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

188 Modelling of Concurrent Reactive Behaviour
6.4.3 Physical and Logical Distribution
Physical distribution is a geographical or spatial distribution of system resources over
interconnected subsystems. Collaborating processors on a network form such spatial
distributed structures.
Logical distribution is the non-physical counterpart of physical distribution. Wegner
defines logical distribution, as a concept in programming languages, based on notion of
name space: ’a system of modules is logically distributed if each module has its own separate
name space’ [Weg90]. In an object based approach this logical distribution is offered in
a natural way by the concept of object itself. The objects are the modules. In this case
objects are the grain of distribution. Each object has its own name space of acquaintances
(identifiers). This name space contains the identifiers of the objects that are visible from
within the object and therefore reachable for direct communication. This space is not
necessarily static. Identifiers of objects can be communicated, so the name space of an
object may be changed dynamically. Objects can also communicate indirectly via other
objects.
The definition of logical distribution based on name space appears to be too restricted,
because objects with channels can communicate without knowing each others identifiers.
Therefore we define logical distribution based on the ability to communicate directly
with each other. A system is logically distributed if each module (subsystem) has its own
limited group of modules with which it can communicate directly. Objects can be such
modules. An object can only communicate with other objects if it knows the identifier
of these objects or if it has channels to these objects.
Logical distribution facilitates the autonomy of subsystems, concurrent execution and
reusability. Autonomously interfacing modules allow the designer to think in terms of
multiple autonomous (conceptually concurrent) activities. Real physical concurrency is
not always required and often unnecessary. Physical distribution usually implies logical
distribution. It is natural to model physical separation using logical separation [Weg90].
In the past logically distributed systems were often implemented by shared-memory
architectures for greater efficiency. Object-based systems are logically distributed but
were usually implemented on non-distributed computers [Weg90]. Problems arise when
object-oriented solutions must be implemented on distributed computers. The communication
between objects on different computers cannot simply be achieved with the
same simple mechanism as the communication between objects in one application on
one computer. We need a stronger notion than logical distribution to describe e.g.
physical asynchronous concurrency of subsystems implemented on different computers.
A method must be able to visualise communication channels between the physically
distributed subsystems and the according communication protocols on those channels.
Physical distribution appears to be an orthogonal concept. Logical distribution may
coincide or not with physical distribution. We will use clusters marked as distribution
boundaries to specify physical distribution. So the meaning of a distribution boundary