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

4.11 Composites and Hierarchies 99
x
w w
x Object A
y z
Object C
a b
c
Cluster
Object B
Figure 4.26: Cluster with Hidden Channels
for the communication with the sensor and hides in fact the sensor for the rest of the
system. The aggregate in Figure 4.27 is thought to encompass Sensor 1 and Actor 1.
However the system boundary intersects the (virtual) cluster. Terminators Sensor 1 and
Actor 1 are considered as belonging to the aggregate, although the cluster boundary
cannot encompass them. By doing this, Object B, for example, can still be considered
to be a separate part of a clustered aggregate. Parts B and C are not visible for other
objects, that are outside the cluster but are internal to the system. Object B and C are
however visible to the terminators, which are external objects. So in this case external
objects require a different treatment than internal (system) objects. External objects do not
respect the rules for inward and outward visibility for clustered aggregates. For reasons
of clarity they must be referred to as terminators and not as objects. They also have a
different graphical symbol distinguishing them from the process objects. The advantage
of a different symbol is that it is always clear that a terminator is an external object. So
it is no problem to draw terminators at various places inside the system boundary. This
is convenient because it prevents diagrams from being cluttered by long connections
to terminators on the edge of a scheme. If needed, the diagrams can be transformed
such that the terminators move outside that system boundary, when system analysis is
finished.
4.11 Composites and Hierarchies
In the previous sections, various forms of hierarchy passed by, such as:
1. whole-part class hierarchy;
2. subtype/supertype class hierarchy;
3. hierarchy of subsystems, cluster hierarchy.
There may be some confusion about the place of composites for modelling hierarchy in
our method. Object-oriented analysis methods use Object Class Diagrams to visualise
Whole-part class hierarchies and Subtype/supertype class hierarchies simultaneously.