Blue-green deployment -- zero-downtime releases

Deploying a new version requires downtime or risky in-place updates. If the new version has a bug, rollback takes time.

Maintain two identical environments (blue and green). At any time, one is live (serving traffic) and one is idle (ready for next release). Deploy to idle, verify, then switch traffic. Rollback: switch back to the previous environment. Variants: (1) DNS switching: change DNS to point to new environment. (2) Load balancer: change target group. (3) Container orchestration: Kubernetes services switch between deployments.

Blue-green eliminates deployment risk by making the switch instant and reversible. The old version remains running until the new version is verified.