Blue-green and canary deploys

Blue-green means running two identical production environments. All traffic flows to blue. You deploy the new version to green. You flip the load balancer. If something goes wrong, you flip it back. No partial rollouts, no mixed state, no downtime.

Canary is the same idea, gradual. Route 1 percent of traffic to the new version. Watch the metrics. Route 10 percent. Watch. Route 50 percent. Watch. Route 100 percent. If any step looks bad, flip traffic back to the old version immediately. Both blue-green and canary reduce the blast radius of a bad deploy.