Feature flags — implementing gradual rollouts and A/B testing

Deploying new features is all-or-nothing. If a feature breaks, you need to deploy a revert. Can't test features with a subset of users. Release and deploy are tightly coupled.

(1) Decouple deploy from release: deploy code with feature behind a flag, enable flag separately. (2) Simple implementation: const flags = { newCheckout: process.env.FF_NEW_CHECKOUT === 'true' }. (3) Percentage rollout: hash(userId + flagName) %% 100 < rolloutPercentage. (4) User targeting: enable for specific users, orgs, or segments. (5) Services: LaunchDarkly, Unleash (self-hosted), Flagsmith, PostHog. (6) Clean up: remove flags after full rollout — flag debt is real. Set expiry dates. (7) Testing: test both flag states in your test suite. (8) Types of flags: release (temporary), ops (kill switch), experiment (A/B test), permission (permanent).

Feature flags let you ship code continuously without exposing unfinished features. They separate deployment (technical) from release (business), enabling gradual rollouts and instant rollbacks.