migration-principles

Migrate legacy code safely by taking a stepwise approach that separates domain, application, and infrastructure while preserving contracts. You catalog current behavior, identify the smallest migratable step, and verify testable equivalence before proceeding.

An anti-corruption layer preserves external contracts during a legacy refactor by isolating new clean architecture boundaries from old code. You need it when migrating modules or services to prevent behavior drift and ensure testable equivalence at each milestone.

Use ports and adapters to enforce explicit separation of concerns, isolating domain logic and application orchestration from infrastructure. This structured migration approach establishes clear boundaries, allowing you to verify risk and equivalence checks at each step.

The best way to prevent behavior drift during a legacy rewrite is to define explicit acceptance criteria and perform testable equivalence checks after each stepwise migration milestone. This controlled approach avoids risky long rewrites by validating behavior continuously.

You can use this stepwise migration approach for both individual modules and entire architectures. It applies to any project where domain logic, application orchestration, and infrastructure must be separated, allowing you to identify the smallest migratable step regardless of scale.

A full legacy rewrite causes behavior drift and delays because it lacks controlled, testable milestones. A stepwise migration with explicit boundaries and risk checks after each step ensures equivalence is maintained throughout the transition.