Plug.ErrorHandler: how to use to return JSON errors?

Questions / Help
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1

I’m looking into returning JSON errors from an API app. I know the specs dictate you must always return HTML on certain error codes but I want to circumvent that because I’ll have complete control both on the calling side and the service itself. Plus the service will send logs and telemetry and I want errors to be machine-parseable.

I tried a few things after searching the forum but I couldn’t manage to make any of them work.

Then I stumbled upon Plug.ErrorHandler — Plug v1.11.1. Call me stupid but I have no clue how to use it?

So as a start I want all 404s and 500s to be wrapped with a JSON key errors in an object, returned alongside a 404 and 500 status.

Is this possible using Plug.ErrorHandler? If so, how exactly?

2 Likes
2

Please bear in mind that all what I am about to suggest is from the top of my mind, therefore not tested, and may be wrong, but hey it doesn’t hurt for you to give it a try :slight_smile:

  def handle_errors(conn, %{kind: _kind, reason: _reason, stack: _stack}) do
    send_resp(conn, conn.status, "Something went wrong")
  end

If I am not mistake the above will override the call to the Error view at lib/yourapp_web/views/error_view.ex.

I think that this will return it in json:

# You may need to use the Jason library if returning only the map doesn't work.
# You also want to put the `application/json` header in the `conn`
send_resp(conn, conn.status,%{error: conn.status, message:  "Something went wrong"})

Maybe you can simplify the above to use in your router:

# file: lib/yourapp_web/router.ex
  pipeline :api do
    plug :accepts, ["json"],
    plug :handle_errors
  end

  # you may need to pattern match in `conn.status` to make this work
  def handle_errors(conn, %{kind: _kind, reason: _reason, stack: _stack}) do
    # You may need to use the Jason library if returning only the map doesn't work.
    # You also want to put the `application/json` header in the `conn`
    send_resp(conn, conn.status,%{error: conn.status, message:  "Something went wrong"})
  end

EDIT: On a second though I think this isn’t reflecting the intended use from the Plug.ErrorHandler

An Alternative that you can try is to change the Error view to always return a JSON object:

defmodule YourAppWeb.ErrorView do
  use YourAppWeb, :view

  require Logger

  # If you want to customize a particular status code
  # for a certain format, you may uncomment below.
  def render(template, assigns) do
    _json_error(template, assigns)
  end

  # By default, Phoenix returns the status message from
  # the template name. For example, "404.html" becomes
  # "Not Found".
  def template_not_found(template, assigns) do
    _json_error(template, assigns)
  end

  defp _json_error(template, assigns) do

    Logger.info("Error template: #{template}")
    # render("500.html", assigns)
    
    # I am not sure, but I think you can just return a Map, otherwise just try to use the Jason library.
    %{
      error: 400, # You need to extract it from assigns or template name
      maessage: "Whoops!"
    }

  end
  
end
3

I still don’t understand where to put this copy-pasted module code in my Phoenix project, and how do I register it to be used – in the router, the endpoint, somewhere else?

The docs kind of assume certain knowledge that I found I don’t have.

4

I update my previous post while you made this one to make it clear, but here it his again:

# file: lib/yourapp_web/router.ex
  pipeline :api do
    plug :accepts, ["json"],
    plug :handle_errors
  end

  # you may need to pattern match in `conn.status` to make this work
  def handle_errors(conn, %{kind: _kind, reason: _reason, stack: _stack}) do
    # You may need to use the Jason library if returning only the map doesn't work.
    # You also want to put the `application/json` header in the `conn`
    send_resp(conn, conn.status,%{error: conn.status, message:  "Something went wrong"})
  end

This is a guess from me, not actually tried it.

EDIT: On a second though I think this isn’t reflecting the intended use from the Plug.ErrorHandler

5

Nope, that didn’t work. :expressionless:

Hitting a non-existent endpoint with curl:

HTTP/1.1 404 Not Found
cache-control: max-age=0, private, must-revalidate
content-length: 1333
content-type: text/markdown; charset=utf-8
date: Mon, 12 Apr 2021 16:33:02 GMT
server: Cowboy

# Phoenix.Router.NoRouteError at POST /does_not_exist

Exception:

    ** (Phoenix.Router.NoRouteError) no route found for POST /does_not_exist (MyAppWeb.Router)
# <snip>

And it’s returning markdown text? Why?

I am still trying to understand how do I just intercept ALL errors and wrap them in keys in a resulting JSON object.

1 Like
6

Sorry for not having given you a good solution :frowning:

But did you try the Error view approach? I think that one should work :thinking:

Because you are in development mode. Go to config/dev.exs and set debug_errors to false.

1 Like
7

No idea what to do exactly there either. :003:

It’s been an absolutely awful day and I’d love complete code examples. At least today. If not, I’ll try formulating a solution on a fresh head tomorrow. :smiley:

8

I showed it to you in my first post:

This file can be found in your app web views folder.

Basically ti’s overriding the render/2 function called to render the error templates for each error status code that is defined in the templates folder.

It also customizes the template_not_found/2 to return the Json error.