The Model/View/Controller Methodology
One of the best-known and most common structures that produce maintainable programs is called Model/View/Controller (MVC). There are plenty of other good methodologies, but we’ll use MVC in this chapter because it’s particularly well-suited to database applications. It’s a very typical three-tier methodology.
MVC splits an application into the Model, the View, and the Controller. Each layer is an independent unit that performs a specific function. If you design the program correctly, you can—at least in theory—completely strip out one layer and replace it with something different without having to change the code in the other two layers.
- Model
The body of the application. Here, all objects that represent real-world “things” within the application are modeled. For example, a book, publisher, and author are represented in code on this layer. The Model is also responsible for making these objects persistent; that is, storing them so they can be retrieved after the program finishes. Therefore, all database interaction is performed by the Model.
- View
The eyes, ears, and hands of the application. The View is responsible for presenting information to the user and collecting any user feedback. In a traditional desktop application, the View draws the screens and reads input from the keyboard and mouse. In a web-based application, the View generates the HTML viewed by the user’s browser and passes down any form data submitted by the user. All I/O that involves the user of the system is done in the View. Any input by the user is passed to the Controller for processing.
- Controller
The brains of the application. Any software logic performed by the application, such as the job discussed in Chapter 6 of authenticating users, is done within the Controller. The Controller is also the communication center of the application. It processes all user input from the View and all data retrieved from the Model.
It’s fairly easy to fit CGI into the MVC methodology. The database maintains the Model, the CGI program (and the web server with which it interacts) is the Controller, and the browser acts as the View. The complete CGI program we’ll show in the following section is also based on the MVC methodology in another way.