Feature flags -- decouple deployment from release

Releasing features requires deploying code. Long-lived feature branches cause merge conflicts. Rollback means redeploying. Cannot gradually roll out to subsets of users.

Use feature flags to toggle features at runtime without redeployment. Deploy code behind flags, then enable gradually. Patterns: (1) Boolean toggle: feature on/off. (2) Percentage rollout: 5% -> 25% -> 100%. (3) User targeting: enable for beta users. (4) Kill switch: instant disable without deploy. Tools: LaunchDarkly, Unleash, Flagsmith, or simple DB/config-based flags. Clean up: remove flags after full rollout to avoid flag debt.

Feature flags separate the technical act of deploying from the business decision of releasing. This enables continuous deployment, canary releases, and instant rollback.