How does a blue-green deployment work, and why use it for model serving?

Blue-green deployment runs two identical production environments: blue (the current live version) and green (the new version). You deploy the new model to green while blue keeps serving all traffic, then run smoke tests and validation against green. When green is verified healthy, you switch the router or load balancer to send all traffic to green instantly. Blue stays running and idle, so if the new model misbehaves you flip traffic back to blue for a near-instant rollback with no redeploy. The benefits for model serving are zero-downtime cutover, fast and safe rollback, and the ability to fully validate the new environment before it takes traffic. The costs are running two full environments (double the resources during the switch) and handling stateful concerns like warm caches or in-flight requests. It differs from canary, which shifts traffic gradually rather than all at once.