Databases Reference
In-Depth Information
wishes to access the information, it may be for other purposes, such as determining when
the topic was added to the library or how much it cost. These issues probably don't
interest the student.
The point here is that a DBMS should supply an interface with a high-level language with
which programmers can program the database to provide specific services—that is, with
which programmers can create database applications. Thus, when a student logs onto a
library's computer to search for a book, he may be accessing a different database
application than the librarian might access. The language that is used for
database
application programming
is the hostlanguage for the DBMS. As mentioned earlier, a host
language may be a traditional programming language, such as C or COBOL, or it may be
an application-level language, such as Microsoft Access or Visual Basic, as it is for the
Jet DBMS.
In fact, the Jet DBMS is so tightly integrated into both of these applications that it is hard
to tell where one leaves off and the other begins. Put another way, it sometimes seems as
though Microsoft Access
is
the Jet DBMS, whereas it is more accurate to say that Access
and Visual Basic are
front ends
, or
host applications
, for the Jet DBMS.
7.8 The Client/Server Architecture
The client/server model of a database system is really very simple, but its meaning has
evolved somewhat through popular usage. The client/server model is shown in Figure 7-
3.
Figure 7-3. The client/server mode example
The server in a client/server model is simply the DBMS, whereas the client is the
database application serviced by the DBMS. (We could also think of Visual Basic and
Access as clients of the Jet DBMS server.)
The basic client/server model says nothing about the location of the various components.
However, since the components are distinct, it is common to find them on different
computers. The two most common configurations are illustrated in Figures Figure 7-4
and Figure 7-5. The
distributed client/server model
(Figure 7-4), wherein the client is on
one computer and the server and database are on another, is so popular that it is usually
simply referred to as the
client/server model
. The
remote database
model (Figure 7-5)
refers to the case in which the client and server are on the same computer, but the
database is on a remote computer.
