Amazon's Route 53 DNS service is well-regarded for its ability to manage complex domain allocations. New features added to the service enable more sophisticated "health checks" to see if your site is up and running.
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Route 53 can now be configured to automatically switch your site to a different server if a defined string isn't returned in the first 5120 bytes of a designated test page. That string could be HTML expected within the page, or text checked for by a pre-programmed task that runs regularly. Check the AWS blog post for examples of how it can be used.