deprecation-and-migration

To deprecate a legacy API safely, you must follow a replacement-first workflow: build or validate a replacement, publish a deprecation notice with a migration guide, migrate consumers incrementally, and remove old code only after verifying zero active usage.

The Strangler pattern is a migration approach that uses parallel routing to incrementally transition consumers from a legacy system to a replacement, allowing you to safely sunset old code by routing traffic progressively rather than migrating everything at once.

A safe migration plan requires evaluating ongoing value and consumer dependency, publishing a deprecation notice with tooling, using feature flags for gradual rollout, and executing safe removal only after confirming zero active traffic to the legacy implementation.

Advisory deprecation warns consumers without forcing immediate action, while compulsory deprecation enforces a hard deadline. You choose between them based on consumer dependency, maintenance costs, and security risks of keeping the legacy API running.

Removing legacy code without measuring active consumers causes unexpected breakage. You must verify zero active usage before safe removal to avoid disrupting consumers still relying on the deprecated API or feature.

Yes, feature flags enable gradual rollout during migration by allowing you to route a percentage of traffic to the replacement system incrementally, which is essential for consolidating duplicates and safely sunsetting the legacy implementation.