Simple authentication in Elixir with Plug

This past week I began building an API in Elixir. Most of the guides and posts I read were focused on the Phoenix framework, and it was a little difficult to find an example on how to perform authentication using Plug.

Here is what I used:

defmodule Api.Authentication do
  import Plug.Conn

  def init(opts), do: opts

  def authenticated?(conn) do
    # Implement authentication logic here, e.g. check auth headers,
    # session creds, etc.

  def call(conn, _opts) do
    if authenticated?(conn) do
      |> send_resp(401, "")
      |> halt

In the call method we can check to see if a request is authenticated. If the request passes our authentication test we send the request downstream to the next plug in our stack for further processing. If the request does not pass our authentication test we immediately terminate the request using halt and respond with an HTTP 401 status.

Once we have the plug we can add it to our router using the plug method.

alias Api.Authentication

defmodule Api do
  use Plug.Router
  plug Authentication # run authentication
  plug :match
  plug :dispatch

  match _ do
    |> put_resp_content_type("text/plain")
    |> send_resp(404, "")

And that is it!

$ curl -I localhost:8085/api
HTTP/1.1 401 Unauthorized
server: Cowboy
date: Fri, 22 Jul 2016 15:32:10 GMT
content-length: 0
cache-control: max-age=0, private, must-revalidate

One gotcha you might encounter is the ordering of plugs. Order does matter when we are creating a router. In this case, plug Authentication should come before the other plugs :match and :dispatch. If we don't order it correctly the server will proccess the authentication check after other route handlers are called.

For example, if we put our authentication plug at the end, like so, the router will try to send a response twice and our process will terminate.

defmodule Api do
  use Plug.Router
  plug :match
  plug :dispatch
  plug Authentication # authentication at the end

The server responds with a 404 and an uncaught exception appears in the server log.

08:15:07.493 [error] #PID<0.262.0> running Router terminated
Server: localhost:8085 (http)
Request: GET /api
** (exit) an exception was raised:
    ** (Plug.Conn.AlreadySentError) the response was already sent
        (plug) lib/plug/conn.ex:458: Plug.Conn.resp/3
        (plug) lib/plug/conn.ex:445: Plug.Conn.send_resp/3
        (office_mate) lib/office_mate/api_authentication.ex:19:
        (office_mate) lib/office_mate/api.ex:3: Api.plug_builder_call/2
        (plug) lib/plug/router/utils.ex:74: Plug.Router.Utils.forward/4
        (office_mate) lib/office_mate/router.ex:1: Router.plug_builder_call/2
        (plug) lib/plug/adapters/cowboy/handler.ex:15: Plug.Adapters.Cowboy.Handler.upgrade/4
        (cowboy) src/cowboy_protocol.erl:442: :cowboy_protocol.execute/4