This trace follows the actual state transitions behind the companion Module Boundaries Should Follow Change. It describes a common execution path; implementation details can vary, so keep the contract separate from the mechanism.
Step 1: Identify a likely product change
A module groups decisions that are likely to change together and hides details other modules do not need. A useful boundary reduces the number of places that must change for one product decision. The public interface should express stable intent rather than expose internal storage or control flow.
Step 2: Assign ownership of the decision
If billing and email share a database table but change for different reasons, each may be better represented by its own module even when both run in one process. A narrow interface can let the implementation change from a local adapter to a remote service without rewriting every caller.
Step 3: Define a stable interface
Keep a decision behind the module that owns it so one policy change does not force callers to know storage or implementation details.
At this point, record the state that changed and check the invariant before advancing. If the operation repeats, make clear which values persist and which are recomputed.
Step 4: Route the call through the boundary
More modules do not automatically mean less complexity: excessive fragmentation adds indirection and coordination cost. A boundary drawn around technical layers may still couple unrelated product rules. Strong boundaries usually emerge from repeated change patterns and explicit ownership.
Step 5: Review the resulting change span
Choose a boundary for a feature that sends a receipt after a purchase. Decide which module owns payment state, receipt formatting, and delivery retries, and explain how dependencies flow.
The trace is complete when the result satisfies the stated contract. Compare this model with the concrete runtime or system you are studying before making a performance claim.