my_router.ex

defmodule MyRouter do
  use Plug.Router

  # Starts the server
  def start do
    Plug.Adapters.Cowboy.http MyRouter, []
  end

  plug :my_handle # custom plug
  plug :match
  plug :dispatch

  def my_handle(conn, _opts) do
    Plug.Conn.register_before_send(conn, fn conn -> 
      # Doesn't show up in console.
      IO.puts "== cleaning up =="
      conn
    end)
    # This is printed to the console.
    IO.puts "== setting up =="
    conn
  end

  get "/" do
    send_resp(conn, 200, "world")
  end

  match _ do
    send_resp(conn, 404, "oops")
  end
end