Yeah a lot of the 90s/early 2000s OOP stuff I learned in school seemed to always result in really tightly coupled systems and bespoke webs of tests and fixtures

Yeah a lot of the 90s/early 2000s OOP stuff I learned in school seemed to always result in really tightly coupled systems and bespoke webs of tests and fixtures that strung along weird dependency chains in unwieldy spaghetti piles that did no good. Following TDD has helped me land at decoupled functional interfaces like the ones you've described, and it all scales and composes so nicely, yet stays very tractable. Robert Martin sketched 2 diagrams in [1] that elegantly illustrate these two different design patterns and how testable usually means composable and more tractable: [1] http://blog.cleancoder.com/uncle-bob/2017/03/03/TDD-Harms-Architecture.html http://blog.cleancoder.com/uncle-bob/2017/03/03/TDD-Harms-Ar...