deprecation-and-migration

To safely deprecate a legacy API, use advisory deprecation notices, set compulsory deadlines, and migrate consumers gradually using strangler routing and adapters before removing the old implementation. This structured migration process prevents breaking active consumers.

The strangler pattern helps with legacy system migration by routing traffic incrementally to a new replacement service while the old system runs in parallel. This allows teams to migrate consumers one by one and verify usage before final removal.

Plan a complete migration by defining a replacement-first strategy, establishing rollout steps with feature flags, executing usage verification checks, and completing the final removal of legacy code, tests, documentation, and configuration once usage reaches zero.

Use feature flags for API sunset and migration rollouts when you need to move users to a new implementation gradually and safely. Feature flags enable incremental rollouts, allowing teams to route specific consumers to the replacement service while maintaining stability.

A legacy code removal checklist should include confirming zero active usage, followed by the complete deletion of the old code, associated tests, documentation, and configuration. This ensures no hidden operational risk remains after migrating consumers.