Blue-green deployment

They operate at different layers. Blue-green swaps the whole environment at once: every user moves to the new build together, and rollback means pointing the router back at blue. A feature flag works one level up, inside a single running environment, toggling an individual feature and serving it to a percentage or a targeted segment of users rather than everyone at once. Blue-green answers "which version of the whole app is live?"; a flag answers "which users see this one feature?". A canary release sits between them, shifting a fraction of traffic to the new version first.

The two compose cleanly, because they solve different halves of a release. Deploy the new build with blue-green while the risky feature is dark-launched behind a flag held at 0%, cut traffic over to green, then ramp the feature with a percentage rollout on its own schedule. That is the deploy-versus-release split: blue-green gets the code safely into production, and the flag controls when users actually see the feature. The rollout strategies guide shows how the serving side is configured.