Preventing Software Degradation with Domain‑Driven Design and the Two‑Hat Refactoring Approach

Software systems inevitably become more complex over time, leading to degradation where code becomes harder to maintain and quality drops after repeated changes. The article traces the root causes of this degradation to mismatched design decisions and the lack of a disciplined approach to handling evolving requirements.

Domain‑Driven Design (DDD) is presented as a solution: by modeling the software closely after the real world and using a clear domain model, teams can guide both initial design and subsequent changes. The article emphasizes that each change should first be reflected in the domain model before any code modifications, ensuring that the underlying business logic remains consistent.

To keep design quality high during changes, the author proposes a "two‑hat" method. The first hat involves refactoring the existing code structure without adding new functionality, creating extensibility points such as interfaces. The second hat implements the new feature using those points. This process respects the Open‑Closed Principle (OCP) and the Single‑Responsibility Principle (SRP), preventing the classic "if‑else" explosion.

The concepts are illustrated with a detailed e‑commerce payment scenario. Starting from a simple payoff() method, successive requirements—discounts, VIP membership, and additional payment methods—are handled by extracting a DiscountStrategy interface, applying the Strategy pattern, and later introducing Adapter patterns for third‑party payment gateways. Each step shows how the domain model evolves and how the code changes remain isolated to a single module.

Finally, the article concludes that by repeatedly applying DDD and the two‑hat approach, software can evolve from simple to complex without degrading, keeping maintenance costs low and design quality high.