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

158 Modelling of Concurrent Reactive Behaviour
Asynchronous communication with reply:
Without waiting for confirmation the sending object continues performing its
own activities, while the receiving object prepares a reply. A time-out may
notify the sender that the receiving object did not reply within a predefined
time interval. The sender must decide what to do with results that come back
too late.
Continuous communication:
The sending object is always (continuously) prepared to deliver a message.
The receiving object must take initiative to read a message from a sending
object.
Interrupting communication:
The sending object forces the receiving object to suspend its current activity.
Suspension may be:
an abort, which means that the current behaviour of the receiver will not
be resumed;
an interrupt, which means that the current behaviour of the receiver can
be resumed after temporarily having performed an alternative behaviour.
Besides pair-wise communication there are also forms of multi-way communication. We
describe two sorts:
Broadcast communication:
A send operation into a broadcast medium causes simultaneous offering of a
message to all objects that are connected to the medium. All receiving objects
can simultaneously accept the message.
Multicast communication:
A generalisation of broadcast where messages are targeted to a subset of the
objects connected to the broadcast medium.
In the next subsections we examine how the presented forms of communication can be
expressed in our method.
6.3.3 Communication between Objects
6.3.3.1 Messages and System Analysis
Messages play a crucial role for the creation of a complete system model. Messages
are needed because objects encapsulate their internal data. This prevents direct access.
There is no sharing of this data with other objects. Any wish to access a data item in
another object needs the indirect mechanism of sending a message to that object. A
message to an object may be a request for giving a data item (which is an object itself),