Hello,

I’m writing my first Elixir program and I’ve managed to to the hardest part of it, but I’m stuck at the ultimate stage and I need some external help now.

The program is an HTTP server that receives requests from the user, analyze the parameters and launches an external executable. This executable spits binary data on it’s standard output and I need to pass this data to the client as a stream.

I’ve managed to write this part like this, but I can’t get any data at the client side. Can you help me understand what’s wrong and how I can fix it ?

# This function opens a Port, start a chunked HTTP response for the client and transmits the data
defp make_response({:ok, datareq}, conn) do
      dataselect = System.find_executable("dataselect")
      port = Port.open({:spawn_executable, dataselect},
        [:stderr_to_stdout, :binary, :exit_status, args: Enum.concat(["-s", selections_file, "-o", "-"], fileslist)])
      Logger.notice("Sending response from dataselect #{inspect(port)}")
      conn = conn
      |> Plug.Conn.put_resp_header("content-disposition", "attachment; filename=fdsnws-dataselect.mseed")
      |> Plug.Conn.put_resp_header("content-type", "application/vnd.fdsn.mseed")
      |> Plug.Conn.send_chunked(200)  # Initiate chunked response
      |> stream_dataselect_output(port)
end

# This function listen to the data from the port and sends it through Plug
  defp stream_dataselect_output(conn, port) do
    receive do
      {:data, data} ->
        Logger.debug("Receiving data chunk for #{inspect(data)}")
        conn
        |> Plug.Conn.chunk(data)
        |> stream_dataselect_output(port)
      :eof ->
        conn
      {:exit_status, status } ->
        Logger.info("Exit status: #{status}")
        conn
    after 3000 ->
        Logger.info("Timeout waiting for dataselect")
        conn
    end
  end

I always hit the timeout and get the error message here:

08:52:05.774 [notice] Sending response from dataselect #Port<0.18>

08:52:05.777 [info] Chunked 200 in 308ms

08:52:08.778 [info] Timeout waiting for dataselect

08:52:08.822 [error] GenServer #PID<0.677.0> terminating
** (FunctionClauseError) no function clause matching in Bandit.HTTP1.Handler.handle_info/2
    (bandit 1.0.0-pre.10) lib/thousand_island/handler.ex:5: Bandit.HTTP1.Handler.handle_info({#Port<0.18>, {:data, <<48, 48, 48, 48, 48, 53, 77, 32, 67, 73, 69, 76, 32, 48, 49, 72, 78, 90, 70, 82, 7, 231, 0, 1, 0, 0, 0, 0, 0, 0, 1, 104, 0, 200, 0, 1, 2, 32, 0, 2, 0, 0, 0, 0, 0, 64, ...>>}}, {%ThousandIsland.Socket{socket: #Port<0.17>, transport_module: ThousandIsland.Transports.TCP, read_timeout: 60000, span: %ThousandIsland.Telemetry{span_name: :connection, telemetry_span_context: #Reference<0.2562483382.3605266436.113476>, start_time: -576460748085329422, start_metadata: %{parent_telemetry_span_context: #Reference<0.2562483382.3605266433.114612>, remote_address: {127, 0, 0, 1}, remote_port: 34208, telemetry_span_context: #Reference<0.2562483382.3605266436.113476>}}}, %{handler_module: Bandit.HTTP1.Handler, http_1_enabled: true, http_2_enabled: true, opts: %{http_1: [], http_2: [], websocket: []}, plug: {Wsdataselect.Router, []}, requests_processed: 1, websocket_enabled: true}})
    (stdlib 4.3) gen_server.erl:1123: :gen_server.try_dispatch/4
    (stdlib 4.3) gen_server.erl:1200: :gen_server.handle_msg/6
    (stdlib 4.3) proc_lib.erl:240: :proc_lib.init_p_do_apply/3
Last message: {#Port<0.18>, {:data, <<48, 48, 48, 48, 48, 53, 77, 32, 67, 73, 69, 76, 32, 48, 49, 72, 78, 90, 70, 82, 7, 231, 0, 1, 0, 0, 0, 0, 0, 0, 1, 104, 0, 200, 0, 1, 2, 32, 0, 2, 0, 0, 0, 0, 0, 64, ...>>}}
State: {%ThousandIsland.Socket{socket: #Port<0.17>, transport_module: ThousandIsland.Transports.TCP, read_timeout: 60000, span: %ThousandIsland.Telemetry{span_name: :connection, telemetry_span_context: #Reference<0.2562483382.3605266436.113476>, start_time: -576460748085329422, start_metadata: %{parent_telemetry_span_context: #Reference<0.2562483382.3605266433.114612>, remote_address: {127, 0, 0, 1}, remote_port: 34208, telemetry_span_context: #Reference<0.2562483382.3605266436.113476>}}}, %{handler_module: Bandit.HTTP1.Handler, http_1_enabled: true, http_2_enabled: true, opts: %{http_1: [], http_2: [], websocket: []}, plug: {Wsdataselect.Router, []}, requests_processed: 1, websocket_enabled: true}}

Somebody get’s what’s wrong here ?

Thanks !