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

78 Abstraction of a Problem Domain
The ROOA method [Mor94] distinguishes objects that have an important role in the
system from objects that only play the role of attributes of other objects. The former
objects are modelled as processes in LOTOS [ISO88] and the latter as values of ADTs
(Abstract Data Types). The objects modelled as LOTOS processes, are similar to our
process objects. The objects modelled in terms of ADTs, however, differ from our data
objects. In fact, ADTs in LOTOS are not expressive enough to model objects. They are
merely collections of data values together with a number of operations defined on these
values. The values of ADTs are not separately identifiable and are therefore not objects.
The other formal description techniques, SDL [BH93] and Estelle [D
89], distinguish
processes from data, just as LOTOS does. Processes in these languages are expressive
enough to model entities that are similar to our process objects. The data part of SDL is
based on ADTs and is not suitable for the modelling of data objects. Estelle uses Pascal
types to model data. Pascal has insufficient support for data modularisation and is also
not suitable for the modelling of data objects. An object-oriented specification method
that makes a similar distinction between process objects and data objects as we do is
ROOM [SGW94]. Process objects in ROOM are called actors.
4.5 Clusters
A model of a complex system contains many different objects. To manage complexity, an
abstraction mechanism is required that groups collections of collaborating objects into
single entities. We offer the concept of ’cluster’ to do this. A cluster is a boundary around
a group of collaborating process objects and clusters. Clusters are not individually identifiable
entities (see Subsection 4.7.3). Therefore clusters are not objects. Nevertheless, a
cluster has a well defined interface, like an object. Another correspondence with objects
is that clusters are grouped in classes. Each cluster is an instance of some cluster class.
Since clusters are not objects, cluster classes are not shown in Object Class Diagrams.
Clusters are typically shown in Object Instance Models. Figure 4.15 shows an object
Object A Object B
Object C
Cluster
Figure 4.15: Clustered Objects
cluster, its interface in the form of external flows, and its internal objects and message