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

4.8 Properties of Composites 95
In general, a data object is separable from a process object, because their lifetimes do not
have to coincide completely. After all, a data object may ’live’ shorter than a process that
encapsulates it. Process objects can never be inseparable from a data object, because the
data object cannot be dependent of a process object. After all data objects cannot send a
message to a process object. A data object can only send replies to process objects.
Data objects can be separable from each other. Inseparability of for instance aggregates
of data objects can be specified by the behaviour description of the whole object class.
The creation or deletion of a whole should lead to a simultaneous respective creation or
deletion of a part, so that their ’lifetimes’ coincide.
Data objects that are passed between process objects, are not always separated from
the sender. Recall that these travelling objects are passed by creation of a deep copy
at the receiver side. The sender has the freedom to remove the original or not. In
case of removal we dissimulate the separation of a data object and the travelling of this
data object to another process. It is dissimulation because the travelling is achieved by
copying the object to a new identity in another process. So travelling is not a separation
and joining of the same identity over different process objects.
Separability in relation to aggregates of process objects raises the question whether it is
possible to add or remove parts. The channel structure of the aggregate in its environment
is defined by the specification. This channel structure is statically determined at
specification-time. So, it may seem as if parts connected with channels to a whole cannot
be separated. However, not only channels determine the separability of a part. Channels
may be shared, and the destination of a message may be determined by the identifier
of a receiving object. Passing of identities of a process object between process objects
creates a logical communication topology on top of the static channel structure. These
dynamic links can be created or forgotten, thereby respectively linking or separating an
object.
4.8.7 Ownership
An entity owns another entity if the former entity depends on the latter one and if this
latter entity is not shared. The following statements are based on this definition.
an entity does not necessarily have an owner;
an entity can have at most one owner (at any point in time);
an entity can change owner;
a data object can be (but is not necessarily) owned by a process object;
a data object can own another data object;
a data object cannot own a process object;