Use Cases and Activity Diagrams

For use cases described using the casual or even the fully dressed template, it is difficult to see the big picture showing all the paths through the use case. The primary path describes just that, the primary path. In order to keep the description simple, it does not describe the extension points or branch conditions for any alternative paths that diverge from it. Similarly, each alternative path is a description of just that path. It does not show in the path description itself how it fits into the overall flow that makes up a traversal of the entire use case.

One useful diagram type from UML that does show this kind of information is the activity diagram. Being based on a flow chart, the activity diagram enhances this basic diagram type with swimlanes, parallel forks and joins, posting of and pending on signals, plus one or two other useful extensions.

An activity diagram can be generated that shows the primary path and all the alternative paths that branch off it, together with the points at which they remerge if any. As such, it is able to show in a single picture the entire functionality of the use case, in a form that is still reasonably digestible by non-technical stakeholders.