gateway.ex

defmodule Gateway do
  use GenServer
  
  @timeout 5000 # timeout in ms

  def init(process), do: %{ process: process, q: :undefined }

  def handle_call(call, from, %{ process: process, q: q }) do
    # add call to priority queue
    newq = add_to_q(q, :call, call, from)
    # exit loop without sending reply to calling process, setting a timeout of 0
    { :noreply, %{ process: process, q: newq }, 0 }
  end

  def handle_cast(cast, %{ process: process, q: q }) do
    # add cast to priority queue
    newq = add_to_q(q, :call, call, :undefined)
    # exit loop without sending reply to calling process, setting a timeout of 0
    { :noreply, %{ process: process, q: newq }, 0 }
  end

  def handle_info(:timeout, %{ process: process, q: q }) do
    # no new messages waiting, process the most urgent message and return
    case next_from_q(q) do
      { :call, from, call, newq } ->
        reply = GenServer.call(process, call, @timeout)
        GenServer.reply(from, reply)
        { :noreply, %{ process: process, q: newq }, 0 }
      { :cast, cast, newq } ->
        :ok = GenServer.cast(process, cast, @timeout)
        { :noreply, %{ process: process, q: newq }, 0 }
    end
  end

  defp add_to_q(q, :call, call, from) do
    # you can inspect the call and insert it into whatever priority queue implementation you like here
  end
  
  defp next_from_q(q) do
    # check the queue and return the next message to handle along with it's type (:cast or :call) and the
    # waiting client if it's a :cast
  end
end