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

80 Abstraction of a Problem Domain
a bottom up approach as well as a top down approach. Clusters can form a composite
of clusters and process objects (bottom up). They can also be used as abstractions of
some complex system part which must be refined into clusters and objects (top down).
A system is usually composed of subsystems, which can be represented by clusters.
Like multiple processes we offer multiple clusters. The graphical notation is the same as
for multiple processes (see Figure 4.13 and Figure 4.14).
Only few current object-oriented methods have a support for clusters. The ROOA
method [Mor94], for instance, supports clusters (called subsystems) to manage model
complexity. Clusters in ROOA, however, are groupings of classes and not of instances.
In our opinion, the clustering of classes is not sufficient for the modelling of complex
systems. We share the viewpoint of de Champeaux [Cha91] who states that objectoriented
analysis without an entity (instance) clustering technique is not a viable method
for the characterisation of large systems. De Champeaux offers ’ensembles’ to cluster
collections of collaborating instances. Ensembles, however, differ from our clusters in
the sense that they have behaviour of their own. The same holds for the clusters (called
blocks) supported by SDL [BH93, Tur93], the clusters (called parent objects) supported
by HOOD and HRT-HOOD [Rob92, BW95], and the clusters (called parent modules)
supported by Estelle [D
89, Tur93]. A disadvantage of clusters that have behaviour
of their own, is that they do not allow flexible and easy manipulation (in the form of
behaviour-preserving transformations, see Chapter 10). Therefore we decided that our
clusters do not have behaviour other than the combined behaviour of their internals.
A specification method that supports a similar notion of clusters as we do is ROOM
[SGW94].
Notice that clusters may be quite opaque or even transparent structures. The structuring
may be completely logical or just for clarity purposes, as a means to improve understanding
by limiting the amount of information presented in one diagram. However,
next to the purpose of reducing complexity, clusters have several other applications.
They let us establish various design decisions. They can be used to formalise properties
of system parts such as concurrency, implementation and distribution. This is discussed
in depth in Chapter 5.
4.6 Aggregates
4.6.1 Definition
We define 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. An aggregate
can be clustered. Aggregate and cluster are orthogonal concepts that enable structuring
a system model.
Complex systems consist of many parts. A complex system is always structured as a
composite of many parts or subsystems. In general, subsystems are again composites.