Canary release

With a flag, the canary is a cohort rather than a separate pool of servers or a second environment you cut over to in a blue-green deployment. You serve the new variation to a small share of users through a percentage rollout, or to a deliberately chosen group such as internal staff through a targeted rollout, watch the metrics for that cohort, and widen exposure when the signal is clean. Because the new code already shipped behind the flag, promoting or aborting the canary is a configuration change with no redeploy, and a kill switch straight back to the old path if the canary turns sick. The rollout strategies guide covers how the serving is configured.

A canary release is the first, smallest step of a wider rollout, the initial exposure whose job is early detection. Progressive delivery is the broader practice that contains it: start with a canary, then ramp through larger tiers (often a beta cohort next) to 100% as the metrics hold. Put another way, a progressive rollout usually opens with a canary, but the canary is just the first move, not the whole sequence. See progressive delivery in depth for how the full sequence fits together.