How Many System Use Cases Are Ideal? Just One!

Question:

The method of transitioning from business sequence diagrams to a system use case diagram is very good. I have a question: to what extent should we make improvements, and how many system use cases are ideal?

Answer:

Strictly speaking, the number of system use cases to focus on in each iteration should always be one.
Even if, while improving the business sequence diagrams using the target system, you happen to improve many aspects and map out multiple use cases for the target system. For example, consider the following two sequence diagrams:

Sequence Diagram 1

Sequence Diagram 2

From the above two sequence diagrams, we map out System E's use case diagram:

System E Use Case Diagram

However, this is only a preliminary exploration. It doesn't mean that the system must have these use cases in the end, nor does it mean that the system only has these use cases.

You need to judge which use case should be implemented first based on the vision, then observe the results of the improvements to determine if the vision's goal have been met and if further improvements are needed. If the goal are met, you can stop; if not, continue improving.

By continuously repeating the above exploration, the number of use cases in the final system does not have a standard answer. The best answer, derived from the vision and stakeholder interests. It could be 1, or it could be 1000.

Understanding the above knowledge, you'll know how to answer similar questions like:

• Is modeling more effective for new projects?
• How does modeling differ when improving legacy systems?
• How can we ensure we've identified all the system's use cases?

These questions don't have fundamental differences.

It's similar to diagnosing a patient. A doctor examines the patient's current condition, considers the treatment plan, tries the treatment, observes the results, and repeats the process. Whether the patient has been treated by another doctor, brought medical imaging records from another clinic, or has leftover medication from another doctor, does not change this approach. If the medical imaging records or leftover medication can still be used, it just saves some money.