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

4.9 Channel Hiding 97
Servant A
Supervisor 1
Cluster X
Servant B
Servant G
Supervisor 2
Cluster Y
Figure 4.24: Supervisors and Servants
Servant H
Servant B is peer independent, because it does not require the other part. Servant A is
not peer independent.
Depending on their communication fashion, cluster X and Y have an inward visibility
that is limited to their internal aggregate objects. In a weakly distributed fashion Supervisor
1 and Supervisor 2 are visible for each other. All parts (Servants) are invisible.
There is no outward visibility for the parts of the aggregates. Servant B may have a
subordinate temporal visibility to Supervisor 1. Servant A is invisible for Servant B.
In general it is possible to decompose a system into a hierarchy of subsystems. It is
recommended to create a corresponding hierarchy of supervisors. Such a structure can
show responsibilities clearly and can lead to reusable and maintainable substructures.
The more independent that subsystems are the better. In Figure 4.25 cluster Y has direct
communication with cluster Z. These clusters become more independent when their
inter-communication is redirected via Supervisor 1. This way they can become peer
independent.
4.9 Channel Hiding
Clusters, aggregates, clustered aggregates, and other composites can hide the channels
that are used for internal communication. Hiding has been described already for clustered
aggregates in Subsection 4.6.4. Hiding can be used to restrict the connection points
to a composite. Only visible channels can be used for connection of the composite to
other objects or other composites. The hidden channels cannot be used for this purpose.
The clustered composite in Figure 4.26 can only communicate with its environment via
channels w, x and z. Channels a, b and c are hidden. They are visible only for the objects
inside the cluster. Channels that pass the cluster boundary may have different inner and
outer names. Figure 4.26 shows that Object A has a channel y that crosses the cluster
boundary. The inner name of this channel is y, and the outer name of this channel is z.