Error in using cowboy in Elixir

Question

I'm using the Erlang web framework :cowboy in Elixir, And I got an error in :cowboy_http_req.reply, here is my code:

mix.exs is:

defmodule Example.Mixfile do
  use Mix.Project

  def project do
    [ app: :example,
      version: "0.0.1",
      deps: deps ]
  end

  # Configuration for the OTP application
  def application do
    [
      # you should start :inets and :crypto first, or it will not start cowboy
      applications: [:inets, :crypto],
      mod: {Example, []}
    ]
  end

  # Returns the list of dependencies in the format:
  # { :foobar, "0.1", git: "https://github.com/elixir-lang/foobar.git" }
  defp deps do
    [
      { :cowboy, github: "extend/cowboy", tag: "0.9.0" },
      # { :lager, github: "basho/lager" }
    ]
  end
end

lib/example.ex is

defmodule Example do use Application.Behaviour

  def start(_type, _args) do
    deps_started

    dispatch = :cowboy_router.compile([
      {:_, [
          {"/objects/[...]", Example.Object, []}
      ]}
    ])

    :cowboy.start_http(:http, 100, [ip: {127,0,0,1}, port: 9080], [env: [dispatch: dispatch]])

    ExampleSup.start_link
  end

  defp deps_started do
    deps = [:ranch, :cowlib, :cowboy]
    Enum.all? deps, &ensure_started/1
  end

  defp ensure_started(app) do
    case :application.start(app) do
      :ok ->
        true
      {:error, {:already_started, _app}} ->
        true
      {:error, {:not_started, dep}} ->
        true = ensure_started(dep)
        ensure_started(app)
      error ->
        IO.puts "Couldn't start #{inspect app}: #{inspect error}"
        error
    end
  end
end

and my lib/example/object.ex is:

defmodule Example.Object do
  @behaviour :cowboy_http_handler

  def init(_type, req, _opts) do
    {:ok, req, :undefine}
  end

  def handle(req, state) do
    {ok, req} = :cowboy_http_req.reply(200, [], "hello world", req)
    {:ok, req, state}
  end

  def terminate(_reason, _request, _state), do: :ok
end

I use mix to manage the reps, and I start the cowboy server with idx -S mix, then I send a http stream use curl:

curl -i http://127.0.0.1:9080/objects/index.html 

and the server got error with error report:

=ERROR REPORT==== 28-Feb-2014::18:03:36 ===
Error in process <0.201.0> with exit value: {[{reason,undef},{mfa {'Elixir.Example.Object',handle,2}},{stacktrace,[{cowboy_http_req,reply,[200,[],<<11 bytes>>,{http_req,#Port<0.4438>,ranch_tcp,keepalive,<0.201.0>,<<3 bytes>>,'HTTP/1.1',{{127,0,0,1},58008},<<9 bytes>>,undefined,9080,<<14 bytes>>,[<<5 bytes>>],<<0 bytes>>,undefined,[],[{<<10 bytes>>,<<11 bytes>>},{<<4 bytes>>,<<14 bytes>>},{<<6 bytes>>,<<3 bytes>>}],[],undefined,[],waiting,undefined...

and the curl client got 500 Internal Server Error.

Could someone help me to finger out what's the matter? Thanks very much.

Answer 1

The error message is broken in three parts:

• reason: undef
• mfa: {'Elixir.Example.Object',handle,2}
• stacktrace: [{cowboy_http_req,reply,[200,[],<<11 bytes>>,...]}, ...]

In a poor way it is saying that you got undef error (which means undefined function) in the function handle/2 in the Example.Object module. And the error has the stacktrace of calling cowboy_http_req.reply/4. My hunch would be that cowboy_http_req does not exist and you should call cowboy_req instead.

I would also suggest for you to use plug, although we don't currently have a tool for nicely showing errors, it is coming soon™. We are also shipping a logger with Elixir soon, which should help with some messages.