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

6.3 Communication 161
6.3.3.4 Messages, Encapsulation, Sharing and Distribution
Encapsulation is a very important concept in the object-oriented paradigm. Encapsulation
creates independence. Of course the communication mechanism must support this
independence. Message passing does so. Data in an object cannot be accessed directly.
Access is performed indirectly via a request in the form of a message.
Analysis methods based on Statecharts [Har87], such as Statemate [H 90], Fusion [C 94]
and OMT [R 91] have a form of communication based on broadcast and on the sharing
of variables. This conflicts with the idea of encapsulation. Sharing is further unattractive
when it must be implemented in a distributed system, where it can become very inefficient.
Besides conventional message passing, the analysis methods described above
have an additional form of communication. They allow an object to change state on a
condition that depends on the states of other objects, without the use of explicit messages.
We decided to stick to pure encapsulation. Only via messages it is possible to get
information about the state of an object. So we have a uniform mechanism that is
always the same, also in case of distribution.
6.3.4 The Basic Primitive for Process Communication
An analysis method must be able to express a variety of communication concepts with
various notations and graphical symbols. This variety is meant to give an easy and
intuitive understanding of a system. Further, there must be a straightforward way to
formalise the various communication concepts in POOSL. Here again, the design of a
method appears to be a struggle with selection problems and concept mapping problems.
On the one side there is a wish to keep the method simple, easy to learn, to understand,
and easy to formalise. So a minimum number of primitives is desirable. Conversely there
is an inclination to add more and more constructs and symbols offering expressive power
for all kinds of communication. This subsection give reasons for the selection of the
concept of one way message passing as the basis for process communication in POOSL.
The following subsections describe how the various sorts of process communication can
be expressed with this single primitive.
One way synchronous message passing between two process objects is defined as an instantaneous
simultaneous directed handing over of a message, where one object plays the
role of sender and the other of receiver. Instantaneous means the message passing is
considered to be happening in an infinitesimal period of time. Simultaneous means that
both partners must be actively taking part in the communication at the same instant in
time. This notion is often verbalised as rendezvous. The direction of the communication
(one way) gives the partners a role. One is sender and the other is receiver. The communication
is directed from sender to receiver, thereby defining the direction of the flow of
the message.
The selection of a simple and adequate communication concept (primitive) has been
a key issue in the design of the method. A first choice to be made is whether the