TTLs, GeoDNS and failover

What you take away

• A TTL is a request, not a rule: browsers, operating systems and some resolvers hold answers longer than you asked.
• Low TTLs buy agility during a change and cost resolver traffic and cold-path lookups the rest of the time.
• DNS routing policies — weighted, latency, geolocation, failover — decide which answer a resolver receives.
• DNS failover has a long tail, so real failover belongs to a load balancer or anycast; DNS is for planned moves.

How the class is laid out

1. The problem
2. The simple version
3. The long tail of a DNS change
4. Choosing a TTL
5. Where the answer comes from
6. Check your understanding