Simple elixir web server with plug

Everybody seems to use Phoenix for their web applications in Elixir. Sometimes,
however, you simply would like to take the easy route and write a small server
at a lower lever because you don’t need all the functionality provided by
Phoenix. It is a layer on top of erlang server Cowboy.
You can find the final app here at github: simple_plug.

Let’s start. First, create a new mix application with a supervisor:

mix new simple_plug --sup

Then add the plug and cowboy dependency into the mix.exs file:

...
defp deps do
    [
     {:cowboy, "~> 1.0.0"},
     {:plug, "~> 0.12"},
    ]
  end
...

Run mix do deps.get, compile to get and compile the dependencies.

We then need a starter for the plug server lib/plug_starter.ex:

defmodule SimplePlug.Plugstarter do

  def start_link() do
    {:ok, _} = Plug.Adapters.Cowboy.http SimplePlug.Router, []
  end

end

which is started through our supervisor at lib/simple_plug.ex:

defmodule SimplePlug do
  use Application

  # See http://elixir-lang.org/docs/stable/elixir/Application.html
  # for more information on OTP Applications
  def start(_type, _args) do
    import Supervisor.Spec, warn: false

    children = [
      # Define workers and child supervisors to be supervised
      # worker(SimplePlug.Worker, [arg1, arg2, arg3])
      worker(SimplePlug.Plugstarter, [])
    ]

    # See http://elixir-lang.org/docs/stable/elixir/Supervisor.html
    # for other strategies and supported options
    opts = [strategy: :one_for_one, name: SimplePlug.Supervisor]
    Supervisor.start_link(children, opts)
  end
end

The last thing we need it the plug-router, which responds to the http request.
In our case we simply show back the entered parameters. If you enter
http://localhost:4000/?aaa=1234 it would return received %{"aaa" => "1234"}.
If you use any other route, it returns a 404 page.
Paste the following code into lib/router.ex:

defmodule SimplePlug.Router do
  use Plug.Router
  require Logger

  # plug Plug.Logger
  plug :match
  plug :dispatch

  def init(options) do
    # initialize your options here

    options
  end


  get "/" do
    # Get the parameters
    conn = fetch_query_params(conn)

    send_resp(conn, 200, "received #{inspect(conn.params)}")
  end


  match _ do
    IO.inspect(conn.params)
    send_resp(conn, 404, "oops")
  end

end

Now you can start the server with iex -S mix and enjoy your lovely plug server at
http://localhost:4000/.