layer than two-tier, thin-client implementations. In general, the isolation of functions in discrete layers, implemented in discrete tiers, means that each tier can be tweaked by using best-of-breed products without much impact on the remaining tiers. As previously mentioned, any technology worth its salt solves old problems while introducing new ones. Some problems caused by implementing applications that follow the n-tier architecture are described below. N-tier architectures are flexible. One result of this flexibility is that the three- or n-tier implementer has to cope with more hardware and software components than its two-tier counterpart. The addition of the application server opens up new system configuration possibilities. While selecting best-of-breed products to implement the system s layers is a good thing, the problems with having a multiple vendor environment, replete with finger pointing, persist. As you might imagine, maintenance costs for a large n-tier system are high. Imagine a large n-tier application, such as a banking/ATM system, with thousands of clients dispersed all over the world securely reading and writing terabytes of data to multiple data stores. The activity between tiers necessary to get the job done must be staggering! The overhead produced by transmitting and receiving all this data across networks that connect hardware and software components that implement the multiple tiers can slow down things, to be sure. The problems I’ve mentioned can be solved for the most part by spending more money for additional hardware not exactly the favorite solution! We ve talked about the benefits of developing software in layers, or tiers. As we ll see here and throughout subsequent chapters, J2EE provides an architecture for constructing n-tier applications. Before we move on to discuss J2EE particulars, we need to take a look at another essential technology instrumental to J2EE application development that has proved its worth in theory and practice: object technology.
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