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

6.6 Dynamic Behaviour 203
processes in software are in general mutually asynchronous.
Currently accepted object-oriented methods [CY91a, CY91b, J 92, Boo91, SM88] model
dynamic behaviour with the use of conventional state machine models that are similar
to the older structured analysis and structured design techniques [WM85, HP88].
OMT [R 91] is explicitly based on Statecharts [Har87]. However, the designers of
OMT seem to have neglected the incompatibility of object-orientation and Statecharts.
In OMT dynamic behaviour is modelled by assigning a state diagram to each object
class. Although this seems to offer modularity and encapsulation, it permits objects to
communicate by direct monitoring of each other states, thereby violating encapsulation.
That this monitoring is allowed is confirmed by the following citation [R 91]: ’Guarded
transitions for one object can depend on another object that is in a given state.’ OMT does
not offer any decent primitive for inter-machine communication [HC91] and hence the
support for concurrency is doubtful.
Alternative approaches
After having observed the above problems we set the goal to find a way of dynamic
modelling that blends with object-orientation. There are various interesting approaches
for the modelling of dynamic behaviour. The methods we studied are LOTOS [Tur93,
Pir92, Mor94], SDL [Tur93, BH93], Estelle [Tur93, D
89] and ROOM [SGW94]. We only
mention some aspects briefly, since most aspects have been described already at other
places in this thesis.
The above methods use autonomous, independent, encapsulated concurrent entities
that are connected in a static topology of channels. They communicate using welldefined
interaction primitives. Communication in SDL, Estelle and ROOM is buffered
asynchronously. A sender can always send a message without knowing the readiness of
the receiver. Messages are either buffered or discarded. Buffered asynchronous message
passing is certainly a valid alternative for synchronous message passing. Recall however
that asynchronous message passing can be expressed with a synchronous primitive.
Expressing synchronous communication with an asynchronous primitive is a lot more
complex.
Comparison with our approach
Our process objects use synchronous message passing for the specification of reactive
hardware/software systems, because of the efficient abstract modelling power. The
dynamic interaction but especially the synchronisation of the independent system components
is modelled very elegantly with synchronous message passing. Synchronous
interaction is also offered in LOTOS by a basic primitive for undirected (multi-way)
action synchronisation. This primitive can be used to express directed message passing
[Mor94], but in a less natural way.
The behaviour of our process objects can be aborted or interrupted. The LOTOS language
supports an abort, but not an interrupt construct. ROOM supports both aborts and
interrupts. SDL and Estelle do not have interrupt facilities. The grain of concurrency in