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

4.4 Data Objects and Process Objects 77
4.4.5 Travelling Objects
A complex system is modelled as statically interconnected communicating processes.
During the lifetime of a system its static channel interconnect structure does not change.
Communication requires that some data can move or flow from one process object to
another. Very often, data must be transformed in a process and must then be passed to a
next process for further transformation. We use data objects to model such data. A data
object that is applied this way is called a travelling object. The idea of travelling objects is
introduced by Verschueren [Ver92]. (He proposes passive and active travelling objects.
We exclusively support passive travelling objects. For details we refer to [Ver92]).
Intuitively a travelling object between processes must be an identity that disappears at
its point of departure, travels via a channel, and appears at its destination. However
the choices we made in the semantics of data objects and process objects prevents that
the semantics of this travelling does not match precisely with the intuitive meaning
of travelling. In fact travelling is ’simulated’ by ’sending’ a copy instead of the real
data object. For this reason a more precise name would be travelling clones, instead of
travelling objects. They are copies (clones). There is no real travelling. The original of
the copy may even persist in the sender. It is a matter of specification (a design decision)
whether the original at the sender side is removed or kept. By leaving this open a sender
can keep the data without further special operations. Note, that the data objects that are
sent are new identities.
Data to be sent may have a complex structure. Complex data structures are modelled
as a composite of data objects, linked by references. The precise semantics of the copy
action are that a deep copy (see Paragraph 4.4.2.6) of the data object to be sent is created
at the receiver side. By making a deep copy, complex data structures can be transported
by simply specifying the sending of the object that holds the references to the rest of the
structure.
The background for this artificial way of ’travelling’ is that data objects should not be
passed by simply passing references. If a process can receive a reference to a data object
contained in another process object, the former process can communicate with this data
object directly. In this way we would allow processes to exchange information without
the use of their channels.
4.4.6 Separation of Data and Processes in other Methods
The design decision to distinguish data objects from process objects offers a very specific
expressive power. In Subsection 9.7.3 we explain how this decision strongly influences
the semantics POOSL, which is divided into two layers of semantics. There are many
other aspects. A short comparison with other methods shows that we had to decide
between many alternative approaches.
Most object-oriented specification methods [R 91, C 94, CY91a, CY91b, J 92, SM88,
Boo91, BW95, Rob92] do not distinguish data objects from process objects, as we do.