Model system
structures
System structures
System context
[interfaces]
Use cases
118 CHAPTER 2 The Pragmatic SYSMOD Approach
In the fi rst step, we will be looking for a central use case. A good candidate for
our system would be start car usage . We pick this use case and select its standard
fl ow to develop a system structure that supports this fl ow.
The pertaining system/actor interaction from Section 2.7.1 shows the fl ow on
message level ( Figure 2.71 ). Since we are now interested in the system structures and
not in the actors, we zoom in to that system element in the sequence diagram. This
??? zooming ??? is formally modeled with a part decomposition ( Figure 2.74 ). The interaction
partners in the detailed sequence diagram are the blocks we are looking for.
What happens when the fi rst message, apply card , arrives at the system
( Figure 2.74 )? The customer card is part of our system, as is the card reader. The
latter reads data from the customer card, but it won ??™ t process the data; it will pass
it on. To whom? Our on-board computer will be equipped with a central control
unit.
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