When to Use Use Cases

Use cases are a valuable tool to help understand the functional requirements of a system. A first pass at use cases should be made early on. More detailed versions of use cases should be worked just prior to developing that use case.

It is important to remember that use cases represent an external view of the system. As such, don't expect any correlations between use cases and the classes inside the system.

The more I see of use cases, the less valuable the use case diagram seems to be. With use cases, concentrate your energy on their text rather than on the diagram. Despite the fact that the UML has nothing to say about the use case text, it is the text that contains all the value in the technique.

A big danger of use cases is that people make them too complicated and get stuck. Usually, you'll get less hurt by doing too little than by doing too much. A couple of pages per use case is just fine for most cases. If you have too little, at least you'll have a short, readable document that's a starting point for questions. If you have too much, hardly anyone will read and understand it.