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Three-Tier Architecture
Two-tier thick client applications are easy to develop, but any software upgrade because
of changes in user interface or business logic has to be rolled out for all the clients. Luck-
ily, the hardware cost became cheaper and processing power increased significantly on
the CPU in the mid-90s. This, coupled with the growth of the Internet and web-based
application development trends, resulted in the emergence of three-tier architectures.
In this model, the client PC needs only thin client software such as a browser to dis-
play the presentation content coming from the server. The server hosts the presentation,
the business logic, and the data access logic. The application data comes from enterprise
information systems such as a relational database. In such systems the business logic can
be accessed remotely, and hence it is possible to support stand-alone clients via a Java
console application. The business layer generally interacts with the information system
through the data access layer. Since the entire application resides on the server, this
server is also referred to as an application server or middleware (see Figure 1-3).
Figure 1-3. Three-tier application
N-Tier Architecture
With the widespread growth of Internet bandwidth, enterprises around the world have
web-enabled their services. As a result, the application servers are not burdened anymore
with the task of the presentation layer. This task is now off-loaded to the specialized web
servers that generate presentation content. This content is transferred to the browser on
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