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

390 Case Study
The specification of the Feeder Controller cluster is as easy as the specification of the
Feeder Station cluster and is not presented here. Instead we will take a closer look at the
Product Input Handler, the Product Info Keeper and the Transporter Image classes.
The first part of the Product Input Handler class is defined as follows:
process class
Product Input Handler myId,productDistance ¡
instance variables
myId
productDistance
productInfo
initialPosition
piKeeperId
: ProductInputHandlerId
: PositionDelta
: PI
: InitialPosition
: PIKeeperId
A Product Input Handler has an identifier myId which is of type ProductInputHandlerId 17 .
The variable is assigned a fixed value when the process is instantiated. The same
holds for the instance variable productDistance. This variable models the minimum
distance between two successive mailing products. The distance is expressed in terms
of a PositionDelta. The variables productInfo and initialPosition and store the PI and the
InitialPosition of the most recently received mailing product. Variable piKeeperId holds
the identifier of the Product Info Keeper in which the current PI is stored.
In the next part of the process class specification the channels and the message interface
must be described. The channels of the class are found in the Instance Structure Diagram
of Figure 12.8. The message interface can be written by combining the channels of the
Instance Structure Diagram of Figure 12.8 with the messages and parameter types of the
Message Flow Diagram of Figure 12.6.
communication channels
i piti wake pihs pihp
message interface
i?accept(PI)
piti?productArrived
piti?currentPosition(InitialPosition)
wake!wakeAt(Position,ProductInputHandlerId)
wake?wakeup(Id)
pihs!schedule(PIKeeperId,InitialPosition)
pihp?free(PIKeeperId)
pihp!store(PIKeeperId,PI)
The behaviour of a Product Input Handler instance consists of an infinite non-terminating
loop modelled by method loopForever. Upon instantiation of a Product Input Handler it
17 Although the language POOSL is formally not typed yet, variables are assigned a type to clarify the
specifications.