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

166 Modelling of Concurrent Reactive Behaviour
Sender
process
Single message flow
Message with reply flow
Continuous flow
Interrupt message flow
Interrupt with reply flow
Buffered message flow
Figure 6.3: Message Flow Symbols
Receiver
process
the data. These class names are added to the textual message description that is placed
as text on the flow in the Message Flow Diagram. (See Figure 6.4.) The meaning is that
Process A
store(ProductInfo)
Process B
Figure 6.4: A Single Message Flow
the message store transports information in the form of an object of class ProductInfo 3 .
Data objects can travel around, but process objects cannot. They are tied in a static
structure of channels. However, it is possible to pass the name of a process in the form
of a process identifier. (See Subsection 6.3.10).
A message without contents can also be a very useful communication primitive. The text
on the single message flow symbol is then a message name without parameters. This
models the notion of an event.
An event is something that happens, and is relevant to the system. The duration of an
event is idealised to be infinitesimal short in time. An event may happen internal or
external to the system. Events may occur simultaneously or they may happen before
or after each other, or they may be completely independent. Events are important for
the description of the flow of control of a system. The dynamic behaviour of a system
3 We recommend to begin class names with a capital letter.