Because many
database queries are performed via applications, a server-based application forces the server to
support both the CPU requirements of the application and the I/O requirements of the database
(see Figure 15-1a). Using a client/server configuration (also referred to as a two-tier architecture)
allows this load to be distributed between two machines. The first, called the client, supports the
application that initiates the request from the database. The back-end machine on which the
database resides is called the server. The client bears the burden of presenting the data, whereas
the database server is dedicated to supporting queries, not applications. This distribution of
resource requirements is shown in Figure 15-1b.
When the client sends a database request to the server, the server 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. The client resources required have caused the
client/server configuration to sometimes be dubbed fat-client architecture. Although workstation
costs have dropped appreciably over recent years, the cost impact to a company can still be
substantial.
The more common, cost-effective architecture used with Oracle Net is a thin-client configuration
(also referred to as a three-tier architecture). The application code is housed and executed using
Java applets on a separate server from the database server.
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