Blue-green deployments is a technique where a new version of an application never gets deployed to the production servers directly. Instead, it gets deployed to another set of servers first. Once this has be done successfully, users are directed to the new deployment.

Let's assume that an application runs on a total of three hosts by default. A typical setup for blue-green deployment would be two sets of three hosts: the blue group and the green group. In front of these two sets, there is a reverse proxy that functions as a load balancer and redirects the incoming requests to the blue group. The following diagram shows how this works:

To deploy a new version of the application in this situation, it needs to be deployed to the green group of servers. Since these servers are not receiving any traffic from end users, this has no impact on them at all.

After the deployment, the new deployment can be verified to ensure it was successful and that the application is running correctly. After this verification, the load balancer is reconfigured to redirect traffic to the green group. Now, the new version of the application is served.

Should there suddenly be any unexpected issues, it is very easy to switch back to the previous deployment by reconfiguring the load balancer back to the blue group. If the deployment is successful and there are no issues, it is possible to start the deployment of the next version by going through the same procedure, but now with the roles of the green and the blue groups switched.