Create A Simple Web Server using Elixir

In this lesson, we will build a simple HTTP server from scratch using the PlugCowboy Elixir library. Cowboy is a simple HTTP server for Erlang and Plug will provide us with a connection adapter for the web server.

Getting started link

Assuming you already have Elixir installed, we will start a simple project by running the command:

  mix new server --sup
cd server
  

Note that server is the name of your project, and you can name it however you want. We also added --sup because our app needs a supervision tree because we will use a Supervision to start up and run our Cowboy server.

Adding the Cowboy dependancy link

Adding dependencies is way simpler than you thought. To use Plug as an adapter interface for Cowboy webserver, we need to install PlugCowboy package. Open your mix.exs file and add the following code

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

Now, your complete mix.exs file should look like this :

  defmodule Server.MixProject do
  use Mix.Project

  def project do
    [
      app: :server,
      version: "0.1.0",
      elixir: "~> 1.12",
      start_permanent: Mix.env() == :prod,
      deps: deps()
    ]
  end

  def application do
    [
      extra_applications: [:logger],
      mod: {Server.Application, []}
    ]
  end

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

Now you can simply install the dependencie by runnig the following command in your terminal:

  mix deps.get
  

Building our router link

To handle requests and send responses, we have to make a router. You can use the example below to build it. Simply create a router.ex file in your lib folder and have this code in it:

  defmodule Server.Router do
  use Plug.Router

  plug :match
  plug :dispatch

  get "/" do
    send_resp(conn, 200, "Hello World")
  end

  match _ do
    send_resp(conn, 404, "Not found")
  end
end
  

Here, we are making GET request to “/” and it will send back “Hello World”. If the URL you are visiting is not defined in the routes, it will send back “Not Found”. You can add as many routes as you want here. You can of course do more than just receiving GET requests. For our demonstation, we will focus only on this one.

Configuring our application module link

We need to tell our application to start up and supervise the Cowboy web server when the app starts up.

We’ll do so with the Plug.Cowboy function. This function expects three options:

• :scheme - HTTP or HTTPS as an atom (:http, :https)
• :plug - The plug module to be used as the interface for the web server. You can specify a module name, like MyPlug, or a tuple of the module name and options {MyPlug, plug_opts}, where plug_opts gets passed to your plug modules init/1 function.
• :options - The server options. Should include the port number on which you want your server listening for requests.

Open your application.ex in the lib/server/application.ex and replace the code with this:

  defmodule Server.Application do
  use Application
  require Logger

  @impl true
  def start(_type, _args) do
    children = [
      {Plug.Cowboy, scheme: :http, plug: Server.Router, options: [port: 4000]}
    ]

    opts = [strategy: :one_for_one, name: Server.Supervisor]
    Logger.info("starting the application...")
    Supervisor.start_link(children, opts)
  end
end
  

Running our server link

No we are finished setting up everything an we can finally run our server. We can use the following command:

  mix run --no-halt
  

Using your browser or postman, you can enter the request localhost:4000/ and you should see the Hello World message.

You have just learnt how to set up and configure a server for your elixir project. You can add more route definitions and try it out unti you get it. Remember, repitition is key.

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