Introduction
to Systems
We build systems like the
Wright brothers built airplanes-- build the whole thing, push it off a
cliff, let it crash, and start over again.R.M.Graham
If builders built
buildings the same way that programmers wrote programs, the first
woodpecker would destroy civilisation.
Gerald
Weinberg
We shall study the characteristics
of a system, its attributes and some basic concepts and strategies for
studying them. Our philosophy is that an accounting
information system is merely a kind of system, and therefore, a study of
the basic principles of systems in general is important for a thorough
understanding of accounting information systems in particular. Our
approach to the study of accounting information systems,
as will become clear, is an engineering one, where one proceeds through a
methodical path of specification, design, construction, testing &
evaluation, operation and maintenance.
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A system is a set of
inter-dependent components (some of which may be systems in their own
right) which collectively accomplish certain objectives.
- Component
- Inter-dependency
- objectives (functions)
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A Contextual view:
Any system operates by interacting
with its environment. The contextual view describes graphically the
interaction of the system with the various entities in its environment.
The interactions consist of dataflows from and to such entities.The
contextual view clarifies the boundary of the system and its interfaces
with the environment in which it operates.
A Control view:
Any system must manipulate certain
variables in order to achieve its objectives. It determines the
manipulation needed by processing its outputs/states in relation to
certain control parameters.
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- Frequently, complexity takes
the form of a hierarchy, whereby a complex system is composed of
interrelated subsyatems that have in turn their own subsystems, and so
on, until some lowest level of elementary components is reached
(Courtois, 1985).
- The choice of what components
in a system are primitive is relatively arbitrary and is largely up to
the discretion of the observer of the system.
- Intracomponent linkages are
generally stronger than intercomponent linkages (Simon, 1985).
- Hierarchical systems are
usually composed of only a few different kinds of subsystems in various
combinations and arrangements (Simon, 1985).
- A complex system that works is
invariably found to have evolved from a simple system that worked..... A
complex system designed from scratch never works and can not be patched
up to make it work. You have to start over, beginning with a system that
works (Gall, 1986).
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- Abstraction:"We
have developed an exceptionally powerful technique for dealing with
complexity. We abstract from it. Unable to master the entirety of a
complex object, we choose to ignore the inessential details, dealing
instead with the generalized, idealized model of the object" Wulf
in Shaw, 1981).
- Formality:Rigor at each
stage in the development of a system.
- Divide and conquer:Divide
a complex problem into a set of simpler problems that can be solved.
- Hierarchical ordering:Order
the simplification of the problem in "divide & conquer" in
hierarchies.
- Cohesion & coupling:Modularise
the system such that interactions within components (cohesion) is
maximised and interactions between components (coupling) is minimised.
This way, the impact of errors, when they arise, is localised and does
not cascade through the system. Diagnosis of offending components is
also made easier.
- Information hiding:Each
module (or subsystem) must have available to it just the information
that is needed by it.
- Conceptual integrity:Consistency
in design.
- Completeness:Ensuring
that the design meets all the specifications.
- Logical independence:Emphasis
on the statement of system objectives in terms of logical functions
independent of physical implementation.
- Correctness &
Efficiency:Correct in the sense that the design meets all the user
requirements. Efficient in that the system accomplishes the objectives
with minimum computing resources.
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- Booch, G. (1994) Object-Oriented
Analysis and Design with Applications, 2nd ed. Redwood City,
California: The Benjamin Cummings Publishing Company, Inc.
- Courtois, P. (1985) On Time and
Space Decomposition of Complex Structures. Communications of the ACM,
vol. 28(6), p.596.
- Gall, J. (1986) Systemantics:
How Systems Really Work and How They Fail, 2nd ed. Ann Arbor, MI :
The General Systemantics Press.
- Simon, H. (1982) The
Sciences of the Artificial. Cambridge, MA : The MIT Press.
- Martin, J. and McClure, C.
(1988) Structured Techniques: The Basis for CASE, Revised ed.
Englewood Cliffs, NJ : Prentice Hall.
- Shaw, M. (1981) ALPHARD:
Form and Content. New York, NY: Springer-Verlag.
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Updated on August 17, 1998 by
gangolly@cnsunix.albany.edu