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

Business rules scattered across handlers, validators, and controllers lead to drift and bugs over time. The solution is an always-valid domain model that enforces invariants directly on the object. Three concrete techniques are covered: blocking construction of invalid objects via private constructors and static factory methods, encapsulating state transitions in methods that own the state they protect, and enforcing aggregate-wide rules on the aggregate root with private child collections. The result is a model that can be trusted to always be valid, eliminating duplicated checks and reducing the risk of new code accidentally bypassing rules.