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

92 Abstraction of a Problem Domain
Subordinate Temporal Visibility
An object can send a message to an acquaintance without being an acquaintance of
that object. In other words the receiver does not necessarily know the identifier of the
sender. The receiver is visible for the sender but not vice versa. If a reply is required
the sender has to report its identifier to the receiver. Process objects can include their
identifier in the message when a reply is expected. When the receiver stores and uses
this information only temporarily, the sender has a subordinate temporal visibility for the
receiver. Data objects have a hidden mechanism that performs this automatically for
their inter-communication. The receiver cannot take initiative for the communication.
A receiver must give a reply on reception of a request and is therefore subordinate to
the sender. We will use the term ’subordinate temporal visibility’ in full, when we refer
to the concept described here. The single term ’visibility’ will always refer to stronger
forms of visibility that imply longer term temporal or static visibility.
Inward Visibility
A cluster is inward transparent if inside process objects can be used as direct communication
partners by process objects outside. Process objects with known identifiers are
visible from the outside. A clustered aggregate limits the inward visibility to the aggregate
object. Communication with parts can only be done via the aggregate object and not
directly. The cluster offers a message interface that represents that part of the message
interface of the aggregate object that is visible from outside. Although an identifier of a
part object can be passed via the aggregate object this does not lead to visibility of that
part. After all, visibility requires that direct communication is possible.
Outward Visibility
A cluster is outward transparent if its internal process objects can directly communicate
with process objects outside it. If a composite in a cluster is an aggregate, its parts are
not allowed to communicate with entities outside the cluster. The entities outside the
cluster are thus invisible to the part objects.
4.8.4 Independence
Civello [Civ93] elaborates on visibility by defining the concepts of whole independence,
of peer independence and of separate part based on visibility. For example: ’a whole
independent part has no visibility to its whole’ [Civ93]. Relating visibility to independence
is not appropriate for our purposes. In our point of view dependence on parts from
wholes is determined by ’who needs who’. Complex tasks are performed by composites.
A request to a composite is always received by some object in the composite. In general,
this object will delegate at least a part of the task to be performed to other objects in the
composite. So there is a (mutual) dependency for performing the tasks of the composite.
An entity is dependent of another entity when it needs the services of this latter entity to
be able to perform its own tasks. Dependency reveals itself by the need to send a request
for a service (a delegated task). Such a request may be simply a command.