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

44 Concepts for Analysis, Specification and Design
Sender
process
Single message flow
Message with reply flow
Continuous flow
Interrupt message flow
Interrupt with reply flow
Buffered message flow
Figure 3.8: Message Flow Symbols
Receiver
process
of each individual object and consider the events that can happen, such as sending and
receiving various possible messages. Behaviour is determined by the events that happen,
and in which order they happen. Based on reception of a message an object changes its
state and possibly performs calculations and produces results, replies or commands.
System behaviour is modelled by a collection of concurrent process objects. However
processes can be specified to wait for a result after sending a message. This way we
can perform the whole behaviour of collaborating processes as sequential behaviour. In
general, a mix of concurrent and sequential behaviour is performed in a complex system.
Humans easily lose the overview on such a system. In practice it is precisely this problem
that causes so many design errors in complex systems. There are no complete solutions
for this problem. Simulation and automatic verification can help to diminish the number
of errors in a design. This is only possible if the behaviour of the objects is formally
described. This implies that a computer can perform the execution of the model. The
method SHE incorporates the language POOSL which can describe system behaviour as
well as system structure. System behaviour can be modelled adequately because POOSL
has the power to describe the communication between parallel processes on an abstract
level. POOSL offers various powerful mechanisms such as non deterministic choice,
interrupt, infinite behaviour, guarded commands, and branching statements such as ’if
then else’ statements.
Process objects communicate via statically interconnected channels. Figure 3.10
visualises this for the Product Input Handler. Note that the message flow
wakeAt(Position,Id)/wakeup(id) from Figure 3.9 represents a message and a reply. So
there are two messages in opposite direction on channel wake of Figure 3.10. Notice
further that both the message flows productArrived and currentPosition(Position) from
Figure 3.9 are mapped on one channel named piti in Figure 3.10.
All message flows that we showed in Figures 3.7 and 3.9 are mapped on such channels.