Databases Reference
In-Depth Information
Figure 12-1
PC
PC
Server
PC
Query
Issued
Here
Printer
PC
PC
PC
Two-tier data.
How these multiple data sources are handled depends on your DBMS and
client application. Some DBMSs support a flexible data dictionary that can make
the process transparent to the database application. With other DBMSs, includ-
ing SQL Server, the DBMS tracks data hosted in its own databases only. Multi-
tiered data environments like those discussed here are treated as
distributed
data environments
because of the heterogeneous data sources, even though all
data sources are physically located on the local network.
As is often the case, you must be careful how you use terms relating to data-
base applications and make sure that you understand the context in which they
are used. When discussing a three-tier configuration, you need to specify whether
you are talking about the data locations or the database application configuration.
The same term,
three-tier,
is sometimes used when discussing a multitiered appli-
cation, referring to the database server, application servers, and client computers
as the three application tiers. The middle tier could involve multiple computers,
such as an application server and a Web server. You can easily have a single-tiered
approach to data storage, with all of the data stored on the database server, sup-
porting a multi-tier (or three-tier) database application.
