Start with ownership

A system becomes easier to reason about when each part has a clear responsibility. Boundaries help teams understand where decisions belong and which component should change when a requirement shifts.

This does not mean every responsibility needs its own service. A well-defined module inside one application can provide a useful boundary without adding network calls or deployment overhead.

Make change visible

Good interfaces expose the decisions a caller must make and hide details that can safely vary. That lets implementation improve without forcing every dependent part to change at the same time.

For a growing codebase, the practical test is whether a developer can change one capability and predict which other areas need review. If the answer is unclear, the boundary may need work.