Database Reference
In-Depth Information
With this alternative architecture, a DBMS runs on the server. A client sends a request to the server, not
for entire files, but for specific data. The DBMS on the server processes the request, extracts the requested
data, and then sends only the requested data back to the client. Compared to a file server architecture, a
client/server architecture reduces communication activity on a network, which reduces delays in supplying
data to users. Because the clients and the server perform different functions and can run different operating
systems, this arrangement of client/server architecture is called a two-tier architecture.
In a two-tier architecture, the server performs database functions and the clients perform the presenta-
tion functions (or user interface functions), such as determining which form to display on the screen and
how to format the form
s data. Which of the two tiers, server or clients, performs the business functions, such
as the calculations Premiere Products uses to determine commissions, taxes, and order totals? When the
clients perform the business functions
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you have a
client maintenance problem. Whenever programmers make changes to the business functions, they must
make sure that they place the updated business functions on every client. For organizations with thousands
of clients, updating the business functions for all clients is an almost impossible task.
To eliminate the fat client maintenance problem, you can place the business functions on the server.
Because clients perform only the presentation functions in this arrangement, each client is called a thin
client. Although you
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each client is called a fat client in this arrangement
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ve now eliminated the fat client maintenance problem by moving the business functions
to the server, you
ve created a scalability problem. Scalability is the ability of a computer system to continue
to function well as utilization of the system increases. Because the server performs both database and busi-
ness functions, increasing the number of clients eventually causes a bottleneck on the server and degrades
the system
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s scalability, some organizations use a three-tier
client/server architecture, as shown in Figure 9-6. In a three-tier architecture, the clients perform the
presentation functions, a database server performs the database functions, and separate computers (called
application servers) perform the business functions and serve as an interface between clients and the data-
base server. A three-tier architecture distributes the processing functions so that you eliminate the fat client
maintenance problem and maximize the scalability of the system. As the number of users increases, you can
upgrade the application and database servers by adding faster processors, disks, and other hardware without
changing any client computers. A three-tier architecture is sometimes referred to as an n-tier architecture
because additional application servers can be added for scalability without affecting the design for the client
or the database server.
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s responsiveness to clients. To improve a system
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