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

4.8 Properties of Composites 93
When a whole needs the services of a part, to perform a task of the composite, the whole
is dependent of the part. This does not imply that a part needs to return a reply to a
whole. A whole can be dependent because a task is delegated to a part, even when the
part performs its tasks autonomously. An autonomous object can perform actions and
take initiative for communication on its own when it needs the service of another object.
An entity is independent of another entity if it does not need a service of the latter entity.
However, other entities can send requests to an independent entity. The others are
then dependent of the independent entity. This independent entity may return a result.
Notice that sending a result is not considered to be a request for a service. So, the sending
of a result does not make an entity dependent.
The parts of a composite that are processes are in principle autonomous entities. In
general, we may expect that a whole needs its parts. A whole is dependent of its parts.
Parts, however, may be independent. Parts may or may not be autonomous entities.
This difference leads to interesting types of composites that we can distinguish.
Parts can be designed such that they either take initiative or not. We call a part whole
dominated when it does not take initiative for communication with the whole without an
explicit request of a whole. So the part may solely give replies as a response on a request,
and not send other messages on its own initiative. Whole dominated parts respond to
commands which may not require an answer, or produce answers on explicit request.
Such parts are passive unless activated by a message from a whole. An example of a part
that is whole dominated is a passive part that has a whole with subordinate temporal
visibility. Such a part can only give answers on an explicit request of this whole.
Based on this approach we redefine Civello’s [Civ93] concepts of whole independence,
of peer independence and of separate part. We define a part of a whole to be whole
independent when it is independent of the whole. A whole independent part can be
either whole dominated or autonomous. An autonomous whole independent object is
an active object that determines by itself what incoming requests will be serviced and
when this will happen.
We call a part peer independent when it is independent of other parts. We define a separate
part as a part that is both whole-independent and peer-independent.
4.8.5 Sharing
Civello defines the concept of sharing based on the concept of reference. ’An object
is shared if two or more objects hold references to it’ [Civ93]. We define sharing based on
communication as follows. Two or more disjoint entities (objects, clusters or composites)
share another entity when each of them is dependent of that entity. The dependent entities
all need the services of the shared entity. Having a reference to an object (temporarily)
is not enough to describe sharing in our point of view. It does not necessarily mean that
a service is needed from that object. Also one does not require a reference to an object
to share this object.