What Invariants Are (and Why a Domain Model Is the Best Place to Enforce Them)

Most ‘DDD-ish’ code I review enforces business rules everywhere except where it should: in the model itself. The same rule ends up duplicated across handlers, validators, and controllers, and each copy drifts a little over time. Here’s how I think about invariants, and why an always-valid domain model is the cleanest way to enforce them.