number_service.exs

defmodule NumberService do
  import Plug.Conn

  @doc "server loop"
  def server(i) do
    receive do
      {:next, caller} ->
        send caller, {:next_num, i}
        server(i+1)
    end
  end

  @doc "call server for the next number"
  def client_call(pid) do
    send pid, {:next, self()}
    receive do
      {:next_num, i} -> i
    end
  end

  def init(options) do
    # start server process
    Map.put(options, :server_pid, spawn_link(__MODULE__, :server, [1]))
  end

  def call(conn, opts) do
    num = client_call(opts.server_pid)
    conn
    |> put_resp_content_type("text/plain")
    |> send_resp(200, "#{num}")
  end
end

{:ok, _} = Plug.Adapters.Cowboy.http NumberService, %{}
receive do
  :term -> :ok
end