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

6.4 Distribution 191
links to each other. They are in the same name space. Objects in X cannot know objects
in Y and vice versa. As a result they cannot collaborate using identifiers of objects from
the other name space.
Wegner [Weg90] describes an approach that differs from our approach. A module in
a strongly distributed system knows only the names of its own communication ports.
’The names of modules with which it communicates, are stored as data in its own communication
ports, and may be viewed as pointers to ports of other modules that cannot be dereferenced to
determine the name of non-local ports’.
We offer channels that connect objects. Channels can enable communication purely by
their existence. A static link is enough to communicate with an unknown process object.
It is enough that an object knows what services can be provided by what message to the
objects on the other side of the channel. It is not necessary to know who offers them.
Figure 6.16 shows objects that know each other within a distribution boundary. The
channel forms a connection surpassing the distribution boundaries. B in module X can
communicate with A in module Y over the channel solely by message passing without
using object identifiers. Notice that we even used the same identifiers A and B in both X
and Y. This is allowed since the name spaces are separated by distribution boundaries.
6.4.4.3 Relation with other Concepts
It is interesting to relate weak and strong distribution to the concepts of physical distribution,
clusters, abstraction and aggregate. Physical distribution is an orthogonal
concept. Physical distribution specifies an independence between modules. Strong
distribution adds another notion of independence. Depending on the implementation
technology a strongly distributed approach may be suitable to realise an adequate model
for a physically distributed system. In practice awareness about these aspects helps in
selecting the proper communication constructs.
We can impose a restriction on the knowledge (name space) of a group of objects when
we cluster them into an aggregate. Recall that an aggregate is a composite whose
communication with the outside is performed by one representative aggregate object. So
the objects, that are outside the cluster do not know the identifiers of the part objects. This
does however not make the cluster a part of a strongly distributed structure. The cluster
can still communicate with the use of object identifiers. Strong distribution requires that
the cluster communicates autistically.
An aggregate is an abstraction boundary which specifies hiding. A cluster that is not
an aggregate does not serve as an abstraction boundary. A cluster then represents the
weakest form of clustering of objects in terms of a so-called domain boundary. This is
independent of the decision to make the cluster strongly distributed from the rest of the
system.
The concepts of weak distribution and strong distribution appeared to be related to the
concepts of static links and dynamic links. Strongly distributed communication uses