Art of Software Reuse

One common criticism against systematic software reuse is the myth that it implies perfection – creating a reusable asset automatically conjures up visions of a perfect design, something that is done once and done right.  Many developers and managers confuse reusability with design purity. However, reusability is a quality attribute like maintainability, scalability, or availability in a software solution. It isn’t necessary or advisable to pursue a generic design approach or what one believes is highly reusable without the right context.

The key is to go back to the basics of good design: identify what varies and encapsulate it.

The myth that you can somehow create this masterpiece that is infinitely reusable and should never be touched is just that – it is a myth and is divorced from reality. Reusable doesn’t imply:

• that you invest a lot in big up front design effort
• you account for everything that will vary in the design – the critical factor is to understand the domain – well enough, deep enough, so you can identify the sub-set of variability that truly matters

In the same vein, reusablility strives for separating concerns that should be kept distinct. Ask repeatedly:

• Are there multiple integration points accessing the core domain logic?
• Is there a requirement to support more than one client and if so, how will multiple clients use the same interface?
• What interfaces do your consumers need? is there a need to support more than one?
• What are the common input parameters and what are those that vary across the consumer base?

These are the key questions that will lead the designer to anticipate the appropriate places where reuse is likely to happen.  Finally, it is important that we don’t build for unknown needs in the future – so the asset is likely to solve a particular problem, solve it well, solve it for more than one or two consumers, and finally has potential to be used beyond the original intent. At each step there are design decisions made, discarded, continuous refactoring, refinements to the domain model – if not re-definition altogether.

Don’t set out trying to get to the end state or you will run the risk of adding needless complexity and significant schedule risk.