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by samprotas 1965 days ago
Having executed several "no-downtime" cutovers between systems via DNS updates, I will warn you that a surprising number of clients never re-resolve DNS, so the TTL is effectively "forever" from their point of view.

For the rare case of lift-and-shift-ing for a system upgrade I felt morally okay about eventually pulling the plug on them, but I'd hesitate to design a system that relied on well-behaved DNS clients if I had a reasonable alternative.
3 comments
tyldum 1965 days ago
Another gotcha would be UDP based services. Since it is packet oriented and not connection oriented, when should it re-resolve? Most will not until the application is restarted.
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gary_0 1965 days ago
When I last updated a domain most clients saw the change within the TTL (1 hour)... except for my cable ISP at home. It took them the better part of a week.
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xorcist 1965 days ago
Moving by DNS change isn't usually that bad. The old system (load balancer) can proxy requests to the new system. Most clients will follow DNS but the laggers won't have too much trouble. Assuming the service already works behind a load balancer of course, that is usually not something than can be fork-lifted in.
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