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

220 Modelling of Concurrent Reactive Behaviour
¢ ¥ Bspec1 Bspec2 Bspec§ Bspec
L and f specify the parallel composition of processes
and/or clusters, hiding and renaming. The meaning of these composition specifications
is analogous to the composition specifications of clusters.
6.6.8 Object Collaboration and Behaviour Styles
In Section 4.8 about properties of composites we showed aspects of object collaboration.
The properties were defined in an abstract way. In the preceding part of this chapter
we defined the possibilities for the description of dynamic behaviour. This enables
to revisit various concepts related to composites from the point of view of behaviour
description and communication. It appears that various sorts of collaboration and
various design decisions lead to the description of the behaviour of objects in particular
styles. Identifying sorts of styles is an interesting issue for future research. In this
subsection we describe some examples of the mutual influence that communication,
behaviour and system structure have on each other.
6.6.8.1 Black Boxes and Glass Boxes
The promise of reuse did not come true with the object-oriented paradigm. It was
recognised that classes are not reusable without additional effort. Approaches such as
frameworks [Joh94] focus on making groups of collaborating objects reusable as a whole.
In general subsystems are not black boxes that you can reuse. The concept of black box is
used for a module that can be used properly based on a description of the interface with
the environment. In general the modules inside a black box need some adjustments
when the black box is used in a new application. Glass boxes or white boxes are modules
designed for reuse. They allow us to inspect their internal structure and behaviour.
Clusters offer hiding of internal structure and behaviour. Aggregates hide their parts.
Strongly distributed modules have autistic communication and enable to construct black
boxes. Processes in the environment can communicate by sending and receiving messages
without knowledge of the internal objects of the module.
6.6.8.2 Collaborating Data Objects
Data objects have sequential behaviour. Collaborating data objects are waiting for each
other. The communication is always in the form of a message with reply and waiting.
Data objects can form very dynamic structures by passing references to each other. A
model which contains only data objects can in principle model a software system. Data
objects are however contained in process objects. Process objects use data objects locally
and transfer data objects with messages. Data objects that are transferred from one
process to another have been defined as travelling objects.
The behaviour of processes can be interrupted or aborted. Notice that the behaviour of
data objects cannot be interrupted. Data objects and process objects have different sets
of statements. The statements defined for data objects can be used as process statements