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

4.4 Data Objects and Process Objects 75
startJob
readyToAccept(PI)
accept(PI)
Job_
Controller
startConfig
Product_
Input_
Handler
startCount/
ShiftCount(Number)
flushFifo
write(PI)
Config_
Handler
PI_Fifo
startConfig
ShiftEndCorr/
shiftCount(Number)
serviceRequested
(ServiceId)/
service(YesNo)
serviceResult
(OkNotOK)
Figure 4.11: A Message Flow Diagram
Product_
Output_
Handler
readyToAccept(PI)
accept(PI)
The example in Figure 4.11 shows a part of a Message Flow Diagram. The goal of
Message Flow Diagrams is to help reasoning about system behaviour. A process of
class Job Controller receives a message startJob. The response of the process object can be
sending a message startConfig to both an instance of class Product Input Handler, and an
instance of class Product Output Handler.
Notice that Message Flow Diagrams do not show when and in what order messages
are exchanged. They only show that processes are able to communicate by exchanging
specified messages. The actual behaviour of processes is specified in POOSL. Various
Sender
process
Single message flow
Message with reply flow
Continuous flow
Interrupt message flow
Interrupt with reply flow
Buffered message flow
Figure 4.12: Message Flow Symbols
Receiver
process
graphical communication primitives are used in the diagram. They are available to
describe various forms of communication between process objects. Figure 4.12 shows
flow symbols that we defined for use in Message Flow Diagrams. They are discussed
extensively in Subsection 6.3.6. It will appear that all flows can be formalised in terms of