Day 1. The Challenge of N-Tier Development

The current trend in enterprise program development is to provide n-tier frameworks aimed at delivering applications that are secure, scalable, and available. To this end, Sun Microsystems introduced Java 2 Enterprise Edition (J2EE), and Microsoft Corporation ventured the .NET framework to help developers build applications that are Web-friendly and frequently used to deliver e-commerce solutions. There are a myriad of application servers available to house enterprise applications, and many service providers are writing modular tools to plug in and extend the rich functionality. The clients that are taking advantage of this distributed architecture can be as simple as a Web browser (a so-called thin client).

This is the overarching vision and the state of the art. But, how did we get here?

To understand this landscape, this chapter investigates the principles of multiple tiers, component environments, and standards that underlie the frameworks. One of the objectives will be to give you a clear understanding of concepts and terminology used when discussing such frameworks. Such terminology can frequently be confusing and inconsistently used. As a start along this road, please note that for the purposes of the following discussions, a tier refers to a physical separation (a different machine), and a layer refers to a logical layer in software terms, such that multiple layers can be on the same machine.