Dissection of an Application Frameworks , Page 4
The actual task of moving the common spot theme into a framework component is not hard. The difficult task is to identify in the analysis phase the common spots throughout the business application that have not yet been developed. It is not always easy to see through the common theme embedded in the application, and it usually takes a few tries to get it right.
Common spots can exist in both the domain-specific framework layer and the cross-domain framework layer. For example, the exchange of business documents is the central theme of B2B applications. A business document object would be considered one of the common themes that can be turned into a framework component for the domain-specific framework layer. Another example is a data cryptography service. Regardless of the type of application, data encryption and decryption are often applied at different parts of the application. A data encryption/decryption component, which simplifies and reduces the amount of code developers have to write to support cryptographic needs in the application, would be considered one of the components in the cross-domain framework layer.
From a technical point of view, implementation of common spots is straightforward. After identifying a common spot, the framework designer can develop the components that encapsulate the theme and logic in the common spot. Such components often take the form of concrete classes or executables. To accommodate the small variations in the component, some configuration data may also accompany the component. For example, you can allocate certain sections in the configuration file for parameterization of the framework components. In terms of adapting such framework components, developers need to write little or no code to use the component within the application.
Common spots capture the themes that are repeated throughout the application; however, each business application is unique, and there are as many spots where each application varies significantly due to the nature of the application as there are common spots. As a framework designer, you need to take account of those variations when designing the framework and make sure that developers can take advantage of the framework not only when there are common themes among applications, but also significant variation and customization between applications. This leads us to the next topic: hot spots.
A hot spot is point of flexibility in the application framework. Another way to look at the hot spot is that it is a placeholder embedded in the framework where application-specific customization occurs.
Hot spots are the opposite of common spots. In a common spot, the framework implements the common themes inside the framework component; however, in the case of hot spots, there is nothing for the framework to implement except to leave an empty placeholder, which is later filled with a custom implementation by the business application built on top of the framework. Because each business application is responsible for providing its own implementation for the hot spot in the framework, the framework will behave differently for each business application. This is how a framework can be designed to suit different business applications in spite of significant variations among these business applications. Figure 4 illustrates how an application framework achieves flexibility through hot spots.
Figure 4. Hot spots
In Figure 4, components in the application framework layer consist of different hot spots, or empty placeholders for customization. Each application may use a number of framework components throughout the application. When the application uses a framework component that contains the hot spot, it needs to provide the implementation only in the hot spot in order to use the framework. The different designs and shading of the hot spot shapes shown in the figure represent the different implementations among various applications. As you can see, by implementing different logic inside the hot spots, each application will elicit different behaviors from the framework components.
As with common spots, identifying the hot spots may not be easy. To identify a potential hot spot, you must understand the business domain inside out and understand which points in the business application will potentially need to be customized. Having too many unnecessary hot spots in the framework will lead to extra coding effort for the developer team. Having too few hot spots makes it harder for the developer team to adapt the framework when desired customization becomes difficult due to the inflexibility of the underlying framework. Although it is possible to achieve such customization by overriding most or all the virtual methods in the base class, doing so would diminish code reuse and the purpose of inheritance.
Creating hot spots in the framework is not as straightforward as creating common spots. There are two approaches on how hot spots are enabled in the framework: the inheritance approach, and the composition approach. Let's look at the inheritance approach first.
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About the Author
Xin Chen is the founder of Xtremework, Inc. Since the inception of .NET, Xin Chen has helped customers in a wide range of industries turn their business ideas into software products using .NET technology. Leveraging his expertise in .NET and EAI, Xin Chen has also worked with several technology leaders, such as Microsoft and Accenture, to bring winning solutions to their customers. When not working overtime, Xin enjoys reading books, writing books, and resting. He is the author of BizTalk 2002 Design and Implementation (Apress, 2003). Xin Chen earned a master's degree in statistics from Columbia University and is currently living in New Jersey.
About the Book
Developing Application Frameworks in .NET by Xin Chen