Running two identical production environments and instantly switching all traffic from the old one to the new one, enabling near-zero-downtime releases and instant rollback.
Example: Flipping traffic from the old live environment to the new standby one instantly.
In practice: Unlike a canary release, blue-green is all-or-nothing per switch, which is simpler to reason about but gives less signal about partial failure than a gradual rollout.
Debated: Debated for stateful systems — an instant full-traffic switch is easy for stateless services, but for anything holding in-flight sessions or writing to a shared database, the 'instant' switch can still cause data inconsistency.