Design Patterns Explained

292 Part V • Handling Variations with Design Patterns
E-tail: A case study
in variation
For example, patients coming to a hospital typically go to the
admitting office first. But when there is a life-threatening situation,
the patient goes directly to the emergency room before having to go
to admitting. These are the variations in the real world, the different
special cases that our system has to deal with.
And this is what creates headaches for us analysts. Can patterns
help us deal with variation more efficiently?
I have used an approach to make explicit the variations in the sys tem
and then use the analysis to identify the patterns I ought to use in my
design. The steps in my approach are as follows:
1. Identify the most important features in one case and organize
them in a matrix. Label each feature with the concept that the
feature represents.
2. Proceed through the other cases, expanding the matrix as nec
essary. Handle each case independently of the others.
3. Expand the Analysis Matrix with new concepts.
4. Use the rows to identify rules.
5. Use the columns to identify specific cases.
6. Identify design patterns from this analysis.
7. Develop a high-level design.
Case Study in Variation: An International
E-Tail System
Suppose an e-tail system must process sales orders in several different
nations. Initially I have to handle just the United States and
Canada. I look at my requirements and see several things that vary. I
note these in Table 20-1.