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

96 Abstraction of a Problem Domain
ownership is not transitive.
In general a process object owns its internal data objects. These data objects are not
shared with other process objects. However an internal data object can be shared with
another internal data object. In this case it is not owned by the process object, according
to the definition.
Data objects can own other data objects. A data object B is owned by data object A, if
B is not shared and A depends on B. An example is that data object B is owned by data
object A, if B is not shared and has subordinate temporal visibility to A. The result of this
definition is that an owned data object works for the owner solely, and does not contract
out work to the owner.
A process object B is owned by process object A, if B is not shared and A depends on B.
However, collaborating process objects are generally autonomous entities that are not
owned. On the other hand aggregates in general own their parts. The condition for a
part to be owned by the whole is that the part is peer independent (see Paragraph 4.8.3).
4.8.8 Collaborations
Composites are formed by collaborating objects. Various composites have various different
properties. In this section about properties of composites we defined various
concepts. As an illustration of the use of the various concepts for the description of
composites (subsystems) we show some examples.
In general, a system model will show a mix of static and dynamic communication
structures. Static structures are formed by autonomous concurrent processes and the
channels between them. These processes may be surrounded by less autonomous
processes that are collaborating in such a way that they perform tasks for the autonomous
objects. They perform the role of servant. The autonomous objects have the role of
supervisor or controller. It is wise to cluster a supervisor with its servants, see Figure
4.23. Clusters can form reusable subsystems. When the communication with other
subsystems is performed by the supervisor only, the cluster is a clustered aggregate.
Although there is no sense of whole-part, they are aggregated. It is rather an artificial
and constructive composite. We will call it a supervised composite. Figure 4.24 shows
two collaborating clustered supervised composites. These composites are aggregates.
Cluster X contains an aggregate consisting of an aggregate object Supervisor 1, and parts
Servant A and Servant B. Servant A is not shared. Servant B is shared by Supervisor 1 and
Servant A, that both depend on Servant B. Servant B is shared within the aggregate but
not with any entity outside the aggregate. Notice that Servant A is owned by Supervisor
1.
Servant A may be inseparable from Supervisor 1, if their lifetimes are specified to coincide.
Servant A is a whole dominated object because it does not take initiative to send messages
to the supervisor on its own. This is also indicated by the message with reply flow. This
flow specifies that Servant A delivers a result only on a request of Supervisor 1.