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

164 Modelling of Concurrent Reactive Behaviour
areYouOpen. The result of the operation is returned to the sending object. This example,
extracted from our case study, concerns a data object that models a mechanical
safety bridge that must be closed before the machine is allowed to run. Another example
concerns a data object that represents personalised product information. Such an object
can be requested whether some service, such as the feeding of a particular magazine,
must be performed. A feeder could need the send expression:
productInformation doYouNeedService(actualServiceOffered)
This expression means: Send to object productInformation, the message doYouNeedService 2 ,
with the parameter actualServiceOffered. The result of the operation is returned to the
sending object.
Recall that data objects do not communicate via channels, and that data objects are not
visualised in Message Flow Diagrams or Instance Structure Diagrams. Their communication
is modelled by a formal behaviour description in the language POOSL. Data
objects are generally used to model relatively simple and dynamic items in the problem
domain. They behave sequentially and can only be used in a sequential behaviour
description. In a system specification they are used within the behaviour description
of the process objects. This is consistent with the fact that process objects are internally
sequential.
6.3.6 Graphical Communication Primitives for Process Objects
This subsection describes the graphical primitives that enable visualisation of communication
between process objects in Message Flow Diagrams. Process objects are
independent concurrent entities that can choose whether they want to communicate or
not. They always perform their communication via channels. Synchronous communication
is a transport between objects via a channel, based on the coinciding readiness to
execute individually specified communication actions. In contrast to data objects whose
message passing is expressed in a single message-send expression, process objects need
two independent descriptions. The one way synchronous message passing primitive is expressed
in terms of two separate statements, one in each process object. There is a
message-send and a message-receive statement. A simultaneous instantaneous execution
of corresponding message-send and the message-receive statements results in a
rendezvous. The statements correspond if they refer to the same channel and the same
message. POOSL offers a clear way to express synchronous message passing. The
sender specification contains a message-send statement of the form:
channel-name ! message-name (parameters)
The receiver specification contains a message-receive statement of the form:
2 We recommend to begin message names and variable names with a lowercase character, and class
names with an uppercase character.