I'm in the design process for a Java web app that I will probably end up deploying to Google App Engine (GAE). The nice thing about GAE is that I really don't have to worry about fortifying my app from the dreaded DDoS attack - I just specify a "billing ceiling", and if my traffic peaks up to this ceiling (DDoS or otherwise), GAE will just shut my app down. In other words, GAE will essentially scale to any amount until you simply can't afford to keep the app running any longer.
So I'm trying to plan a contingency whereby, if I do hit this billing ceiling and GAE shuts my app down, my web app domain DNS settings "fail over" to another, non-GAE IP address. Some initial research has shown that certain CDNs like CloudFlare offer services for this exact situation. Basically, I just keep my DNS settings with them, and they provide an API I can hit to automate a failover procedure. Thus, if I detect that I'm at 99% my billing ceiling for my GAE app, I can hit this CloudFlare API, and CloudFlare will dynamically change my DNS settings to point away from the GAE servers to some other IP address.
My initial contingency would be to failover to a "read-only" (static content only) version of my web app hosted somewhere else, maybe by GoDaddy or Rackspace.
But then it suddenly dawned on me: if DDoS attacks target the domain name, what difference does it make if I rollover from my GAE IP address to my (say) GoDaddy IP address? In essence, the failover wouldn't do anything other than allows the DDoS attackers to bring down my backup/GoDaddy site!
In other words, DDoS attackers coordinate an attack on my web app, hosted by GAE, at
www.blah-whatever.com, which is really an IP address of 18.104.22.168. They cause my traffic to spike to 98% my billing ceiling, and my custom monitor triggers a CloudFlare failover from 22.214.171.124 to 126.96.36.199. The DDoS attackers don't care! They're still launching an attack against
www.blah-whatever.com! The DDoS attack continues!
So I ask: what protection do CDNs like CloudFlare offer so that - when you need to fail over to another DNS - you aren't at risk for the same, continued DDoS attack? If such protection exists, are there any technical restrictions (e.g. read-only, etc.) that are placed on the failover site? If not, what good are they?! Thanks in advance!