Information Technology Reference
In-Depth Information
Drawing the boundaries of the problem world demands a judgment based on
the responsibilities and the scope of authority of the customer for the system
(we return to this topic in Section 2.1).
One view is that it is the customer's responsibilities that bound the effects
to be evaluated in the problem world, while the customer's scope of authority
bounds the freedom of the developers in aiming to achieve those effects.
The customer's requirement is that the gate should be open or closed accord-
ing to a certain regime intended to ensure appropriate irrigation of the fields.
The problem is to develop the controller that will impose this regime. The prob-
lem is depicted in the problem diagram in Figure 3. The two rectangles represent
the two physical domains of this problem. One is the Control Machine, which is
the computer executing the control program that we are to develop. It is marked
with a double stripe; this indicates that it is the machine domain in the problem.
The other is the Sluice Gate with its sensors and drive motor, the plain rectangle
indicating that it is a problem domain , which in the software development we
regard as given. 4
Fig. 3. The machine, the problem world and the requirement
In this diagram there is only one problem domain; it is frequently the case
that there are two or more, interacting with each other and with the machine
domain. We refer to the problem domains collectively as the problem world ,
distinguishing them from the machine. The requirement is represented by the
dashed ellipse; the requirement is to impose the desired regime on the gate. The
requirement phenomena -that is the phenomena in terms of which the require-
ment is expressed- are represented by the arrow marked a , and listed in the text
below the diagram. The specification phenomena -that is, the shared phenomena
of the interaction between the machine and the problem world — are represented
by the line marked b , and listed in the text below the diagram. The notations
“CM!” and “SG!” indicate that the Control Machine and Sluice Gate respec-
tively control the annotated phenomena: the machine can switch the motor on
and off and set its direction , while the top and bottom sensors are controlled by
the sluice gate. The requirement phenomenon is expressed in terms of periods
in which the gate is either open or closed .
4 It is important that we are concerned with software development, and that we regard
the problem domain as given : that is, we are not free to replace the sluice gate
equipment with different equipment better suited to our needs. We must develop a
control program for the sluice gate with which our customer presents us.
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