In red-black deployment, before standing up a new version of a system, first, perform canary testing. The canary replaces around 1% of its existing production system with the latest version of the application and monitors the newest version for errors. If the canary clears this initial test, the system is deemed ready for deployment.
In preparation for the switchover, a new version of the system stands up side by side with the old version of the system. The initial capacity of the new system is set manually by examining how many instances are currently running in production and setting this number as the desired capacity for the new auto scaling group. Once the new system is up and running, both systems are red. The current version is the only version accepting traffic.
Using the DNS service, the system is then cut over from the existing version to the new version. At this point, the old version is regarded as black; it is still running but is not receiving any traffic...