Feature Flags: Decouple Deploy From Release Without Losing Control

A **feature flag** (feature toggle) gates runtime behavior without shipping a new binary. Martin Fowler's feature toggle taxonomy (release, experiment, ops, permission) remains a useful framing on `martinfowler.com`.

Flags need **lifecycle management**: create, default off in production, roll out by percentage or cohort, monitor, then remove dead code. Long-lived flags become branching debt.

Store flag state in a centralized service (LaunchDarkly, Unleash, Flipt, vendor-specific offerings) or configuration with audit trails. Client-side flags alone are not security boundaries; sensitive entitlements belong on the server.

Pair flags with observability: tag metrics and traces with flag variants to detect regressions during gradual rollout.