UML Weekend Crash Course™ - To Parent Directory

Session 8—Identifying the Use Case Scenarios 83
the logic expressed in the Use Case narrative and the scenarios; the places where you grapple
with business objectives; and your plans to cope with all these issues to ensure the success
of the system.
Having said this, I want to voice a caution: You can dissect Use Cases endlessly looking
for anything and everything possible. But you want to avoid analysis paralysis by recognizing
that you won’t get everything right the first time through. It just isn’t possible. Allow
for a number of passes through the problem with limited time frames. Move on to other diagrams
and revisit the Use Cases and scenarios after looking at the problem from the unique
perspectives that the other diagrams provide. Let the diagrams reveal information and
inconsistencies, or prove you right. Above all, allow time for practice. Trial and error can be
excellent teachers.
How to find Use Case scenarios
So how do you find these scenarios Reading a narrative and following every possible path
can be difficult sometimes. One very simple and practical tool to use to find the scenarios
visually is an Activity diagram. I almost always draw Activity diagrams rather than rely
solely on text. Visual models provide a valuable complement to text. Together, the two perspectives
can reveal insights not visible from only one perspective. Given that people learn
in different ways, having different tools to explain the problem can help everyone grasp the
important issues more easily.
One major benefit of an Activity diagram is its ability to quickly reveal dead-end segments
and incomplete paths. The Activity diagram is still grounded in textual description
for each activity, so there is significant overlap that helps ensure that your information is
consistent.
Cross-Ref
If you aren’t comfortable with flowcharts or Activity diagrams, you may want
to skip ahead to Sessions 14 and 15 before finishing this chapter.
To find a scenario, start at the beginning. It usually works best to follow the path of the
successful scenario first since it will usually be the most comprehensive. Trace the steps
until you come to a decision, represented by a diamond. Now you have to make a choice.
Select one of the possible paths leading out of the diamond, preferably the path that leads
to the successful completion of the scenario, and continue to trace the steps. Continue the
process until you reach an end point. That is Scenario 1. Now return to the top and retrace
the first scenario to the first branching point (decision). Start the second scenario at the
branch, but follow a different path leading out of the decision. Continue tracing the steps
as before. If you loop back to a place you have already been, then stop. Avoid creating
redundant segments.
Repeat the process until you have traced a path through every segment of the diagram.
You should now have a set of segments that together account for every element of the
Activity diagram.