Databases 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 sys-
tems, 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 cli-
ents perform the business functions—each client is called a
fat client
in this arrangement—you have a cli-
ent 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 cli-
ents, 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'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 utili-
zation of the system increases. Because the server performs both database and business functions, increasing the
number of clients eventually causes a bottleneck on the server and degrades the system's responsiveness to clients.
To improve a system's scalability, some organizations use a three-tier client/server architecture, as shown in Fig-
ure 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 database server. A three-tier architecture distributes the process-
ing 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 proces-
sors, 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
impacting the design for the client or the database server.
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