Drupal uses routes, which define a path, that returns content. Each route has a method in a controller class that generates the content, in the form of a render array, to be delivered to the user. When a request comes to Drupal, the system tries to match the path to known routes. If the route is found, the route's definition is used to deliver the page. If the route cannot be found, the 404 page is displayed.

The HTTP kernel takes the request and loads the route. It will invoke the defined controller method or procedural function. The result of the invoked method or function is then handed to the presentation layer of Drupal to be rendered into the content that can be delivered to the user.

Drupal 8 builds on top of the Symfony HTTP kernel to provide the underlying functionality of its route system. It has added the ability to provide access requirements, cast placeholders into loaded objects, and provide partial page responses.