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

74 Abstraction of a Problem Domain
objects and communication flows are visualised in Message Flow Diagrams, Process
objects and their Channels are visualised in Instance Structure Diagrams. The symbol
that denotes a process object is shown in Figure 4.9. The class name of the process, and
its possible identifier are denoted in the symbol. In our examples we will often dissent
from this rule and denote an object name and the purpose of the object.
4.4.3.4 Instance Structure Diagrams
Process X a Process Y
b
Process U Process V
Figure 4.10: Instance Structure Diagram
c
Process W
Process objects communicate via channels. Channels and messages accepted on channels
are defined in the message interface of a process class definition. The channel
interconnect topology of process objects can be visualised in so called Instance Structure
Diagrams (see Figure 4.10). The static interconnect structure both models a static structure
of communication between concurrent instances (process objects), and a structure
that may be imposed by architecture requirements. Figure 4.10 shows channels such as
a and c between pairs of process objects. Channel b connects four objects as a backbone,
and typically reflects an architecture structure. All messages between V and W are
transported via channel c. Instance Structure Diagrams are useful as a visual aid for
changes in topology. During the analysis and design process Instance Structure Diagrams
are transformed so that they can represent and formalise an architecture and/or
implementation structure of a system.
4.4.3.5 Message Flow Diagrams
Besides Instance Structure Diagrams we offer Message Flow Diagrams. Together they
form an Object Instance Model of a system. Message Flow Diagrams visualise process
object instances and their communication flows. A class in an Object Class Diagram
visualises the collection of all possible instances on the same spot. Relations visualise
only a possible need for communication with an instance of another (related) class. In
contrast, instance models visualise instances in a concrete topology of flows or channels.
This topology can be designed in such a way that real system constraints, such as
implementation topology and distribution topology, are met.