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

6.2 Concurrency and Synchronisation 155
at the same time. However it also introduces more non-determinism. A problem is that
processes can act on the same objects and can even execute the same method in the same
object at the same time. The non-determinism is caused by the sharing of resources.
To reduce non-determinism it is common to use semaphores for synchronisation and
mutual exclusion.
The sharing problem is an indication that processes as described above do not match very
well with the concept of object. Objects should prevent sharing by strong encapsulation.
The object’s message interface should be used as the only entrance to resources of the
object.
6.2.3.2 Restriction of Non-Determinism
Non-determinism makes it very hard for designers to keep overview on the behaviour
of a concurrent system. The problem is very well described by Selic et al. [SGW94]
when they state that ’It is impossible to get an adequate mental model that keeps overview of
the relative progression of the threads. When we also take into account real-time aspects and
the fact that in a distributed system the modules tend to have different execution speeds...’ In
other words the huge number of possible orderings of actions and the huge possible
state space is the biggest problem in the design of concurrent systems.
We restrict synchronisation of concurrent threads to one mechanism: synchronous message
passing via the message interface of process objects. The composite behaviour of
processes is completely determined by the synchronisation achieved by message passing.
The already mentioned coincidence of the grain of abstraction (encapsulation) and
the grain of concurrency, removes sharing and makes that synchronous message passing
is enough to perform all necessary synchronisation and communication between
process objects.
The problem of modelling and understanding behaviour is now restricted to:
the local (internal) sequential behaviour of processes;
the composite behaviour of collaborating processes.
So the interactions can be studied on two levels: inside and outside of processes. The
inside behaviour must be able to handle all possible sequences of messages. The outside
behaviour is determined by observation of messages on channels. All the interactions
via the object’s message interface are analysed by playing scenarios of sending and
receiving messages between collaborating objects. That is why our method pays a lot of
attention to the modelling of message flows. The various forms and their visualisation
are described in Section 6.3.
Where on the one side the amount of non-determinism is limited by making the message
interface the place for all possible communication there are other aspects that determine
non-determinism inside objects. First we offer a non-deterministic choice statement.
This statement requires to consider all possibilities of alternative behaviour. Secondly we