deprecation-and-migration

Legacy system migration involves building replacement APIs, providing migration tooling and documentation, migrating consumers incrementally using feature flags or adapters, verifying behavior with metrics, and removing the old system once usage reaches zero.

Incremental migration uses patterns like strangler and adapter strategies to replace legacy systems. You build the replacement, migrate one consumer at a time using feature flags, verify behavior with metrics, and coordinate final removal when usage is zero.

Sunsetting a feature is appropriate when maintenance costs outweigh user impact. A decision framework evaluates whether to maintain, migrate, or sunset a system by weighing user impact, maintenance cost, and operational risk to reduce zombie code.

Verify consumer API migration by monitoring usage metrics and running tests to confirm behavior parity. You safely coordinate the removal of the old implementation only when metrics verify that legacy API usage has reached zero.

During API migration, feature flags enable incremental consumer rollout. You migrate one consumer at a time from the legacy system to the replacement, verify behavior with metrics, and use the flags to safely control the transition before final removal.

Zombie code in consolidated systems creates security and operational risks. Governance and risk controls mitigate this by enforcing advisory or compulsory deprecation, providing migration tooling, and coordinating removal when consumer usage reaches zero.