Why True Decoupling Requires Isolating Data, Not Just Logic

A software architecture analysis argues that most engineers misunderstand decoupling by focusing on code structure while ignoring how data is shared in memory. The core argument holds that if multiple modules point to the same mutable memory block, they remain tightly coupled regardless of how cleanly their interfaces are designed. The piece introduces the concept of 'stop-and-start boundaries,' where data is serialized and handed off across explicit boundaries rather than shared via live pointers. Languages like Erlang, Clojure, and Rust are cited as practical examples of enforcing true data isolation through process-level copying, structural sharing, and compile-time ownership rules respectively. The author contends that adopting data-centric decoupling not only improves system reliability but also addresses limitations in how both human developers and AI tools reason about large codebases.