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

6.3 Communication 181
whether the system actually ever executes them, because this can depend on events from
the environment of the system. Therefore we choose for a syntactically based definition,
where we can check the existence of a link statically (at compile-time).
A static link between two process objects exists when the specifications of the message
interface of these two objects contain matching information. This means that the objects
are connected to a common channel and both have at least one message in common for
that channel.
Processes with a static link have a so-called corresponding message interface. They can at
least communicate one message. Notice that if there are more messages in common for a
channel, then there is still only one link. If there are more channels between two objects,
any of these channels can offer an additional link.
The communication between process objects is possible without the use of object names
(identifiers). A sender can simply send a message to a channel without knowing the
identifier of the receiving object. We define an object to be autistic 4 when this object
communicates solely without the use of identifiers (names) of its communication partners.
Autistic communication is message passing without the use of identifiers for the
determination of sender or receiver (see also Subsection 6.4.4).
Actual communication means the actual usage of a static link. Communicating process
objects execute matching message-send statements and message-receive statements.
These statements can be constructed in various ways. The communication can be
defined to happen conditionally depending on the name of sender or receiver. There
are differences between the communication of data objects and of process objects. The
next paragraphs describes some aspects.
6.3.9.2 Dynamic Links with Process Objects
The use of identifiers of objects in the description of communication gives the ability
to send a message to one specific object from a collection of objects connected to a
common channel. When an object A has the identifier of another object B, then B is an
acquaintance of A. Having an identifier of another object can be a necessary condition
for communication with that object. Identifiers can be passed between objects. So an
object may become an acquaintance. This opens a way to dynamic linking of objects.
Local changes of information can influence the communication behaviour in time.
Direct communication between process objects is not possible without a channel. The
first intuition about a dynamic link is that it can be created dynamically, starting from
the absence of a link. However, channels are defined statically. So a dynamic link
between process objects needs a static link to exist. A dynamic link between a pair of
process objects is a directed relation that is determined by the fact that one object has
the other object’s identifier for the purpose of communication. Process objects have user
4 Source of term autistic: anonymously communicating (autistic) processes [Weg90].