The client resource requirements
become very low, and the cost is reduced dramatically. The application code becomes isolated
from the database. Figure 15-2 shows the thin-client configuration.
The client connects to the application server. Once the client is validated, display
management code is downloaded to the client in the form of Java applets. A database request
is sent from the client through the application server to the database server; the database server
D
Chapter 15: Oracle Net 513
FIGURE 15-1 Client/server architecture
then receives and executes the SQL statement that is passed to it. The results of the SQL statement,
plus any error conditions that are returned, are then sent back to the client through the application
server. In some versions of the three-tier architecture, some of the application processing is
performed on the application server and the rest is performed on the database server. The
advantage of a thin-client architecture is that you have low resource requirements and maintenance
on the client side, medium resource requirements and central maintenance on the application
server, and high resource but lower maintenance requirements on one or more back-end database
servers.
In addition to client/server and thin-client implementations, server/server configurations are
often needed. In this type of environment, databases on separate servers share data with each
other. You can then physically isolate each server from every other server without logically isolating
the servers.
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