NOTE
Single Responsibility Principle
Grouping behavior around a coherent responsibility and a focused reason to change.
This is a historical learning note and may contain outdated or incomplete understanding.
The Single Responsibility Principle asks whether a module owns one coherent responsibility rather than unrelated concerns that change for different reasons.
Do not interpret it as “every class must contain one method.” The useful boundary is change and ownership: if business policy, persistence, transport, and formatting evolve independently, separating them often improves maintainability.