Specification of Reactive Hardware/Software Systems - Electronic ...

386 Case Study
communicated. For this purpose Instance Structure Diagrams are used. These diagrams
formalise the architecture structure (in the form of boundaries) of the control system and
are used as a preparation to create a formal POOSL description. An Instance Structure
Diagram of a complete production line directly follows from the Message Flow Diagram
of Figure 12.3. Each pair of successive stations is equipped with a private communication
channel. The resulting diagram is shown in Figure 12.7. Notice that each of the clusters
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Separator/
Feeder
Merger
Feeder
Packer
Stacker
i o c1 i o c2 i o c3 i o c4 i Printer o c5 i o
Station
Station
Station
Station
Station
Station
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Feeder
Station
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Feeder
Station
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Product_
Information_
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Figure 12.7: Instance Structure Diagram of the Control System
represents a distribution boundary. These boundaries indicate that each of the stations
and the product information server are physically distributed. The Feeder Station cluster
in Figure 12.7 hides its internal structure. This structure is visualised in the Instance
Structure Diagram of Figure 12.8. We use the heuristic that each pair of dependent entities
is connected by a private communication channel. An exception is formed by the
wake channel which connects the Product Input Handler, the Feeding Unit Image and the
Product Output Handler to the Transporter Image. The Transporter Image exports a wakeUp
service that is used by each of the three clients. If a shared channel is used, the POOSL
formalisation of the Transporter Image is simplified substantially 10 . Furthermore, the
process class of the Transporter Image will become better reusable when a single channel
is used.
12.3.7 Conceptual Model
The Message Flow Diagrams of the previous paragraphs contain several references to
data classes in the form of parameter types of messages. The diagrams do not show how
the data classes are built and how they relate to each other. Data classes and relations are
typically shown in Object Class Diagrams. A (rather incomplete) Object Class Diagram
of the mailing machine is shown in Figure 12.9. Next to the most important data classes
10 Notice that the channels can always be split using the behaviour-preserving transformations of
Chapter 10.
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