor])
    ]

    {:ok, pid} = Supervisor.start_link(children, strategy: :one_for_one)
  end
end

server.ex

defmodule Pubsub.Server do
  use GenServer

  def start_link do
    GenServer.start_link(__MODULE__, [], name: {:global, __MODULE__})
  end

  def subscribe(pid) do
    GenServer.call({:global, __MODULE__}, {:subscribe, pid})
  end

  def publish(message) do
    GenServer.call({:global, __MODULE__}, {:publish, message})
  end

  def handle_call({:subscribe, pid}, _from, existing) do
    subscribers = [pid | existing]
    {:reply, :ok, subscribers}
  end

  def handle_call({:publish, message}, _from, subscribers) do
    Enum.each(subscribers, fn subs -> send(subs, message) end)
    {:reply, message, subscribers}
  end
end