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

6.3 Communication 177
process A process B process C
m
m
sender
Figure 6.12: Multi-Way Communication
receivers. We denoted m three times at the diverging flows. An alternative is to write it
only once, at the common part of the flow.
We identify two types of multi-way communication:
Broadcast communication. A send operation of a sending object on the broadcast
channel causes the delivery of the message at all objects that are connected to the
channel, and that are willing to receive the message.
Multicast communication. A generalisation of a broadcast, where messages are
targeted at a subset of objects connected to the broadcast channel.
Multicast is a generalisation of broadcast because we can describe broadcast as a multicast
to all receivers.
The common notion of multicast is asynchronous; the sender can always send, and
neither knows who is receiving, nor gets a confirmation about reception. The fact that
the sender can always send was the main item in the definition of asynchronous flow.
Synchronous Message Passing Model
We searched for a possible way to model multicast with synchronous message passing.
We would like to express the senders behaviour as a single message-send statement.
However, if the receivers all have a matching message-receive statement we have a
problem. The execution of the message-send statement in the sender is paired with
only one matching message-receive statement. So this construction describes a nondeterministic
behaviour. Non-deterministically one of the receivers will accomplish the
rendezvous, while all others cannot finish the communication. So we cannot realise a
synchronous multi-way communication straightforwardly this way. The construct is
however interesting for a completely different application. It is useful when we want
m