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

178 Modelling of Concurrent Reactive Behaviour
to send a message to one arbitrary object out of a collection of objects (see Subsection
6.3.8).
Delay Model
The solution for the problem had to wait until the semantics of POOSL was extended
with a delay primitive [Gei96]. The delay operator can define a stop criterion for multicast
behaviour. Until a specified period of time is over an arbitrary number of send statements
can be performed. Hence, during that period of time an arbitrary number of rendezvous
with the receivers can be performed.
Weakly Distributed Model
If the receivers can be known or if the number of receivers can be known, another way to
construct a multicast is possible. Although this is not a natural multicast we can express
the behaviour of the sender as a sequence of send statements performing the rendezvous
with all receivers. The messages can be sent with an explicit use of the identifiers of the
receivers. In this case the sender must know all these names.
Willingness
Our concept of multicast is that receivers that do not want to accept, or that are not ready
to accept, do not receive the message. A sender can only continue its behaviour, when
all the message passing with all potential receivers is completed. So we have to make
sure that receivers that are not ready or are not willing to receive do immediately finish
the rendezvous and discard the message. We can force objects to do this by defining
an interrupt message that finishes the rendezvous of the multicast. When the object is
willing to receive a multicast message on channel ch, the object is ready for the execution
of a receive statement:
ch?multicast message name
When the object is not ready or willing to receive, the actual behaviour must be specified
with an interrupt:
(S1; ¡ ¡ ; Sn) interrupt ch?multicast message name
After the interrupt the behaviour is resumed where it was interrupted. The message
is discarded. It is simply ignored, because there is no additional behaviour for the
interrupt.
A Buffered Model
Another possible way to model multicast communication comes from the recognition
that a multicast is rather asynchronous. We can model a buffered message flow to all
potential receivers, with a one place buffer. Now the sender can finish all the rendezvous
with all potential receivers without waiting. This model, however, allows a receiver to
read its buffer after some time. This does not match to the intuitive notion of an
instantaneous character of a multicast. We would need some real-time primitive again
to clear the buffer when the receiver does not react immediately.