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

6.3 Communication 163
Synchronous message passing is a quite primitive concept. We would like to have
primitives that are more expressive. During analysis we must be able to show and
recognise the various forms of communication easily. In the next subsections we will
show how we use this concept of synchronous message passing to define and build the
various forms of communication we need.
6.3.5 Data Object Communication
The communication with a data object is strictly limited to one sort. This sort is synchronous
communication with reply and waiting. The sender may be either a process object
or a data object. POOSL offers a simple way to express this. The sender specification
contains a message-send expression in the form of:
destination-expression message-name (parameters)
The destination-expression is evaluated by the object that plays the role of sender. The
message-name is the name of a service offered by the receiver. The message may contain
parameters in the form of expressions that are evaluated before the message is sent.
The sending object suspends its operations and waits until the receiver sends the result
of its operations to the sender of the message. So the result of the evaluation of the
message-send expression is that the result from the receiving object is returned to the
sending object. See Figure 6.1. A formal semantics for synchronous communication with
Data object A
(sends and waits
for reply)
Message
Reply
Figure 6.1: Communicating Data Objects
Data object B
(activated after
message
reception)
reply and waiting could be built from two one way message passing primitives. A first
rendezvous is necessary to hand over the message from sender to receiver. A second
rendezvous is necessary to hand over the result from receiver to sender. In reality the
formal semantics is described using another operational model (see Chapter 8).
Users of the method do not have to know about the details of the formal semantics. A
behaviour specification may be for instance:
bridgeContact areYouOpen
This message-send expression means: send to object bridgeContact, the message