deprecation-and-migration

Safe API retirement requires a replacement-first approach: build the new API, publish deprecation notices, migrate consumers incrementally with verification, and remove the old API only after metrics confirm zero active usage.

Feature deprecation planning starts at design time by evaluating unique value, consumer dependency, and migration cost, then applies patterns like the Strangler pattern or feature flags to incrementally migrate users and reduce downtime.

The Strangler pattern is used for system migration when you need to incrementally replace legacy functionality by building new services alongside the old system, reducing behavioral mismatch and downtime during the transition.

Decide whether to maintain or remove a redundant service by evaluating its unique value, consumer dependency, replacement availability, migration cost, and ongoing maintenance cost to determine if deprecation reduces long-term technical debt.

Feature flags manage API deprecation by allowing incremental consumer migration with verification, gating new behavior while old code remains active until metrics confirm zero active usage before final removal.

Removing legacy code before confirming zero active usage breaks production because active consumers still depend on the old API or feature, making post-migration removal verification critical to avoid breaking contracts and workflows.