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

5.8 Boundaries 139
In Chapter 10 we introduce Behaviour Preserving Transformations. By mapping boundaries
on clusters, these transformations can be used as basis for the formal restructuring
of boundaries as well.
In general the various forms of boundaries will coincide at various places in a model.
Coinciding boundaries specify that corresponding properties coincide. This will impose
more constraints on the internal behaviour description. Transformations on clusters that
represent coinciding boundaries, require precise and independent consideration of the
various properties. New clusters may be necessary to prevent intersection.
5.8.3.3 Reuse
The big advantage of an implementation-oriented description style as we propose here
is the fast and easy path to implementation. A disadvantage however is that it hinders
reuse of clusters and process objects.
Abstraction boundaries that represent an aggregate will not cause problems. In general,
being an aggregate is simply a property. There will be no conflict in reuse. Concurrency,
distribution and implementation boundaries possibly prescribe a specific behaviour,
that gives reuse problems. A cluster behaving internally sequential, can never be reused
for a situation where internal concurrency is required. For various implementations it
depends on the differences in style and the specific constraints on the use of POOSL
whether the modules can be reused or not. In general, reuse can be improved by
minimising the number of classes. We observed that classes with only one instance are
not rare in the systems we aim at. In this case there is no reuse problem. However, reuse
is interesting enough for future research.
5.8.3.4 Black and Glass Boxes
During analysis domain boundaries may be preliminary enclosures of subsystems that
must still be refined. A domain boundary may become a cluster or an object. A domain
boundary appears to be a concept for analysis of the problem domain or so-called
domain of discourse. Finally domain boundaries dissolve in the model into clusters
or objects. During our research we produced alternative names for the concept of
domain boundary. Illustrative, but rejected names were ’simple cluster’ and ’primitive
cluster’. These names denote that clusters are in fact rather weak boundaries. There is
a strong inward visibility. They do not encapsulate or hide their internal objects until
their properties are restricted. An example of such a restriction is: define that a cluster
represents an abstraction boundary. Such a cluster limits the inward visibility.
Traditionally an abstraction of a module without inward visibility is called black box. A
black box is a module that can be applied when the knowledge about the component is
restricted to its external interface. Black boxes are also called ’plug-in components’. The
black box model is useful until its reuse requires knowledge of its internal structure.
In contrast to black boxes there are white boxes, also called more ’clearly’: glass boxes