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

160 Modelling of Concurrent Reactive Behaviour
be sent at some moment in time from one process object to another. Channels between
process objects are the vehicle for message transport. We do not show them in a Message
Flow Diagram. When we need a message between two process objects we implicitly
define the need for at least one channel between them. The channels are visualised
in alternative graphical representation called Instance Structure Diagrams. They show
channels, process objects and clusters. More details about diagrams and the heuristics
to make them are described in Chapters 11 and 12.
6.3.3.2 Messages and Methods
Usually the concept of message is strongly related to the concepts of service or operation
(implemented by a method). Many object-oriented analysis methods are based on
concepts from object-oriented languages. Therefore the semantics of the reception of a
message is usually that the execution of a method with the same name is started by the
receiving object. We choose to implement this approach only for the data objects in SHE,
see Paragraph 4.15.3.1.
The choice for a strong relation between message and service is a design decision in
the development of a specification method. For process objects we decided to separate
these concepts. This has consequences for the semantics of both messages and methods
(see Subsection 4.15.3 for details). The separation enables to make process objects more
independent, and what is more important to let them continue their activities between
subsequent sending of messages. Objects can decide whether or not, and when, they
want to send or receive a message. Their internal behaviour may have a finer grain of
behaviour than classical methods have.
By offering asynchronous concurrency we allow behaviour of objects to take time independently
of each other. Individual processes may perform complex and time consuming
behaviour. Asynchronous concurrency enables a natural modelling of distributed
systems. Distributed subsystems are relatively independent, and usually run asynchronously.
6.3.3.3 Messages and Concurrency
Data Objects in SHE are passive entities that can only be activated by a message. A
sender waits until a receiving data object returns a result or a confirmation of ’message
handled’. So the sender is active or the receiver is active, not both of them. Their
composite behaviour is still sequential.
Concurrent process objects can individually perform infinite behaviour and synchronise
with each other when they exchange a message. In real-time concurrent systems
messages may come during the execution of some behaviour. The consequence is that
objects are not always prepared to listen to a request for communication. In real systems
this is not always acceptable. Therefore we needed and introduced an interrupt and an
abort primitive.