Elixir Plug order matters!

I ran into this gotcha today as I was playing with Plug of Elixir.

The issue I was facing was that I defined a super simple plug that added a header to the underlying connection:

defmodule HelloPlug.Plug do
  import Plug.Conn
  def init(options) do
    options
  end

  def call(conn, options) do
    put_resp_header(conn, "x-plug", "PlugPlug")
  end
end

And it was plugged into the router definition like so:

  plug :match
  plug :dispatch
  plug HelloPlug.Plug # here it is!

In the router test I was getting errors that said that the response was already sent:

     test/hello_plug_test.exs:7
     ** (Plug.Conn.AlreadySentError) the response was already sent

Then it hit me: Plugs are executed in the order they are defined!. The problem was that the :dispatch function (from Plug.Router) was sending the request down the wire, and after that I was trying to set the header to the connection.

Simply plugging my Plug before :match and :dispatch did the trick:

  plug HelloPlug.Plug
  plug :match
  plug :dispatch