Moodle 3.x Developer's Guide

Moodle application layers

In terms of application architecture, Moodle can be thought of as having three layers--diagrammatically looking rather like a layer cake:

By shine Oa (originally posted to Flickr as wedding cake) [CC BY 2.0 (http://creativecommons.org/licenses/by/2.0)], via Wikimedia Commons

At the top is the presentation layer--this displays output to the user but also accepts data from users through HTML forms. Next is the business layer and, beneath that, the data layer. The data layer is relatively easy to understand: it is where user data is maintained, where quiz questions would be stored, where grades are recorded, and so on. ...