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

364 SHE Framework
Behaviour-Preserving Transformations can affect clusters that carry boundaries. In
such a case we must verify whether the new structure does appropriately specify the
properties required. Otherwise the affected boundaries must be redesigned carefully.
By definition of boundaries we can minimise the number of iterations in the design
process. The boundaries are important directives for the specification style of the classes
in the Unified Model.
If two processes from the same class are enclosed in different boundaries it can be
necessary to define separate classes for these processes. These separate classes can
perform the same observable behaviour, but can have interesting differences also. A
particular style, for example, can fulfil the requirements for a straightforward translation
to a particular implementation description language, whereas another style may be
inappropriate.
In Chapter 10 we offered a collection of basic Behaviour-Preserving Transformations for
the restructuring ISDs, as well as an example. In this subsection about ISDs we showed
that various additional aspects specified as boundaries can influence the desirability of
particular transformations.
11.4.8 Formal Description in an Essential Unified Model
The behaviour description of the system in POOSL is called the Unified Model. During
the essential modelling phase a lot of effort is put into the creation of Message Flow
Diagrams, Object Class Diagrams and Instance Structure Diagrams. These various parts
of a model all offer a particular view on the system that is a part of the preparation for
the creation of Unified Model. From the point of view of the preparation of the Unified
Model, we summarise the use of the various models. Notice however that the creation
of a POOSL description is not always a last step in the process. It is recommended to
perform some try outs, especially for the core of the conceptual solution. It appears that
intermediate attempts offer a lot of additional understanding that can prevent costly
iterations in a later phase.
The various models prepare the Unified Model as follows:
Object Class Diagrams. They show the process classes as well as the data classes that
must be described in the Unified Model. Attributes are in general implemented
as variables. The messages are defined in message interfaces of classes. (The
dynamic use of messages is described in the methods.) Relations between classes
are in general implemented as properties of classes that must be implemented in
its internal behaviour.
Instance Structure Diagrams. The hierarchical structure of clusters found in ISDs is
used to create the System Specification in the Unified Model. The ISDs channel
names are used as outer channel names that must be coupled to inner channel
names of instances. Boundaries must be used to determine the style of behaviour
description of a class.