Architecting the System
Architecting consists of deciding what tiers are needed for the application, what services are required at each tier, and how the application logic will be spread across different tiers. As you learned on Day 15, “Understanding J2EE Architecture,” the J2EE architecture is designed for multitier applications. In a multitier architecture, the business logic can be split into more than one layer. In the sample application, logic is partitioned into the business logic tier and the presentation logic tier. The user interface is partitioned into the client tier and the presentation tier. The application-persistent data is stored in the EIS tier. Figure 21.3 shows the architecture of the system.
The sample architecture consists of the following tiers:
Client tier— Students and administrators use the client tier to interact with the system. The client tier is provided by a Web browser, such as Internet Explorer or Netscape Navigator. The client tier communicates with the Web tier by using the HTTP protocol.
Web tier— This tier accepts user requests and generates responses using the presentation logic. The sample application uses both servlets and JSPs in a Web container. The Web tier communicates with the business logic tier using RMI/IIOP protocol.
Business logic tier— This tier handles the core business logic of the application. The business components are implemented as EJB components with support from an EJB container.
EIS tier— The EIS tier consists of the database in which the sample application's permanent data is stored. The business-logic tier communicates with the EIS tier using JDBC.
Note
J2EE offers flexibility in partitioning the application logic across tiers. For example, you have a choice between Web-centric and EJB-centric design. In the Web-centric design, the Web tier components are responsible for most of the application's functionality. The Web tier components communicate directly with the EIS tier using container services such as the JDBC API. In the EJB-centric design, the enterprise beans encapsulate the core application logic. Web tier components communicate with EJB tier components instead of accessing the EIS tier directly.
The decision between the Web-centric and EJB-centric approaches depends on factors such as application functionality and scalability requirements. The Web-centric approach offers a quick start for small applications, but can rapidly become complex and difficult to maintain for large applications. The EJB-centric approach offers advantages such as automatic handling of transactions, distributed processing, security, and so on, and can stay manageable as applications grow more complex. The sample application uses the EJB-centric approach.