Systems are interfaces, not implementations

When building a program, the implementation is important. It must perform a specific task. Otherwise, the program has little value.

When building a system, it is the interfaces that are important. Interfaces exist between the components (the implementations) that perform specific tasks.

Interfaces define the system's architecture. Good interfaces will make the system; poor interfaces will break it.

It is much easier to fix a poorly-designed component than a poorly-designed interface. A component hides behind an interface; it's implementation is not visible to the other components. (By definition, anything that is visible to other components is part of the interface.) Since the other components have no knowledge of the innards, changing the innards will not affect them.

On the other hand, an interface is visible. Changing an interface requires changes to the one component *and* changes to (potentially) any module that uses the component. (Some components have large, complex interfaces; a minor change may affect many or few consumers-components.) Changes to interfaces are more expensive (and riskier) than changes to implementations.

Which is why you should pay attention to interfaces. As you build your system, you will get some right and some wrong. The wrong ones will cost you time and quality. You need a way to fix them.

Which doesn't mean that you can ignore implementations. Implementations are important; they do the actual work. Often they have requirements for functionality, accuracy, precision, or performance. You have to get them right.

Too many times we focus on the requirements of the implementations and ignore the interfaces. When a development process is driven by "business requirements" or "functional requirements" then the focus is on implementations. Interfaces become a residual artifact, something that "goes along for the ride" but isn't really important.

If you spend all of your time implementing business requirements and give no thought to interfaces, you will build a system that is hard to maintain, difficult to expand, and providing a poor user experience.