The use case diagram is not dead. In fact, business people love it. It’s simple, clear and business-friendly. In has three self-explanatory elements – the boundary, the actors and the use cases. Without getting into specifics of relationships such as extended and included use cases, a high-level use case diagram is perfect for depicting a scope of a solution for all audiences.
A classic use case diagram has a single system boundary. The focus is on one solution or system. It may be sufficient for discussing the system itself. In real life, however, whenever we capture the scope of a solution or a system, we need to keep the context in mind. And the context usually involves other systems.
A use case diagram that focuses on one system only will leave some questions unanswered. What about other use cases that are not in the diagram? How will those be accomplished?
Many organizations manage complex landscapes of legacy systems, and new applications and COTS implementations with various degrees of interconnectedness.
In these circumstances, a use case diagram that includes several systems can help understand who does what and where. Adding more to the model can save you a lot of explaining and additional narrative.
Let’s look at this example. The project is going to build a new Online Ordering System. The boundary of the new system has been defined, and use cases in scope have been captured:
Now, do you understand how the order will be processed end-to-end? Not really, as the new system clearly deals with only subset of use cases. What happens with the orders that are ready for delivery? How are the deliveries scheduled? Who ensures that all required ingredients are purchased?
This is where an expanded use case diagram is handy.
In the expanded diagram, we can see other systems that are involved, and what use cases each system supports. Even without explicitly showing integrations between systems, we have more useful information about the context of the new solution.
Advantages of the expanded use case diagram:
- Provides a view of use cases across the end-to-end process
- Captures multiple systems required to support the complete user experience
- Highlights actors that must use multiple systems to do their job (“swivel-chair” user experience)
- Brings attention to interfaces (automated or manual) that must exist to exchange data between systems
- Shows the complexity of the system interconnections at high level – can be used as a communication tool with business stakeholders and executive sponsors
- Remind the stakeholders that the new system will not replace all the old clunkers
A business analyst must use a variety of tools to create a shared understanding of business requirements. A slightly unconventional diagram that helps to bring clarity is better that strict adherence to the rules in the models that the audience does not understand.
For more on use case diagrams and other models, check out my book “Business analyst: a profession and a mindset”.