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

6.3 Communication 183
6.3.9.4 Modelling Aspects
Where data objects are very flexible with their dynamic links, process objects require
static channels. The rigidity of the channel structure may give the feeling the SHE
method is not powerful and flexible enough to describe a wide variety of systems, especially
software systems. The channel concept is introduced to describe static structures
that are especially needed in hardware designs. There is however no fundamental
constraint on our method. Nothing forbids us to make all process objects fully interconnected.
Even one channel is sufficient to do this. As long as we define enough unique
message identifiers we can perform the necessary communication. If we do so, the object
identifiers can be used for full dynamic communication. When all process objects are
connected to a common channel we create in fact a common address space. In the next
subsection we show a conditional message-receive statement that enables addressing of
objects in the common space. So the concept of static channels does not limit the descriptive
power of the POOSL language. Of course this is a pure theoretical discussion.
We do not want to recommend to structure a model this way. On the contrary, we expect
that the use of channels can also structure a software system in a useful way.
6.3.10 Behaviour Description of Communication
6.3.10.1 Description of Process Object Communication
Because process objects can have static links as well as dynamic links, we can identify
the following situations:
1. There are precisely two objects connected to a channel. They can communicate:
a) pair-wise without the use of identifiers (autistically);
b) pair-wise with the use of identifiers. The sending object must know the
receivers identifier, and must use it explicitly in the send statement.
2. There are more than two objects connected to a channel. These objects can communicate:
a) pair-wise non-deterministically (without the use of identifiers). Any receiving
object that accepts the message can finish the rendezvous.
b) pair-wise with the use of identifiers. The sending object must know the
receiver’s identifier, and use it explicitly in the send statement.
The communication in cases 1.a) and 2.a) can be performed on static links. Cases 1.b)
and 2.b) require dynamic links as well.
The communication between a pair of process objects requires the simultaneous execution
of a pair of matching statements: a message-send statement in the sender and a
message-receive statement in the receiver. There are various POOSL constructs that can
be used to describe communication. The main difference between the various constructs
is in the determination of the actual receiver among all potential receivers on a channel.