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

180 Modelling of Concurrent Reactive Behaviour
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Figure 6.14: Common Channels on Multiples
The first is non-deterministic acceptance of a message to a multiple by one of the objects
in the multiple. An object can send a message without a destination-identifier via a
channel to a multiple. A channel to a multiple is always connected to all objects of
the multiple (see Figure 6.14). However, only one of the objects in the multiple can
really do a rendezvous with the sender and thus receive this message. A receiver can
possibly reply its identifier to the sender. After that the sender knows which particular
object from the multiple has received its message. This object becomes a (temporary)
acquaintance of the sender this way.
The second way to communicate with multiples is that all free objects from a multiple
can take initiative themselves to advertise their availability, and thereby send their
identifiers to an object that may want to use them. Sender and receiver both can know
each other’s identifiers now. This is what will appear to be a fully weakly distributed
description approach in Subsection 6.4.4.
6.3.9 Communication Links
6.3.9.1 Static Links
The communication between process objects is set up via channels. However the existence
of a channel between two process objects does not mean that they will ever
communicate. An additional condition is that the object interfaces contain corresponding
messages. To describe whether both conditions are fulfilled for communication we
define the concept of link. A link defines a conceptual connection between two objects,
which means that the conditions for communication are fulfilled. However, it is not
required that communication actually happens.
For a more precise definition we considered to require that the objects must have the
adequate behaviour description besides enough matching information in their message
interface to actually communicate. A message interface specifies which messages can
be sent or received on which channel. We can base the definition of a link purely on
the message interface. This offers the possibility to verify quite easily whether a link
exists or not. If we define a link based on behaviour it is difficult to verify that a link is
available. Even if there are statements to perform the behaviour we still do not know
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