How to Do a Canary Rollout With Feature Flags

Target the flag at a small canary segment, watch metrics, then widen the audience or turn it off.

Canary releasing with flags means enabling a feature for a tiny slice of traffic first. You expand exposure only after the canary looks healthy, and revert instantly by flipping the flag off.

Steps

Define a canary segment (internal users or 1% of traffic).
Enable the flag for that segment only.
Monitor error rate and latency for the canary.
Widen the rollout or disable the flag based on the result.

Canary targeting rule

flags.yaml

flags:
  new-checkout:
    defaultVariation: false
    rules:
      - segment: canary   # internal users + 1% bucket
        variation: true

Promote after the canary passes

Terminal

# Terminal: widen once canary metrics are clean
flagctl update new-checkout --segment beta --variation true
flagctl update new-checkout --rollout 25

Gotchas

A canary segment too small to produce traffic tells you nothing; size it for signal.
Pair the canary with alerts so you catch regressions before you widen exposure.