Databases Reference
In-Depth Information
The color of the board square is not represented explicitly. The
external processor may infer it by adding the numbers of the row
and the column: If the result is even, the board square is black;
otherwise, it is white.
·
Using that internal schema, the partial contents of the internal DB corre-
sponding to Figure 2.7 would be
R1
8
2
R1
8
4
K1
8
7
Q1
7
3
¼¼
The internal processor receives the commands issued by the information
processor and executes them, possibly accessing the internal DB. For exam-
ple, if the internal processor receives the command (or, as in this case, ques-
tion) Is the board square of column 6 and row 2 free? it will check whether
there is a record, in the above file, such that Row
6. If
there is not such a record, the answer to the question will be positive, and
negative otherwise. To perform its task, the internal processor needs to know
the internal schema, including its correspondence with the conceptual
schema.
Modern architectures of ISs are layered, with three logical layers: pres-
entation, domain, and data management. The equivalent to the external
processors is located in the presentation layer, the information processor in
the domain layer, and the internal processor in the data management layer.
=
2 and Column
=
2.4
Requirements Engineering
Section 2.3 discussed the role of conceptual schemas in the architecture of
ISs. Now, we are going to see their role in the development of the systems.
Conceptual schemas are the common base for external and internal
schemas, as well as for their processors. Therefore, it is clear that it is not pos-
sible to design the architecture of an IS without the conceptual schema. Con-
ceptual modeling must precede system design.
