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

6.3 Communication 171
(normalCourseOfBehaviour) interrupt channel!temperature(real)
In this construct (normalCourseOfBehaviour) is a collection of one or more process statements
that perform some behaviour. This behaviour can be interrupted any time to
finish a rendezvous with the requester of a temperature(real) message.
Recall that runk is an example of one of the unknown objects. These are built-in primitive
objects in POOSL. When a message transports more complex objects, we are ourselves
responsible for the definition of unknown values.
Summary
The continuous flow symbol is useful for the modelling of a continuous stream of
messages or values. The meaning of the flow is that the producer produces continuously,
and the consumer can consume at any moment. This means that the producer:
sends a message instantly on any initiative for a rendezvous by a consumer of the
flow;
refreshes the message instantly after each rendezvous with a consumer;
sends a message containing the instantaneous value of the continuous source at
the moment of the rendezvous, during the time interval that the flow must be
produced;
sends a message containing an unknown value, during the time interval that the
flow must not be produced.
6.3.6.4 Buffered Message Symbol
The buffered message symbol models asynchronous communication. Asynchronous communication
means that a sender can send without knowing whether the receiver is willing
to receive. This sort of communication is modelled by the introduction of a buffer.
Therefore the symbol is a flow with a buffer symbol near the arrow head, see Figure 6.7.
A problem with this communication concept is that the receiver may or may not accept
client retrieve(PersonName) dataBaseA
Figure 6.7: A Buffered Message Flow
the message within a given time period. The sender, however, can always accomplish
a rendezvous with the buffer; the buffer always listens. The receiver cannot always
perform a rendezvous. This depends in the first place on the state of the receiver, but