Akamai Phased Release
At my day job I am migrating one of our websites to AWS. It is fronted by Akamai and we are using the Phased Release Cloudlet to split traffic between the on-premise instances and the AWS instances during our migration. One of the options in the configuration is to enable failover. If the new origin returns a error code, it stops sending traffic to the new origin for a configurable duration.
But how do you know it is failing? It isn't immediately obvious through the Akamai control panel, but there is a way. The HTTP response headers hold the secret.
aka_cd_failover_INSTANCELABEL = 1
If you are using Chrome, you can find them with the Development Tools.