How to create a web server using Cowboy without Plug or Phoenix - Part 01

Cowboy is an amazing web server that is used by Plug/Phoenix out of the box, I don’t think Phoenix supports any other web servers at the moment. However, the plug adapter is fairly abstracted, and plug implements this adapter for cowboy through the plug_cowboy hex package. In theory, you should be able to write a new adapter if you just implement the Plug adapter behaviour. The plug cowboy adapter has a lot of interesting code and you’ll learn a lot from reading it. Anyway, this blog post isn’t about Plug or Phoenix. I wanted to show off how you can create a simple Cowboy server without using Plug or Phoenix (I had to learn how to do this while creating my side project webpipe)

We want an application which spins up a cowboy server and renders a hello world message. Here is the required code for that:

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defmodule Hello do
  # The handler module which handles all requests, its `init` function is called
  # by Cowboy for all matching requests.
  defmodule Handler do
    def init(req, _opts) do
      resp =
        :cowboy_req.reply(
          _status = 200,
          _headers = %{"content-type" => "text/html; charset=utf-8"},
          _body = "<!doctype html><h1>Hello, Cowboy!</h1>",
          _request = req
        )

      {:ok, resp, []}
    end
  end

  def start do
    # compile the routes
    routes =
      :cowboy_router.compile([
        {:_,
         [
           # { wildcard, handler module (needs to have an init function), options }
           {:_, Handler, []}
         ]}
      ])

    require Logger
    Logger.info("Staring server at http://localhost:4001/")

    # start an http server
    :cowboy.start_clear(
      :hello_http,
      [port: 4001],
      %{env: %{dispatch: routes}}
    )
  end
end

And, here is a quick test to assert that it works!

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defmodule HelloTest do
  use ExUnit.Case

  test "returns hello world" do
    assert {:ok, {{'HTTP/1.1', 200, 'OK'}, _headers, '<!doctype html><h1>Hello, Cowboy!</h1>'}} =
             :httpc.request('http://localhost:4001/')
  end
end

Full code on GitHub

I am currently working on LiveForm which makes setting up contact forms on your website a breeze.