Chapter 2 DATABASE DESIGN FOR SQL SERVER 2000
Chapter 2 DATABASE DESIGN FOR SQL SERVER 2000 95 interacts with the user. The database server manipulates the data, but there also exists a middle tier to control some of the operations. The middle tier can be represented by one or more machines that offload some processing from the database server, which allows for a very large number of users. There is usually no differentiation between three or more tiers and instead they are all categorized as n-tier systems. In an n-tier system, processing is divided into three categories. Each category represents one of the three main tiers in the three-tier system, which is also carried forward in an n-tier system regardless of the number of layers of processing. The presentation or client tier contains the components of the system that interact directly with the user. The sole purpose of this tier is to focus on the end user and present data in an attractive, organized, and meaningful fashion. The middle tier, or business tier, is responsible for communicating with the database server and also sets up the rules by which communication will be established. The idea behind the business tier is to provide mechanisms to implement the business rules that need to be applied to validate data and also perform intermediate processing that may be needed to prepare the data for presentation to the user. For this reason the business tier is often separated into two divisions: interactions with the user and interactions with the database server. With this approach business rules can be separated from data access processes. The final tier is the data tier, which is responsible for the execution of the data engine to perform all manipulations of the data. Access to the data tier is made through the middle tier. The data tier doesn t directly interact with the presentation layer. Internet Applications Internet applications can be said to fall under a two- or three-tier model, depending on the complexity and physical design. In an Internet application, the web server prepares the presentation elements to be displayed on the user s browser. If a middle tier server exists, then the web server is configured to interact with that server. If no middle tier server exists, then the web server interacts directly with the database server.
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