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

6.8 Summary 227
They can graphically represent communication between process objects and clusters
in so-called Message Flow Diagrams. These symbols are designed to be understood
intuitively. All forms of communication can be formalised by mapping them on the one
way synchronous message passing primitive of POOSL. Channel names and identifiers
of process objects can be used to create static and dynamic links between process objects.
The concepts of physical and logical distribution support system structure design. Physical
distribution is specified by distribution boundaries on clusters. The specification
of distribution is related to communication, more specific to name spaces of process
object identifiers, and to the use of channels. Weakly distributed modules communicate
using object identifiers to determine the receiver of a message. In contrast, strongly distributed
modules communicate autistically via static links. Channels, message names
and process object identifiers are used to specify either static or dynamic links between
collaborating entities. Considered use of these concepts is necessary to specify a distribution
structure.
Scenarios are used to conquer complexity and to integrate a multidisciplinary approach
into our method. Scenarios are an approach that extend the interpretations incorporated
in other methods. The use of scenarios shortens system development time. A considered
set of scenarios offers adequate abstractions that greatly improve the efficiency of the
communication with various experts.
Various concepts to reason about state on various levels of abstraction have been defined.
Named states represent behaviour that produces responses on messages representing
events. This approach integrates aspects of event-orientation found in Structured Analysis
with the object-oriented paradigm.
An introduction to the expressions and statements that POOSL offers for the definition
of data classes, process classes, cluster classes and systems completed the description
of the modelling of dynamic behaviour. Instances of process classes can perform concurrent
and/or infinite behaviour. Behaviour is defined as a collection of methods.
Interrupt, abort and choice statements are constructs that give our method the properties
to describe systems adequately on a high level of abstraction. Clusters and systems
are described as the parallel composition of instances of process classes and cluster
classes using channel hiding and renaming. Data objects enable the description of dynamic
data structures. Data is stored in instance variables and local variables. On the
reception of messages, corresponding methods are activated. Interaction of data objects
is sequential. Various statements such as assignment statements, ’if’ statements and ’do’
statements have been presented.
Real-time requirements can be specified on system architecture level. The incorporation
of time primitives and the development of tools will be covered in a separate Ph.D.
project.
The material of this chapter, together with Chapters 4 and 5 forms the basis for the
method framework that is described in Chapter 11. All concepts have been defined and