We will follow the mostly classic Smalltalk MVC model, with a slight adaption to the web. There are many derivations in use today, which are often conflicting, so we’ll describe what this means for a web-based application.
Definition: "MVC consists of three kinds of objects. The Model is the application object, the View is its screen presentation, and the Controller defines the way the user interface reacts to user input. [Other] user interface designs tended to lump these objects together. MVC decouples them to increase flexibility and reuse." – from "Design patterns" book by the "Gang of Four"
Note that the term "Controller" is sometimes mis-used for code that drives actions that were not initiated by the end user, but initiated by the application or outside input. According to Design patterns book, "the Controller defines the way the user interface reacts to user input". Thus, to avoid confusion, we will instead use the term "Services" for components that initiate non-user actions. This is in line with Mozilla terminology.
Lower level tiers must never access higher level tiers. But rather, higher level tiers drive and call lower level tiers.
Specifically, the model must never ever access the view, but the view accesses the model (both read and write), and the view watches changes in the model by subscribing observers.