Design Patterns at Different Scales
  
R.J.A. Buhr, Department of Systems & Computer Engineering, Carleton
University, Colonel By Drive, Ottawa K1S 5B6, Canada

buhr@sce.carleton.ca

Abstract

Patterns in the style of the "gang-of-four" book or the ACE framework
are becoming popular as a way of enabling developers to reuse good
design solutions to implementation problems. However, systems
constructed from combinations of many general patterns of these kinds,
in which the specifics are in the code-customization details, are not
easy to explain or understand as a whole. In particular, it can be
hard to understand how stimuli propagate through a specific
implemented system as a whole, through many components that may come
from many different, overlapping patterns. This is an unsatisfactory
state of affairs for real-time and distributed systems, for which
stimulus-response behaviour is very important. This paper proposes
that a solution to this problem requires a paradigm shift in
description techniques at some scale, similar to the paradigm shift
patterns provide from code. In the new conceptual domain above this
paradigm shift, description techniques would provide a coherent view
of a system as a whole and an organizing framework for understanding
the combination of patterns from which it is constructed. The prospect
also exists of discovering patterns of a different scale and kind in
this domain. This paper proposes that a diagramming technique called
use case maps provides a suitable paradigm shift and illustrates the
proposal with examples that span the range from an object-oriented GUI
to a distributed client-server system that processes end-to-end
transactions across a network. The motivation for this work is
high-level understanding and design of real-time and distributed
systems.