INTRODUCTION

Much has been accomplished by designing information systems architecture as independent tiers that interoperate only with those that are directly adjacent. In this way, logic is boxed into a logical hierarchy that makes solutions more scalable and easier to understand, maintain, and change.

This thinking started with the client/server model. This model was used successfully at first in two different environments and for two different purposes:

• as a way to separate the database services from an application that uses them (in this case, the client is the application and the server is the database server),

• as a way to separate the presentation services on a workstation from an application that uses them (in this case the client is the application and the server is the presentation server).

As more modern enterprise applications evolved, the need for separating both the presentation layer and the data layer from the “business-logic” layer became self-evident, giving way to the so-called three-tier client/server model. The three-tier architecture has had many different implementations and has gotten more and more sophisticated.

Initially, the layers were simply the data layer, which consisted fundamentally of the database server; the presentation layer, which was basically a presentation server; and the application layer in the middle, which consisted of everything else.

The application layer still implemented many generic reusable services that could be distilled from it. As these services started being abstracted from the application logic, the concept of application servers as “business logic containers” became more popular.