alt Hacker NewsThe biggest issue with DNS is not the protocol, or even the reference implementation. It's the people who think they are clever and try to make things better by making them worse.
The most egregious of course is ISPs rewriting TTLs (or resolvers that just ignore them). But there are other implementation issues too, like caching things that shouldn't be or doing it wrong. I've seen resolvers that cache a CNAME and the A record it resolves to with the TTL of the CNAME (which is wrong).
I'm also very concerned about the "WHY DNS MATTERS FOR SYSTEM DESIGN" section. While everything there is correct enough, it doesn't dive into the implication of each and how things go wrong.
For example, using DNS for round robin balancing is an awful idea in practice. Because Comcast will cache one IP of three, and all of a sudden 60% of your traffic is going to one IP. Similar issue with regional IPs. There are so many ways for the wrong IP to get into a cache.
There is a reason we say "it's always DNS".
ISP DNS servers really ought to be banned, they are always so bad. I've seen traffic days later on a record with 1 hour TTL. In general I see like 50% traffic move after the initial 1-2x TTL interval, another 40-45% over next several hours up to one day, and then the last 5-1% can take forever.
For round-robin, I've actually had it work reasonably well for API usage. Of course it's not ideal, but when I wanted to roll out new things slowly over several days and could not use a load balancer or reverse proxy, it kind of worked. I think most API users are just running with a reasonable resolver and not residential ISP ones.