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

194 Modelling of Concurrent Reactive Behaviour
can be specified once for that class even if the instances are scattered over distributed
subsystems. The price for the relative independence and autonomy of a distributed
subsystem is that sharing of code becomes expensive and inefficient. So, although from
a specification point of view the behaviour of a collection of objects can be specified in
one single class, this may not be feasible in a distributed implementation. Distribution
considerations can force to make identical but separate classes for each module, because
this is the only way to share the code of classes efficiently.
6.4.7.2 Inheritance
We consider inheritance as an implementation of a strategy to enable sharing of code.
Inheritance is in fact a completely orthogonal concept for object-based methods. Inheritance
is a problem in physically distributed systems. Instead of being a mechanism that
realises sharing reasonable efficiently, it will become an inefficient mechanism that must
collect shared information from remote places via a communication channel. A communication
protocol on the channel may cause substantial overhead. Zondag [Zon90]
refers to [Her90] where properties that make that inheritance conflicts with distribution
are described. A treatment of this subject is beyond the scope of this thesis.
6.4.7.3 Travelling Objects
The concept of travelling objects that has been introduced in Subsection 4.4.5 has consequences
in physically distributed systems. Where logically distributed systems consist
of objects that can reach each other efficiently, this is more difficult for physically distributed
systems. Travelling objects typically represent items that must be transformed
at different places in the system. It is efficient when they can communicate as directly as
possible with the objects that collaborate to perform the required transformations. However
these collaborating objects may be dispersed over the distributed system. In this
case they would like to share the object to be transformed. Travelling objects can be used
typically to move objects in a distributed system from one subsystem to another. This
solves the problem of inefficient communication over distribution boundaries. Notice
however that the classes needed to create deep copies need to be locally available.
6.4.7.4 Summary
Various concepts that characterise distributed systems have been defined such that they
support our method. The concepts of physical and logical distribution support system
structure design. Clusters are used to denote modules of distribution. Physical distribution
is specified by distribution boundaries on clusters. To what extent a system
is distributed depends on the sort of links between entities. Distribution is related to
communication, and more specific to name spaces of process object identifiers, and the
use of channels. Weakly distributed modules communicate using object identifiers to
determine the receiver of every messages. In contrast, strongly distributed modules
communicate autistically via static links. Channels, message names and process object
identifiers are used to specify either static or dynamic links between collaborating