How to gracefully timeout a web request

Hi all,

I have an intermittent bug in production which I am trying to track down which causes requests to take a really long time:

Heroku times out all requests after 30 seconds, meaning the response never get’s sent for these slow requests and I never get alerted of the error.

Ideally I would force my own 30 second timeout to kill the request early. I’ve looked into cowboy’s idle_timeout option but it seems to have strange behaviour, not returning an error message to the user but rather a chrome error page and the client automatically attempts a refresh.

I am thinking of setting up a genserver that does a Process.send_after(__MODULE__, {:kill_if_open, request_pid}, 30_000)
and run Process.exit(pid, :kill) in the :kill_if_open handler.

Does anyone have any feedback on this approach? If it is bad practice, I could simply log/report that request and then investigate later.

I’ve now implemented it:

defmodule Myapp.RequestTimeout do
  use GenServer

  require Logger

  @timeout :timer.seconds(2)

  def start_link(_) do
    GenServer.start_link(__MODULE__, %{}, name: __MODULE__)
  end

  @impl true
  def init(_) do
    {:ok, nil}
  end

  @impl true
  def handle_cast({:register_request, log_meta, request_pid}, state) do
    Process.send_after(__MODULE__, {:kill_if_open, log_meta, request_pid}, @timeout)
    {:noreply, state}
  end

  @impl true
  def handle_info({:kill_if_open, log_meta, request_pid}, state) do
    Logger.metadata(log_meta)

    Logger.error("RequestTimeoutError: #{Logger.metadata()[:request_id]}")

    Process.exit(request_pid, :timeout)

    {:noreply, state}
  end

  #
  # API
  #

  def register_request(log_meta) do
    GenServer.cast(__MODULE__, {:register_request, log_meta, self()})
  end
end

defmodule Myapp.RequestTimeout.Plug do
  def init(opts) do
    opts
  end

  def call(conn, _) do
    IO.inspect(Logger.metadata()[:request_id], label: "request_id")

    Myapp.RequestTimeout.register_request(Logger.metadata())

    conn
  end
end

But there is no response sent to the user, the browser just hangs.

If anyone has any feedback or suggestions I’d be grateful to hear.

Thanks