Front Controller

Context

You have decided to use the Model-View-Controller(MVC) pattern to separate the user interface logic from the business logic of your dynamic Web application. You have reviewed the Page Controller pattern, but your page controller classes have complicated logic, are part of a deep inheritance hierarchy, or your application determines the navigation between pages dynamically based on configurable rules.

Problem

How do you best structure the controller for very complex Web applications so that you can achieve reuse and flexibility while avoiding code duplication?

Forces

The following are specific aspects of the forces from Model-View-Controller that apply to the Front Controller pattern.

If common logic is replicated in different views in the system, you need to centralize this logic to reduce the amount of code duplication. Removing the duplicated code is critical to improving the overall maintainability of the system.

The retrieval of data is also best handled in one location. A series of views that use the same data from the database is a good example. It is better to implement the retrieval of this data in one place as opposed to having each view retrieve the data and duplicate the database access code.

As described in MVC, testing user interface code tends to be time-consuming and tedious. Separating the individual roles enhances overall testability. This is true not only for the model code, which was described in MVC, but also applies to the controller code.


The following forces might persuade you to use Front Controller as opposed to Page Controller:

A common implementation of Page Controller involves creating a base class for behavior shared among individual pages. However, over time these base classes can grow with code that is not common to all pages. It requires discipline to periodically refactor this base class to ensure that only common behavior is included. For example, you do not want a page to examine a request and decide (based on request parameters) to transfer control to a different page, because this type of decision is more specific to a particular function, rather than common among all the pages.

To avoid adding excessive conditional logic in the base class, you could create a deeper inheritance hierarchy to remove the conditional logic. For example, in an application that has three functional areas, it might be useful to have a single base class that has common functionality for the application. There might also be another class for each functional area, which inherits from the overall application base class. This type of structure, at first glance, is straightforward, but often leads to a very brittle design and implementation, and to a morass of code.

The Page Controller solution describes a single object per logical page. This solution breaks down when you need to control or coordinate processing across multiple pages. For example, suppose that you have complex configurable navigation, which is stored in XML, in your Web application. When a request comes in, the application must look up where to go next, based on its current state.

Because Page Controller is implemented with a single object per logical page, it is difficult to consistently apply a particular action across all the pages in a Web application. Security, for example, is best implemented in a coordinated fashion. Having security handled by each view or page controller object is problematic because it can be inconsistently applied and can lead to security breaches. An additional solution to this problem is also discussed in Intercepting Filter.

The association of the URL to the particular controller object can be constraining for Web applications. For example, suppose your site has a wizard-like interface for gathering information. This wizard consists of a number of mandatory pages and a number of optional pages based on user input. When implemented with Page Controller, the optional pages would have to be implemented with conditional logic in the base class to select the next page.



http://msdn2.microsoft.com/en-us/library/ms978723.aspx