WebSphere Application Server - IBM Redbooks

In the following sections, we discuss some of the properties of each of these
layers.
Model
The model layer manages the application domain’s concepts, both behavior and
state. It responds to requests for information about its state and responds to
instructions to change its state.
Just like any software component, the model should have a public interface that
is well defined and as simple as possible. This is usually achieved through the
use of a facade. The intent of facades is to provide a simple and unified interface
to the otherwise complex model that lies behind it. By doing so, we reduce the
dependencies between the model classes and its clients. Fewer dependencies
means more freedom to adapt to new requirements.
Note: Facade is a documented design pattern. For more information, refer to
Design Patterns: Elements of Reusable Object-Oriented Software.
As an example, consider an appliance with which you are certainly familiar: a
television set. You will probably agree that changing TV channels is quite a
simple task. Now ask yourself these questions:
► Do you know how the television set represents channels internally?
► Do you know exactly what goes on when you change TV channels?
► Do you think you should know these things?
Chances are that you have answered no to all of the questions above. What is
the advantage of not knowing how TVs represent their data and implement their
operations? You could buy a new TV, which does these things internally in a
completely different way, and you could still change channels.
Figure 2-8 on page 31 shows the model layer with its encapsulated business
domain objects and the exposed facade object.
30 IBM WebSphere Application Server - Express V5.0.2 Developer Handbook