Databases Reference
In-Depth Information
3.
Get the two parents gp3 and gp4 of p2.
4.
The grandparents of gc are gp1, gp2, gp3, and gp4.
Derivation rules may be specific of a given domain (e.g., a bank), applicable
to all domains of a certain class (e.g., banking), or domain independent (e.g.,
statistical concepts). The conceptual schema must include all derivation rules
that can be used in a particular system, but we should explicitly define only
those rules that are specific to our domain. The other derivation rules could
be shared by all conceptual schemas for domains of the same class or by all
conceptual schemas.
In practice, most derivation rules infer new facts of the information
base, and then the rules are included as part of the conceptual schema of the
state. However, nothing prevents the inference of events from other events,
and then the corresponding derivation rules are part of the conceptual
schema of the behavior. For example, a derivation rule referring to events of
type Travel could define Long travels as those travels such that the distance
traveled is greater than 1000 km.
2.3
Abstract Architecture of an IS
Section 2.2 presented conceptual schemas. This section shows the essential
role these schemas play in the architecture of ISs. By architecture, we mean
the main components and their relationships. In principle, there are many
possible architectures, and choosing the most convenient for a particular IS
depends on many factors, including the preferred architectural style and the
hardware and software platform on top of which it must work. However, we
do not need to take such diversity into account here. For our purposes, it will
suffice to consider the ANSI/SPARC abstract architecture proposed in the
ISO report [10] (Figure 2.6).
To illustrate this architecture and the role played by conceptual sche-
mas in it, we will use the example of a chess-playing system that can play with
persons or with other systems.
The conventional representation of the state of a chess game is a draw-
ing like the one shown in Figure 2.7. However, not everybody uses exactly
the same representation; different icons can be used to denote the same
piece. Some users may prefer other graphical representations (e.g., the three-
dimensional view), and in some cases text-based representations may be
preferred (e.g., in machine-machine communication).
