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

154 Modelling of Concurrent Reactive Behaviour
The modelling of interleaving semantics define concurrency formally as a statement
about the arbitrary ordering of actions:
a ¢ b ab ba
The parallel composition ( ¢ ) of actions a and b is interpreted as the behaviour in which
action a precedes action b or ( ) in which action b precedes action a.
This semantic model can be thought to use a time scale with a fine enough grain. Actions
can be mapped upon this time scale, so that each action has an unique point in time. The
only actions that can be performed simultaneously are the joined actions that together
result in a synchronous communication.
This model does not work for the parallel composition on the level of processes. In
general the parallel composition of processes A and B cannot be modelled by interleaving
of the behaviour of the processes as a whole:
¢ A ¡ B AB BA
We cannot accept that the complete behaviour A is performed before B or vice versa.
Such processes would not be concurrent at all. Notice also that the behaviour of a process
may be non-terminating. In this case it would be possible that the other process could
never perform any action. So the interleaving should not be on the level of processes
but on the level of the atomic internal actions performed by processes.
In contrast to events or actions that happen in an arbitrary order, the actions performed
by parallel processes in general have a real overlap in the time. To make the semantic
model more realistic, it has to be extended with a notion of time. We come back to this
subject in Section 6.7.
6.2.3 Synchronisation
Communication between both process objects and data objects is realised by synchronous
message passing. If two objects pass a message, they are engaged in a rendezvous.
This is the moment that both objects synchronise their activities.
6.2.3.1 Non-Determinism
America [AR89a] identifies the main problem of concurrency to be non-determinism. Synchronisation
and mutual exclusion are mechanisms that can reduce non-determinism. A
well known solution to extend sequential object-oriented languages with parallelism is
to add processes as orthogonal concepts 1 . America states that the addition of processes
as orthogonal concept is not a proper way to solve the non-determinism problem.
Having processes as an orthogonal concept enables having more than one object active
1 This notion of process is another concept than our process objects.