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Comment by nathanaldensr | Hacker News Reader

The article posits why:

Why are DNS records set with such low TTLs?

- Legacy load balancers left with default settings

- The urban legend that DNS-based load balancing depends on TTLs (it doesn’t - since Netscape Navigator, clients pick a random IP from a RR set, and transparently try another one if they can’t connect)

- Administrators wanting their changes to be applied immediately, because it may require less planning work.

- As a DNS or load balancer administator, your duty is to efficiently deploy the configuration people ask, not to make websites and services fast.

- Low TTLs give peace of mind.

- People initially use low TTLs for testing, and forget to crank them up later.

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The DNS based load balancing isn’t a myth if you want to do any kind of load balancing that isn’t round robin. If you want to, say, send 10% of traffic to data center A and 90% to data center B, and you don’t want to use up 10 IPs to do that.

But those are no valid use cases which was my question. So there are no valid use cases at all?

> Administrators wanting their changes to be applied immediately, because it may require less planning work.

Why is this not valid?

But 5 minutes should be fine? It surprised me that so many has 1 second or 20 second TTL.

If I need to get a web service up and I can save 4 minutes by setting a low TTL when I configure my DNS record why wouldn't I?

Because you're pushing the cost on to someone else.

If your DNS hosting provider charged you per query (some do, especially when adding features like health checks & load balancing), then it might make a big difference.

Valid migration plan should be able to handle it without very short TTL.

The major cases revolve around failure recovery, and traffic distribution. A 5 minute outage is not acceptable in many industries or at scale.

If a load balancer or DC fails we need to ensure traffic moves away fast. Similarly if you want to take a system out for maintenance or perform migrations.

> A 5 minute outage is not acceptable in many industries or at scale.

Well, if that's the case, you better have your redundant systems on your normal DNS entries, because there is no chance you will distribute new entries over the internet in 5 minutes, whatever value you specify at the TTL.

There are tons of things that don't follow TTLs, but a large majority of normal people traffic does.

Easily 90% of new connections will move following the TTL. Of course, some traffic got a DNS result once in 2003 and is going to use that forever. If it's important traffic, you can trace it and follow up with them. If not, you do the best you can and let the rest go.

Found an earlier HN discussion about this question from November 2019: https://news.ycombinator.com/item?id=21437160

A low DNS TTL for testing purposes is a valid use case.

This is about production systems of large enterprises.

...

Please tell me you're not testing in production.

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