Api Rest With Cowboy

Jan 9 2019

At Elixir world the well know framework Phoenix is the main tool to develop the client side interface in any project. This framework allows us to develop complex real-time web systems simply with a lot of integrated features such as websockets. But in the case of just pretending to build a simple api rest a good option is to use the Cowboy library, which makes quite more lightweight the final application than using Phoenix.

In this post I will expose how to build a simple Rest Api Service with plug_cowboy. This library makes even easier to use the Cowboy library, which is native from Erlang.

The code of this post is availabe at this github repository.

Setup project

Create New project with Elixir

mix new cowboy_example

Add cowboy dependency at mix.exs

  defp deps do
    [
      {:plug_cowboy, "~> 2.0"}
    ]
  end

Let's get and compile this dependency:

mix deps.get && mix deps.compile

Application entrypoint

Let's use the lib/cowboy_rest.ex file as application entrypoint for the supervision tree. We need to indicate this on our mix.exs file:

And set lib/cowboy_rest.exs as an application module so his content will be:

defmodule CowboyRest do
  use Application

  def start(_type, _args) do
    children = [
    # No childrent yet
    ]

    opts = [strategy: :one_for_one]
    Supervisor.start_link(children, opts)
  end
end

We will create the folder lib/cowboy_rest where submodules will be included. In our case the only submodule will be the web one at lib/cowboy_rest/web.

Cowboy files structure

At this point we can start working on Cowboy on the Web submodule. We will need:

lib/cowboy_rest/web/supervisor: The module defined in this file will be the Supervisor of the Web submodule. At start it will launch the http_listener that behaves as entrypoints for all the requests.

defmodule CowboyRest.Web.Supervisor do
  @moduledoc false
  use Supervisor

   def start_link(_arg, _opts) do
     Supervisor.start_link(__MODULE__, [], [name: __MODULE__])
   end

   def init(_arg) do
     children = [
       worker(CowboyRest.Web.HttpListener, [[], []])
     ]

     supervise(children, strategy: :one_for_one)
   end
 end

lib/cowboy_rest/http_listener: This file will keep the routing to the api Rest Handler functions. This will be also in charge of launching Cowboy Listener at start.

defmodule CowboyRest.Web.HttpListener do
  alias CowboyRest.Web.Handler
  alias Plug.Cowboy
  require Logger

  def start_link(_state, _opts) do
    Logger.info ("API Rest is working ...")
    options = [
      port: 4000,
      dispatch: [ { host(), routes() } ],
    ]
    Cowboy.http(__MODULE__, [], options)
  end

  def host(), do: :_

  def routes() do
    [
      {"/cowboy_rest/[...]", Cowboy.Handler, { Handler, [] }},
    ]
  end
end

lib/cowboy_rest/rest_handler: It is a Rest Handler in charge of handling and answering the incoming requests.

defmodule CowboyRest.Web.Handler do
  use Plug.Router
  require Logger

  plug :match
  plug :dispatch

  @content_type_header_key    "content-type"
  @html_header_value          "text/html"
  @entrypoint                 "/cowboy_rest/"

  get @entrypoint <> "/welcome" do
    Plug.Conn.fetch_query_params(conn) # populates conn.params
    |> put_resp_header(@content_type_header_key, @html_header_value)
    |> send_resp(200, "Welcome to CowboyRest Service")
  end

 end

But in order to launch this submodule we need to launch our new Web supervisor from the application root, so the file ib/cowboy_rest.exs should look like this:

defmodule CowboyRest do
  use Application

  def start(_type, _args) do
    children = [
    # No childrent yet
    ]

    Supervisor.start_link(children, [])
  end
end

Running it

Now if we run the application with iex -S mix, we can try a http request trough the command line with curl:

> curl localhost:4000/cowboy_rest/welcome
Welcome to Cowboy Rest Services

We can see how it is really easy to have a lightweigth API Rest application on elixir using Cowboy.

Api Rest With Cowboy

Table Of Contents

Setup project

Application entrypoint

Cowboy files structure

Running it