Persistence: The Final Tier

As has been covered elsewhere in this book, the typical BEA WebLogic Server configuration consists of three tiers:

Tier 1: Web Server Tier
Tier 2: Application Server Tier
Tier 3: Database Tier

The Database tier is responsible for storage and retrieval of attributes and properties of the enterprise application through persistence. The application achieves the following goals when persistence is used correctly:

Durability ” The state of the application can be restored after system shutdowns or crashes.
Reliability ” The state of the application is accurate. The data is left in a consistent and known state.
Scalability ” The application can grow to support more objects without changing or affecting the architecture.

A durable, reliable, and scalable application is exactly what is provided by entity Enterprise JavaBeans. In Chapter 6, "Transitioning from Software Design to J2EE Technology Components and Services," the Model-View-Controller design pattern was introduced. This design pattern maps directly to an enterprise application, with JSP and servlets providing the view, session EJBs providing the control, and entity EJBs providing the model. This chapter focuses on the model and how it is implemented using entity EJBs. Specifically, it focuses on the components of an entity EJB and how persistence is implemented.

Note

J2EE also allows for stored data to be represented within an XML document, JMS message, via Cookies (on client), and through JDBC.