This is literally the 'O' in SOLID. The key idea is to break code into "chunks" that each do one thing. Then, if you have to add a new feature, it goes into a

This is literally the 'O' in SOLID. The key idea is to break code into "chunks" that each do one thing. Then, if you have to add a new feature, it goes into another chunk, instead of editing/modifying existing code. The same logic applies to system design at different scales, whether fine-scale OOP or coarser-scale (micro)service architecture. The ideal size of an individual "chunk" is somewhat subjective & debatable, of course. It's like Haskell-style immutable data structures, but applied to writing the code, itself.