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

168 Modelling of Concurrent Reactive Behaviour
6.3.6.2 Message with Reply Flow Symbol
The message with reply flow symbol represents a synchronous message passing with reply,
which is a two way communication. The flow symbol has two arrowheads (see Figure
6.5). The end with the arrow heads is at the receiving object. The backwards directed
second arrowhead represents the reply. This flow symbol simplifies the Message Flow
Diagrams because it replaces the two single message flows that would be necessary
otherwise. The semantics of the symbol are restricted to synchronous communication with
reply and waiting. The message name is denoted on the flow. This message name can be
followed by a list of parameters, and a list of possible answers (message names) which
is preceded by a slash symbol. The parameters are data objects which are denoted by
their class names. Figure 6.5 shows that a message with message-name serviceOffered can
serviceOffered(ServiceName)/
process A process B
required,notRequired
Figure 6.5: A Message with Reply Flow
be sent. The message contains a data-object of class ServiceName. The sender expects
one of the two possible answers which is either the message required or the message
notRequired.
The message with reply flow symbol is very convenient to describe a client-server model.
We need only one flow to connect the client to the server and we can specify the request
together with the accompanying information and the possible answers.
Notice that there is a difference with the description of communication with data objects.
There we use a message-send expression, and do not have a channel. The sort of
communication seems to be similar: synchronous communication with waiting and
reply. However there is a very fundamental difference. A message to a data object
is tightly coupled with the activation of the corresponding behaviour of the object
(correspondence between message-name and method in the data object). Recall that
process objects have a conceptual separation between the readiness to receive a message
and the possible activation of a method. By doing so the receiver is not forced any more
to give an immediate answer. The message with reply flow symbol specifies waiting. So
although this is a powerful primitive for analysis, the consequence is that we must be
careful when writing the corresponding behaviour.
The POOSL behaviour description of the message with reply flow needs two messagesend
and message-receive statement pairs. The designer must be aware of the fact that
he is responsible for the explicit specification of the waiting of the sender (= client).This
can easily be realised. The message receive statement for the reply must be the first