The best way to think of a Web application is as a series of interactions between a client and a particular Web site. The entire Web interaction process begins with a single page displayed in the browser. The user clicks on a button or link on the page causing a request to be sent to the Web application server. The request is processed on the Web application server and a new page is sent back showing the results of the request and presenting buttons or links for the next request. Thus the Web application consists of a set of processing steps or interactions, each of which gets a request generated from a page and each of which must produce a response in the form of a page that will serve as the input for subsequent interactions.

If we examine a single interaction at the next level of detail, we see a common set of processing requirements that can be nicely mapped to the classical model/view/controller paradigm as follows:

• Model - the elements of logic that actually record and process the user's input. For example, the logic that does a database update to add an item to a user's shopping cart or the logic that transfers funds from one bank account to another. We refer to this part of the interaction as the business logic.
• View - the elements of logic that construct the HTML page that is sent back to the user, thus determining the presentation form and style of the results of the interaction. We refer to this part of the interaction as the user interface (UI) logic.
• Controller - the elements of logic that control the other elements. For example, processing the HTTP request, selecting the correct model logic to run, and then, based on the results of the business logic, selecting the correct view logic to prepare the response page. We refer to this part of the interaction as the interaction controller.