On February 26, 2017, I presented a session at the Pacific Northwest Drupal Summit entitled "Anatomy of a Drupal Page Request. Description:
Have you ever wondered how Drupal responds to a request for a URL within a site? This session will describe how the request process works in Drupal. Some background on how web servers work will be included, and both an overview and some of the gritty details of Drupal's part in serving the request will be covered.
The focus will be on Drupal 8, but Drupal 7's process will also be covered. Non-programmers should come away with a better understanding of how Drupal works "under the hood"; programmers should come away with a better understanding of how their modules and themes can interact with and influence the process.
Slides are attached below.