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

6.4 Distribution 189
is that the inside is physically separated from the other system parts. A distribution
boundary has a constraint-oriented character. It notifies the designer that the behaviour
and implementation of the system must realise a sort of communication that passes
physical distribution boundaries. It depends on the realisation technology what the
consequences are. A complex communication behaviour (a protocol) can be needed
when we pass a distribution boundary.
Logical distribution is not denoted as a distribution boundary. We feel that logical
distribution is a natural intrinsic concept in object-orientation. When we use the term
distribution it will be context dependent what it means. When we use the term distributed
system however we refer to a system that is physically distributed. Logical distribution
is implicitly represented by objects or by clusters of objects. However there are sorts
of logical distribution that influence the modelling. These are described in the next
subsections.
6.4.4 Weak Distribution and Strong Distribution
6.4.4.1 Weak Distribution
We have presented a redefinition of the notion of logical distribution based on communication
between modules in the previous subsection. Analogously the concepts of weak
distribution and strong distribution can be redefined. [Weg90] defines them as ’A system
is weakly distributed if its modules know the names of other modules. It is strongly distributed
if its modules cannot directly name other modules.’ ’Traditional object-oriented languages are
weakly distributed.’ ’Objects can know names of other objects and send messages to these known
objects. The weak distribution reflects that implementation is usually in a shared-memory architecture’
[Weg90]. As stated before we try to avoid sharing. We redefine the concepts
with the emphasis on communication instead of on knowing each other.
A (sub)system is weakly distributed if its modules can communicate using object identifiers
to determine the receiver of every message. We call this form of communication weakly
distributed communication. Identifiers form the name space of the sending objects. So
communication based on the use of dynamic links (see Subsection 6.3.9) is typical for
weakly distributed systems. The idea of name space is further clarified with Figure 6.15.
An object B is an acquaintance of object A, if A has the identifier of object B. In other
words: A knows the name (identifier) of object B, or A has a dynamic link with B. We
say that the identifier of B belongs to the name space of object A. the identifier of B must
be unique within this name space. Notice that object A can be constructed such that the
identifier of B is passed to another object such as C. This extends the name space of C
with the identifier of B. This enables a communication provided that the new dynamic
link is supported by a static link. K and L are in a separate name space. These objects
do not have a dynamic link to A, B or C.