08-process-ring.exs

# The goal here was to learn about Elixir's (Erlang's) lightweight processes.
# The following module gets spawned a few times connected in a ring where messages are sent around in order.

# http://elixir-lang.org/getting-started/basic-types.html
# http://elixir-lang.org/docs/stable/elixir/Process.html
# http://elixir-lang.org/getting-started/processes.html
# http://benjamintanweihao.github.io/blog/2013/06/25/elixir-for-the-lazy-impatient-and-busy-part-2-processes-101/
# http://benjamintanweihao.github.io/blog/2013/07/13/elixir-for-the-lazy-impatient-and-busy-part-4-processes-101/

defmodule Pinger do
  @doc "Send message (echo) to the next process in the ring."
  def ping(echo, limit) do
    receive do
      # got a message, send another message to the next process in the ring
      {[next | rest], msg, count} when count <= limit ->
        IO.puts "Received: #{inspect msg} (count #{count})"
        :timer.sleep(1000)
        send next, {rest ++ [next], echo, count+1}
        ping(echo, limit)

      # over our limit of messages, send :ok around the ring
      {[next | rest], _, _} ->
        send next, {rest, :ok}

      # someone told us to stop, so pass along the message
      {[next | rest], :ok} ->
        send next, {rest, :ok}
    end
  end
end

defmodule Spawner do
  def start do
    limit = 5
    {foo, _foo_monitor} = spawn_monitor(Pinger, :ping, ["ping", limit])
    {bar, _bar_monitor} = spawn_monitor(Pinger, :ping, ["pong", limit])
    {baz, _baz_monitor} = spawn_monitor(Pinger, :ping, ["pung", limit])
    send foo, {[bar, baz, foo], "start", 0}
    wait [foo, bar, baz]
  end

  @doc "Waits for all processes to finish before exiting."
  def wait(pids) do
    IO.puts "waiting for pids #{inspect pids}"
    receive do
      {:DOWN, _, _, pid, _} ->
        IO.puts "#{inspect pid} quit"
        pids = List.delete(pids, pid)
        unless Enum.empty?(pids) do
          wait(pids)
        end
    end
  end
end

Spawner.start