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📐 System Design medium ⚡ 55 XP

With DNS-based failover between two regions, why can users keep hitting the dead region for a while after failover triggers?

DNS changes propagate only as fast as caches expire, so a 300-second TTL means up to five minutes of misdirected traffic — and some resolvers and applications cache longer than they should. Low TTLs on failover records and client-side retries mitigate this.

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