What is Blue-Green Deployment?

Blue-Green Deployment, explained

In a blue-green deployment, the current version of an application runs in one environment, called blue, while the new version is deployed and tested in a second environment, called green. When the green environment passes checks, a load balancer or router sends all traffic to it. Blue stays available for a quick rollback.

For example, an online banking portal could deploy an update to the green environment overnight, run automated tests against it, then switch users over in seconds. If an error appears, traffic moves back to blue almost immediately. This approach reduces downtime and release risk. Its drawbacks include the cost of running two environments and the difficulty of handling database changes, which must work with both versions during the switch.