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

6.3 Communication 165
channel-name ? message-name (parameters ¡ condition)
The exclamation mark denotes the notion of offering a message. The question mark
denotes that the statement asks for a message. The channel name can replace the need to
know the name of the object at the other side. The simple existence of a channel is enough
to be able to communicate. (See Figure 6.2 POOSL rendezvous). The channel with name
a connects a sender process and a receiver process. The sender’s behaviour description
contains a message-send statement a!ready. The receiver’s description contains a?ready.
The message ready is handed over on the moment that both POOSL statements can be
executed simultaneously. It is possible to pass the name of a destination process as a
Sender:
a!ready
a
Receiver:
a?ready
Figure 6.2: POOSL Rendezvous
parameter. In Section 6.4 about distribution, various aspects related to this subject are
covered. The condition in the message-receive statement gives the expressive power
to perform a conditional message reception. In this case the message is only received
if the condition expression evaluates to true. The message-receive statement waits for
execution if the condition expression evaluates to false.
Notice that we did not redefine one way synchronous message passing. We use this
concept consistently with the definition in Subsection 6.3.4. We showed how it can be
expressed and used on channels. The next question is: how can we use this primitive to
show and describe the various sorts of communication on channels? We offer a collection
of so-called message flow symbols for use in Message Flow Diagrams. The message flow
symbols are presented in Figure 6.3. Each of the symbols represents a possible flow of
messages between process objects.
6.3.6.1 Single Message Flow Symbol
The single message flow symbol (see Figure 6.4) represents a very basic communication
primitive, which is synchronous communication without reply. This basic primitive corresponds
directly to one way synchronous message passing. Notice that without reply does
not mean that the sender misses confirmation about the success of the message passing.
The mere fact that the communication took place makes sure that the other party participated
successfully. The information in the message may be substantial in the form
of data (objects), or may be simply the arrival of the message with a particular message
name without data.
Data transport is modelled by denoting the class names of the data objects that contain