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

6.4 Distribution 187
force to take decisions in an early phase of the analysis process to prevent iterations that
would definitely occur later otherwise. Most conventional design methods postpone
the mapping of processes or objects into an architecture structure to an implementation
phase that follows the specification phase. At that time designers may very well be confronted
with need to change the structure of the analysis model because it is impossible
to perform this mapping.
A complex system may be decomposed into several forms of logical as well as physical
distribution simultaneously. So there may be various layers of distribution structures,
and there may be inter-dependencies. Each layer of distribution must be designed at the
appropriate level of abstraction. In order to gather enough information to take a liable
design decision for distribution we must have performed some preliminary refinement.
At a specific level of abstraction, this refinement must be performed to one or two levels
deeper, than the current level. This preliminary refinement is subsequently changed
and made consistent with the distribution structure that was decided upon.
The design of a distribution structure itself is influenced by a lot of issues. Distribution
can enhance availability, reliability and performance. On the other hand distributed
architectures produce design cost that is not present in non-distributed (unitary) systems.
An example of an attempt to find the considerations and critical design issues for a
class of distributed systems is published by Adler [Adl95]. The following issues are
mentioned:
locating programs and data resources distributed across the network;
establishing and maintaining inter-program communication on the network;
co-ordinating the execution of distributed applications;
synchronising replicated programs or data to maintain a consistent state;
detecting and recovering from failures in an orderly and predictable manner;
securing resources by limiting remote access to authorised users.
Another complicating factor is that distributed systems can be quite heterogeneous systems.
This means that ’different processor types and different programming languages use
different representations for the data they manipulate. In distributed systems we see that a
message is sent from one process, written in one programming language and running on one
processor type, to another, written in another language and running on another processor type,
the contents of the message will not be understood unless there has been prior agreement on the
representation of the data in the message’ [Mul93].
We do not focus specifically on the design issues of distributed systems. However we
wanted to show here that distribution is a non-trivial design problem. In the following
subsections we present definitions, concepts and primitives to describe various aspects
and sorts of distribution.