The object tier usually maps to a WebLogic cluster that houses the heavier J2EE services such as EJB and RMI objects, as well as the JMS resources. Because the EJB objects typically need JDBC access to the backend DBMS (data tier), the object tier in our multi-tier application setup also hosts the JDBC resources. Remember, a clustered architecture need not have an object tier. If your application doesn't use EJBs to encapsulate the business logic or doesn't require any asynchronous message handling, you easily can bypass the object tier. In that case, you could deploy the JDBC resources to the presentation tier directly.
Even if your application does make use of EJB objects and/or JMS resources, you still need not construct a separate object tier cluster. You could deploy your entire application (servlets, JSPs, EJBs, JMS destinations, etc.) to a single WebLogic cluster. We examine the trade-offs of this combined-tier architecture later in this chapter.
This section examines the object tier by first exploring the interplay between replica-aware stubs and WebLogic's clustered JNDI service. Then we review how EJB and RMI objects, JDBC, and JMS resources behave in a clustered environment.
Establishing the object tier cluster is
no different from setting up the web/presentation tier. The object
tier cluster, named
OCluster here, is composed of
two Managed Servers,
ServerG. Both servers coexist on another ...