At this stage, you should have the following script, which is a barebone, simple-yet-complete web application.

class App
  def call(env)

    # a hash containing response headers
    headers = {
      "Content-Type" => "text/html"
    }

    # an array containing the HTML response string
    response = ["<h1>Hello World</h1>"]

    # an array containing
    # 1. HTTP status code
    # 2. HTTP headers 
    # 3. HTTP response
    [200, headers, response]
  end
end

Right now, our application does only one thing. Whenever a request arrives, it returns the response Hello World to the browser. It sends the same response, regardless of the request URL.

➜ curl localhost:9292
<h1>Hello World</h1>

➜ curl localhost:9292/posts
<h1>Hello World</h1>

➜ curl localhost:9292/posts/new
<h1>Hello World</h1>

Having a web application that returns the same response for every request isn't very exciting... or useful! Let's make it smart by returning a different response based on the incoming HTTP request's path.

To keep things simple, I'll make the following assumptions:

• Our application only supports HTTP GET requests,

• It only needs to handle a specific set of routes shown below.

• For all other URLs, it returns no route found message.

Here're the URL patterns and corresponding actions our application will perform:

• / : shows the home page

• /articles : shows all articles

• /articles/1 : shows a single article

• /random : "no route found"

Let's get to it.