Aspect-Oriented Programming: Road to a Resilient Architecture

Apr 8, 2005

📄 Contents

␡

What Is Architecture?
What Is a Good Architecture?
Steps to Establish an Architecture Baseline
Begin with a Platform-Independent Structure
Overlay Platform Specifics on Top
Summary and Highlights

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Aspect-Oriented Software Development with Use Cases

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11.4 Begin with a Platform-Independent Structure

The way you structure the system is an important architectural decision. You structure the system such that concerns are kept separate. You achieve this structure first from a platform-independent perspective and then refine it with platform specifics. A platform-independent structure is driven by functional requirements (as modeled with use cases).

The tools you use to achieve a resilient structure are classes and use cases. Classes help you keep the elements in a system separate, and use cases help you keep the tasks of each element separate. Accordingly, there are two orthogonal structures in the system—the element structure and the use case structure.

The element structure identifies where elements of the system are located in the element namespace. It structures the elements hierarchically in terms of layers, packages, classes, and the features within these classes.
The use case structure defines how you overlay functionality onto the element structure. It comprises slices—both use-case slices and non-use-case-specific slices—that add the actual classes and class features onto the element structure.

You want your structure to be resilient along both structures. This means that if there are changes in requirements, their impact should be localized to a few packages and classes in the element structure. Their impact must also be localized to a few use-case slices. Localized means that there are few changes, and the changes do not propagate beyond those packages or use-case slices that require change.

11.4.1 Element Structure

The element structure for a model is a hierarchical structure of packages and classes. It uniquely identifies each element. Since the goal is to achieve resilient structure, you naturally locate classes that are used for the same purpose together.

Layers

You normally use layers as the first-level partitioning in a model. Layers are used to group software elements that are on the same level of abstraction. You place more abstract and reusable elements in lower layers and more concrete or less reusable elements at the top. Normally, two high-level layers are sufficient to refine the functional requirements for the system: the application layer and the domain layer.

Application Layer

The application layer contains elements that realize workflows in the use cases supporting the primary actors of the system. The elements in this layer normally use the elements in the domain layer to realize use cases. You can organize packages in the application layer according to the following criteria.

Classes that support one or more particular actors.
Classes that are involved in one or more particular use cases.
Classes that are involved in some functional area in the system.

Domain layer

The domain layer contains elements representing significant domain concepts. They capture information to be maintained, tracked, or manipulated by the system and the associated behaviors for doing so. These elements are normally shared across use case realizations. They are more reusable and so reside in a lower layer than the application layer. However, since they are shared by use-case realizations, use-case realizations frequently cut across domain elements.

Figure 11-1 depicts the initial structure of the Hotel Management System that realizes the functional requirements of the system. The packages in the application layer are grouped according to actors—the customer, the hotel counter staff, and the management. The packages in the domain layer group classes related to rooms and classes related to reservations.

Figure 11-1 Initial layers and packages in the element structure.

The structure in Figure 11-1 is an initial one. It is refined further into classes and so forth as you analyze the use cases for the system.

Sidebar 11-1. How Use Cases Help to Structure the Application Layer

We mentioned earlier that classes that are involved in a particular use case can be placed within packages in the application layer. Figure 11-1 shows a relatively simple case of identifying packages in the application layer. For larger systems, you can partition your application layer based on the principles illustrated by the figure below. Before we go on, we want to emphasize that this is but one way for you to structure the application layer.

In the example above, we split the application layers further—application-specific and application-generic. Classes participating in peer use case realization are allocated to application-specific packages. Classes that participate in generalized or included use cases are allocated to application-generic packages. There is a dependency from application-specific packages to application-generic packages. This preserves the relationship from the use case model into the analysis element structure.

Note that there is a limit to how much you can keep the realization of the use cases separate in the element structure. That is why we need the use case structure that defines overlays on the element structure.

Sidebar 11-2. How to Organize the Domain Layer

In Figure 11-1, we show only two domain-layer packages. You definitely expect more from larger systems. The example below shows how you can structure packages in the domain layer.

In essence, you organize entities that are common across industry domains within domain-generic packages. For example, classes like Address, Customer, and Company are used in many industry domains and are, therefore, highly reusable. Domain-specific entities can then either specialize these entities or reference them through associations. Again, we like to emphasize that this is but one way for you to organize your domain layer. The key idea is to distinguish between what is domain-specific and what is domain-generic or independent. This is a way to achieve understandability and also reuse.

11.4.2 Use-Case Structure

As mentioned, the element structure is simply about identifying elements in a namespace. It is the slices in the use-case structure that overlay the actual content for each element. There are two kinds of slices: use-case slices and non-use-case-specific slices.

The convention is to depict the element structure (comprising layers, packages, and classes) vertically such that at the top you find application-specific layers and packages, and at the bottom you find application-independent ones (as per Figure 11-1). To emphasize the orthogonality of the use case structure, we depict the use case structure horizontally with the non-use-case-specific slices on the left and the use-case–specific slices on the right (see Figure 11-2). The arrows in Figure 11-2 show the dependencies between the use-case slices and non-use-case-specific slices.

Figure 11-2 Use-case structure.

Non-use-case-specific slices are derived by exploring the commonalities between use-case realizations. They normally have a close correspondence to the element structure, especially to the lower layers. After all, lower layers in the element structure and non-use-case-specific slices are for the purpose of grouping things that are shared—though shared from a different perspective. This is exemplified by the slices on the left of Figure 11-2. The Hotel Reservation slice adds the domain packages into the element structure. The slices Customer Application and Counter Application add classes to the corresponding packages in the element structure.

The use case slices in Figure 11-2 are derived directly from use cases in the use-case model. Thus, on the right of Figure 11-2, there are use-case slices for Reserve Room, Check In Customer, and Check Out Customer.

Note that the Customer Application and the Counter Application non-use-case-specific slices do not extend the Hotel Reservation non-use-case-specific slices. The former contains classes that depend on or makes use of classes contained in the latter. The former do not extend the latter. Hence, there is no «extend» relationship between them.

Sidebar 11-3. How Use Case Slices Improve Reuse

Without aspect orientation, you normally attempt to achieve reuse by pushing reusable things into lower layers. For example, if a class is reusable, you push the class down to a lower layer or a lower package. If some operations in a class are reusable, you may factor the common operations into a generalized class and push this down to a lower layer or package. This seems to work well, but the problem is this: you need to push down complete operations or complete classes.

On your project team, you may find a good programmer. Every boss likes her and gives her different things to do. Soon, she has so many different things to do that she ceases to be effective. Working with reusable elements is similar. As you attempt to make these elements more reusable, you inevitably get them to do more things, and they quickly become heavyweight and entangled.

This problem is solved with aspect orientation and use-case slices. In the use-case structure, you can push only those reusable extensions into lower slices—not the entire class. For example, the Hotel Reservation non-use-case-specific slice contains partial elements that are needed by all slices that are on top of it. In this way, you achieve reuse without the heavyweight problem.

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