Chapter 25. From Use Cases to Implementation

Key Points

• Some requirements map well from design to implementation in code.

• Other requirements have little correlation to design and implementation; the form of the requirement differs from the form of the design and implementation (the problem of orthogonality).

• Object orientation and use cases can help alleviate the problem of orthogonality.

• Use cases drive design by allowing all stakeholders to examine the proposed system implementation against a backdrop of system uses and requirements.

• Good system design is not necessarily optimized to make it easy to see how and where the requirements are implemented.

We have been building complex software systems for over 40 years . And yes, we have struggled and had our share of failures, but we have also achieved an extraordinary degree of success: online trading, the Internet, desktop productivity tools, lifesaving medical equipment, and safe power plants, to name a few.

It's clear that we have somehow managed to move from the world of requirements to the world of design and implementation. We have implemented many complex systems that conform to their requirements. However, when it comes to building complex systems that require a high degree of safety or reliability assurance, it hasn't always been a pretty (or at least a rigorously scientific) matter. The reason is that requirements do not lend themselves to being readily exposed for inspection within the implementation. Proving that any particular requirement is fulfilled in the code is a nontrivial matter. In this chapter, we explore this particular problem to see if we can gain some insights into a potential solution.