Cfrgtkky

Background

I have an app that is a webserver with cowboy and uses Plugs. Since this app was inherited, using Phoenix is out of the question unless we remake the entire thing, which is not happening.

My objective is to instead of having everything inside one huge file, to have several plugs and connect them via pipelines.

Code

Let’s assume I have a main router Plug, that looks like this:

defmodule MyApp.Web.Router do

  use Plug.Router



  plug(:match)



  forward "/check", to: MyApp.Route.Check

  forward "/dispatch", to: MyApp.Plug.Dispatch      

end

So here I have 2 things. A Route for the endpoint /check, which looks like this:

defmodule MyApp.Route.Check do

  use Plug.Router



  plug(:dispatch)



  get "/", do: send_resp(conn, 200, "ok")

end

And a Plug pipeline for /dispatch that looks like this:

defmodule MyApp.Plug.Dispatch do

  use Plug.Builder



  plug(Plug.Parsers, parsers: [:urlencoded])   #parses parameters

  plug(MyApp.Plug.Metrics)                     #exposes /metrics path

  plug(Cipher.ValidatePlug)                    #typical URL validation

  plug(MyApp.Route.Dispatch)                   #forwards to dispatch Route 

end

This pipeline parses the parameters, notifies a metrics service, validates the request and then sends it to the proper Router, which looks like this:

defmodule MyApp.Route.Dispatch do

  use Plug.Router



  plug(:dispatch)



  get "/", do: send_resp(conn, 200, "Info dispatched")

end

Problem

The problem here is that nothing works. Quite literally if I launch the application and try to access even the dummest endpoint ( /check ) the code blows with errors:

17:44:03.330 [error] #PID<0.402.0> running MyApp.Web.Router (connection #PID<0.401.0>, stream id 1) terminated

Server: localhost:4003 (http)

Request: GET /check

** (exit) an exception was raised:

    ** (Plug.Conn.NotSentError) a response was neither set nor sent from the connection

        (plug_cowboy) lib/plug/cowboy/handler.ex:37: Plug.Cowboy.Handler.maybe_send/2

        (plug_cowboy) lib/plug/cowboy/handler.ex:13: Plug.Cowboy.Handler.init/2

        (...)

I have now spent the entirety of my day reading documentation and this was as far as I got. The app is super simple, it is pretty much the hello world for plugs:

https://elixirschool.com/en/lessons/specifics/plug/

But with MyApp.Web.Router instead of the one they use.

A MWE can be seen here:

https://github.com/Fl4m3Ph03n1x/plug-pipeline-problem

What am I doing wrong?

Route was not matching on Example.Route.Dispatch and Example.Route.Check. To fix that you need to make two changes:

defmodule Example.Route.Dispatch do

  use Plug.Router



  plug(:dispatch)



  get "/*glob", do: send_resp(conn, 200, "Info dispatched")

end

and

defmodule Example.Route.Check do

  use Plug.Router



  plug :match

  plug :dispatch



  get "/*glob" do

    send_resp(conn, 200, "ok")

  end

end

Alternatively, you can do this: https://github.com/Fl4m3Ph03n1x/plug-pipeline-problem/pull/1

To understand more about why it is not matching you can use Plug.Router.match_path/1 on a "catch all" match.