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

186 Modelling of Concurrent Reactive Behaviour
We defined a collection of message flow symbols. They can graphically represent communication
between process objects and clusters in so-called Message Flow Diagrams.
These symbols are designed to be understood intuitively.
We showed that all forms of process communication can be formalised by mapping
them on the one way synchronous message passing primitive of POOSL. We described
various alternatives to specify the destination of messages between process objects, and
multiples. We described how channel names and identifiers of process object can be
used to create static and dynamic links between process objects.
We described the communication between data objects. They always use a double
rendezvous to realise synchronous communication with reply and waiting.
6.4 Distribution
6.4.1 Introduction
Distribution is a key issue in present-day research for complex system design. Production
systems, industrial control systems and automated teller networks, for instance, are very
often distributed systems. Even many consumer products are distributed systems internally.
A distributed computer system has its resources dispersed across independent
computers (subsystems) connected through a communication network. These resources
may be programs, pieces of hardware, data resources, peripherals or specific interfaces
to the environment. Distribution may be logical, physical or both, it may be weak or
strong, and the subsystems may be statically or dynamically linked. In this section we
describe the concepts for distributed modelling.
6.4.2 Distribution and System Design
The architecture model of a system can define a distribution structure. Physical distribution
is often simply prescribed by a real physical topology, e.g. topology of an
existing infrastructure, or by the physical (geographical) embedding of the system to
be designed. In these cases structure is a strong requirement that should influence
the development process as early as possible. This is an important reason to integrate
system specification with system design. During analysis of a complex system we use
abstract entities such as objects as the vehicles that enable a dispersion (distribution) of
the overwhelming amount of details in the problem domain. A method must be able
to match the two completely different forms of distribution: the conceptual (logical)
distribution and the implementation-oriented physical distribution.
A distributed architecture requires a specific sort of decomposition of a system into
subsystems. This can be specified by defining subsystems with distribution boundaries.
Any object that is found during continuing analysis must find its place in one of the subsystems.
We must prevent that objects appear to be cut by a distribution boundary. We