The Business Logic Layer

If the control layer orchestrates communication with the outside world and marshals a system's response to it, the logic layer gets on with the business of an application. This layer should be as free as possible of the noise and trauma generated as query strings are analyzed, HTML tables are constructed, and feedback messages composed. Business logic is about doing the stuff that needs doing—the true purpose of the application. Everything else exists just to support these tasks.

In a classic object-oriented application, the business logic layer is often composed of classes that model the problems that the system aims to address. As we shall see, this is a flexible design decision. It also requires significant up-front planning.