DNS Is Not a Load Balancer#

This needs to be said upfront: DNS was designed for name resolution, not traffic management. Using DNS for failover is a pragmatic hack that works well enough for most use cases, but it has fundamental limitations.

DNS responses are cached at multiple levels (recursive resolvers, OS caches, application caches, browser caches). You cannot force a client to re-resolve. You can set a TTL, but clients and resolvers are free to ignore it (and some do). Java applications, for example, cache DNS indefinitely by default in some JVM versions unless you explicitly set networkaddress.cache.ttl.