Modern N-tier Architectures

If three tiers are better than two tiers, which are better than a monolithic program (generally speaking), then is there any benefit to adding a fourth or fifth layer?

The answer is yes. Additional software layers can be identified from good modular design and separated out onto different physical tiers at suitable boundaries. Deciding what is a suitable boundary is the hard part.

The downside to additional tiers is that extra communication overhead can inflict performance bottlenecks. The upside of multiple tiers is that additional hardware (multiple servers and network bandwidth) can be used to alleviate the bottlenecks and improve reliability using techniques such as server farms and failover clustering.

The modern emphasis on the Internet is driving n-tier architectures and many enterprise service vendors are seeking to exploit this market. Web services are the next generation of n-tier development, allowing applications to be created from components distributed across the Internet. As this model evolves, the distributed Internet becomes the computer. This book shows how you can use J2EE technologies to provide Web services (see Day 20, “Using RPC-Style Web Services with J2EE” and Day 21, “Message-Style Web Services and Web Service Registries”).

Enterprise applications can be Webcentric, but need not be. To cover Webcentric programming, this book shows how to integrate Servlets (see Day 12) and JavaServer Pages (JSPs) (see Day 13,) into Enterprise applications. Within an organization, or even when creating business-to-business (B2B) links, enterprise applications need not use Servlets or JSPs. In this case, clients may connect directly to business components, in the shape of Enterprise JavaBeans (see Day 4, “Introduction to Enterprise JavaBeans”), over RMI (Remote Method Invocation) or CORBA (see Day 19, “Integrating with External Resources”).

As the provision of functionality over the Internet gains importance it becomes important to maintain the integrity of the data in the corporate systems. Transactions and security provide a common mechanism for doing this. Transactions are covered in detail in Day 8, “Transactions and Persistence” and security in Day 15, “Security.”