Multiple-Tier Architecture

Multiple- Tier Architecture

The multiple-tier architecture is a true enterprise-caliber solution (Figure 4-3). By breaking out the EJBs onto their own dedicated servers, you incrementally increase the availability and performance of the cluster as a whole. The architecture is reliable, scalable, and complicated to administer.

In order to take advantage of multiple-tier architecture, your EJB application must be designed with replica-aware stubs. The actual implementation of these stubs is outside the scope of this book. However, for purposes of this discussion, it's worth providing a brief background description. (Also see the discussion on EJBs in the Introduction.)

Replica-Aware Stubs

To the caller, a replica-aware stub appears to be a normal RMI stub. However, the stub must contain program logic for locating a replica of an EJB or RMI class on any WebLogic server in the cluster in which it is deployed.

In a multitier architecture, all the presentation logic occurs on the standard servers. Consequently, no client ever gets a replica-aware stub that is located on the same server as the clustered object. Therefore, the calls to the clustered objects are load balanced according to the logic implemented in the replica-aware stub.