ring.exs

defmodule Ring do
  def run(member_count, loop_count, message) do
    log(:ringleader, "building ring with #{ member_count - 1 } more nodes...")
    next_pid = spawn_link(__MODULE__, :build_ring, [member_count - 2, self])

    log(:ringleader, "sending initial message to #{ inspect(next_pid) }...")
    next_pid <- {:message, message, loop_count}

    message_loop(next_pid)
  end

  def message_loop(next_pid) do
    receive do
      {:message, message, 0} ->
        log(:ringleader, "got last message: #{ message }; terminating...")
        :ok
      {:message, message, m} ->
        log(:ringleader, "got message: #{ message }; forwarding to #{ inspect(next_pid) }; #{ m - 1 } loops to go...")
        next_pid <- {:message, message, m - 1}
        message_loop(next_pid)
    end
  end

  def relay(next_pid) do
    receive do
      {:message, message, m} ->
        log(:node, "got message: #{ message }; forwarding to #{ inspect(next_pid) }")
        next_pid <- {:message, message, m}
        if m > 0, do: relay(next_pid)
    end
    :ok
  end

  def build_ring(0, ringleader_pid) do
    log(:final, "ring is complete; connecting back to ringleader (#{ inspect(ringleader_pid) })")
    relay(ringleader_pid)
  end

  def build_ring(remaining, ringleader_pid) do
    log(:node, "extending ring; #{ remaining } more nodes to go...")
    next = spawn_link(__MODULE__, :build_ring, [remaining - 1, ringleader_pid])
    relay(next)
  end

  def log(role, message) do
    IO.puts "[#{ inspect(self) }][#{ role }] #{ message }"
  end
end

ring_output.log

iex(1)> Ring.run 5, 3, "hello"
[#PID<0.41.0>][ringleader] building ring with 4 more nodes...
[#PID<0.41.0>][ringleader] sending initial message to #PID<0.44.0>...
[#PID<0.44.0>][node] extending ring; 3 more nodes to go...
[#PID<0.44.0>][node] got message: hello; forwarding to #PID<0.45.0>
[#PID<0.45.0>][node] extending ring; 2 more nodes to go...
[#PID<0.45.0>][node] got message: hello; forwarding to #PID<0.46.0>
[#PID<0.46.0>][node] extending ring; 1 more nodes to go...
[#PID<0.46.0>][node] got message: hello; forwarding to #PID<0.47.0>
[#PID<0.47.0>][final] ring is complete; connecting back to ringleader (#PID<0.41.0>)
[#PID<0.47.0>][node] got message: hello; forwarding to #PID<0.41.0>
[#PID<0.41.0>][ringleader] got message: hello; forwarding to #PID<0.44.0>; 2 loops to go...
[#PID<0.44.0>][node] got message: hello; forwarding to #PID<0.45.0>
[#PID<0.45.0>][node] got message: hello; forwarding to #PID<0.46.0>
[#PID<0.46.0>][node] got message: hello; forwarding to #PID<0.47.0>
[#PID<0.47.0>][node] got message: hello; forwarding to #PID<0.41.0>
[#PID<0.41.0>][ringleader] got message: hello; forwarding to #PID<0.44.0>; 1 loops to go...
[#PID<0.44.0>][node] got message: hello; forwarding to #PID<0.45.0>
[#PID<0.45.0>][node] got message: hello; forwarding to #PID<0.46.0>
[#PID<0.46.0>][node] got message: hello; forwarding to #PID<0.47.0>
[#PID<0.47.0>][node] got message: hello; forwarding to #PID<0.41.0>
[#PID<0.41.0>][ringleader] got message: hello; forwarding to #PID<0.44.0>; 0 loops to go...
[#PID<0.44.0>][node] got message: hello; forwarding to #PID<0.45.0>
[#PID<0.45.0>][node] got message: hello; forwarding to #PID<0.46.0>
[#PID<0.46.0>][node] got message: hello; forwarding to #PID<0.47.0>
[#PID<0.47.0>][node] got message: hello; forwarding to #PID<0.41.0>
[#PID<0.41.0>][ringleader] got last message: hello; terminating...
:ok