We're trying to make a decision between using our hosting company for DNS or using Amazon's Route 53.
One of the benefits of touted by Amazon about Route 53 is that it uses Anycast which means that their DNS servers are distributed throughout the globe, and when a DNS request needs to be resolved, the request will be sent to the closest server, which will reduce the amount of time the lookup takes.
My question is - doesn't the DNS cache of whatever DNS the user is using make this irrelevant? Don't all secondary DNS servers cache local copies of the DNS so that they don't need to forward the request to the primary DNS server? And isn't that information updated automatically (not when the user requests it) based on the SOA records "refresh" value?
I have a similar question about uptime. Let's say we choose our hosting company's DNS, and it goes down for a few hours. Won't secondary DNS servers just continue to serve the cached content for as long as the SOA's "expire" value is set to (1 week, in our case)?