Today, we had our first drill with dual homing on reCAPTCHA. In Pittsburgh, the water main that serves the Carnegie Mellon area broke today, causing a complete water outage on campus. This has resulted in many servers being shut down. reCAPTCHAs servers were kept up, as they are production servers, however we were told that it was possible they'd be shut down.
It's times like these when you just love having a backup. We have a DNS service that does automatic health checking and routes away from unplanned outages. However, with DNS it takes a few minutes for these sorts of changes to take affect. We proactively switched away our traffic off of the pittsburgh servers.
One of the funny things about using DNS for Dual Homing is how long it takes to really kick in. We're still getting requests to our pittsburgh servers even hours after we made the switch. This is one reason it's important to have DNS not be the only load balancing solution (you need a L7 or L4 load balancer as well)