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

84 Abstraction of a Problem Domain
boundary. Figure 4.22 shows channels x and y that connect the aggregate to the cluster,
and subsequently connect the cluster to objects outside the cluster. The cluster hides
channels r, s, and t. These channels cannot be connected to objects outside the cluster.
An aggregate is not necessarily abstracted into a cluster. In case it is, we call it a ’clustered
aggregate’. In this way we can distinguish them from the plain clusters which group
a collection of autonomous process objects that are all free to have channels to objects
outside the cluster.
So far we presented the concept of aggregate, and related its application to the cluster
concept. The semantics of aggregation may seem to be clear. However, a more in depth
study shows that there is a lot of confusion about aggregation. There is no agreement
on what an aggregate is and what properties they have. Therefore we elaborate on this
subject.
4.6.5 Aggregate Semantics
4.6.5.1 Aggregate and Composite
In Subsection 4.6.1, we defined an aggregate as a composite of a whole object with its
part objects, where the whole hides the parts for all objects that collaborate with this
composite. We call objects that have a whole-part relationship a composite. According to
our definition an aggregate is a special kind of composite. Our definition of aggregate
is relative narrow when compared to other methods such as [R
91, CY91a]. We require
hiding, where others allow for instance sharing of parts objects among aggregates.
Sharing is only one example of the wide variety of aspects that determines the precise
meaning of the concept of aggregate. Various authors add various aspects to the meaning
of ’aggregates’. Comparison of their work leads to a broader understanding of the
concept, but also shows that there is some confusion.
4.6.5.2 Various Sorts and Interpretations
To start with, there are various slightly different conceptual notions, that are described
as ’aggregates’ (notice that when we refer to others we often write aggregate in this
paragraph, when we mean the more general concept of composite). From the whole’s
perspective a composite or aggregate can be described as a has a or ’consists of’ relation,
and from the parts perspective as a ’component of’, or part of relation. Besides wholepart
relations, various other relationships between classes are used to denote results
of problem domain analysis. Relationships between classes may indicate that there
is possibly communication between instances of the classes. Communication implies
collaboration. Collaborating instances form composites. A composite can often be
interpreted as a whole composed from parts. So it seems as if not only explicitly found
whole-part relations lead to aggregates. In an Object Instance Model, the majority
of objects seems to be involved in whole-part relations. There are various forms of
composites and hierarchical clustering that do all lead to whole-part relations between
objects.