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elixirphoenix-frameworkplug

How does Plug.Exception work?

I’m having some trouble getting a simple example to work. I’m not using Phoenix FWIW, just plug

defmodule Unauthorized do
  defexception message: "not authorized", plug_status: 401
end

defmodule Foo do
  use Plug.Router
  plug :match
  plug :dispatch

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

  get "/boom" do
    raise Unauthorized
  end

  match _ do
    send_resp(conn, 404, "not found")
  end
end

This is kind of a silly example, but I’m just trying to see if it will work like I think it is supposed to work.

I was hoping that Plug would handle the exception raised in GET /boom and return a 401 status

However, when I do try to GET /boom it is returning a 500 status, so apparently the exception isn’t being handled by Plug

Solution

  • You need to add use Plug.ErrorHandler if you want to catch these exceptions and send an HTTP status code based on the exception that was raised:

    defmodule Foo do
  use Plug.Router
  use Plug.ErrorHandler

  ...
end

    With this change, I get the correct response:

    $ curl -i http://localhost:4000/boom
HTTP/1.1 401 Unauthorized
server: Cowboy
date: Wed, 17 May 2017 19:59:57 GMT
content-length: 20
cache-control: max-age=0, private, must-revalidate

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