Separation of Concerns (Software Design)

Separation of Concerns is a design principle in software development and can be considered fundamental for creating maintainable and clear structured software systems.

A Concern represents a (functional) task in a system: It can be a single unit responsible for a simple calculation, or a conglomerate of functionality providing a service with a given interface.

Once it's identified and clear-cut and refined into boundaries facilitating physical or semantic (or both) tools and methods (such as modules and packages, or Bounded Contexts), a single Concern becomes separated from other concerns co-existing in the system. Concerns do not overlap, and as a consequence they become special and distinctive to this system.

In his paper On the role of scientific thought[📖DIJ, p. 61] , Edsger W. Dijkstra writes:

"It is what I sometimes have called "the separation of concerns", which, even if not perfectly possible, is yet the only available technique for effective ordering of one's thoughts, that I know of. This is what I mean by "focussing one's attention upon some aspect": it does not mean ignoring the other aspects, it is just doing justice to the fact that from this aspect's point of view, the other is irrelevant. It is being one- and multiple-track minded simultaneously." Edsger W.Dijkstra