Smalltalk's MVC implementation was created with traditional desktop GUI systems in mind. The separation of responsibilities that it represents makes a lot of sense for web-based software; the model is the representation of the business and persistence layers, the controller is the server-side glue, and the view is the HTML rendered for the client browser.

However, in traditional MVC, the view observes changes in the model in order to reflect its current state by responding to events that the model issues. In a standard HTTP request/response situation, this isn't viable.

Model 2 is a derivative of MVC that was implemented in the Java Struts framework, which introduced a potential solution to this issue. Rather than the view and model directly communicating, the controller becomes a marshaling point for changes. It responds to changes in the view and passes them to the model and vice versa, as shown in the following diagram:

MVC/Model 2 on the Web

This is the way in which...