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

4.8 Properties of Composites 89
for them. Hence, a cluster is not a behaviour identity that can be given an identifier in
the form of an attribute.
Nevertheless, if we would like to model behaviour that is private to a cluster we could
do so by introducing a new process object representing the cluster as a whole. This
process can be given an identifier. In this case it does not harm to consider the identifier
of an aggregate object to be the identifier of the cluster too. A typical example where
such an approach can be used is a clustered aggregate. The identifier of the aggregate
object can be considered to be the identifier of the cluster. The cluster hides the internal
objects except for the aggregate object.
4.7.4 Identification of Data Objects
Data objects tend to have a rather dynamic life with changing connections to other
objects. Communication with a data object can be performed only by objects that
have a reference to it. The value of such a reference is a unique identifier of a data
object. Data objects have implicit identifiers. These identifiers are hidden. They are not
named explicitly such as identifiers of process objects, that are chosen during analysis
as free unique readable names. Identifiers of data objects are automatically generated
as unique numbers. These identifiers do not have to be unique on the global level of
a system specification. Process objects are the units that determine the name space of
data objects. Different process objects may contain data objects with the same identifier.
However, these identifiers must uniquely identify the data objects in each process object.
Data objects can form complex structures that may travel from process to process (see
Subsection 4.4.5). Travelling data objects are handed over between process objects by
making a deep copy at the receiver side. Therefore new unique identifiers are required
at the receiver side.
In contrast to process objects data objects are not visualised in instance diagrams during
analysis. The classes of data objects, however, are visualised in an Object Class Model.
Identifiers can be modelled as attributes of classes in Object Class Models. Since data
objects have implicit identifiers, they are in general not shown in Object Class Models.
The identifiers of process objects on the other hand are shown, but this is only done if
processes communicate in a weakly distributed fashion. Processes that communicate in
a strongly distributed fashion do not need identifiers.
4.8 Properties of Composites
4.8.1 Introduction
A complex system consists of many collaborating objects. Handling complexity requires
that we can reason about them and recognise various forms of composites. This can
only be done when we are able to distinguish various forms of composites. We already