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I'm reading about the differences between a load-balancer implemented at the DNS level vs having a single DNS entry which forwards to a load-balancer.

I found this Q&A particularly useful:

I'm getting the impression from the top answer that DNS based load-balancing isn't reliable. When I run nslookup on some big sites most of them seem to have multiple IP entries:

nslookup google.com - 1 IP

nslookup amazon.com - 6 IPs

nslookup netflix.com - 8 IPs

Do the results from Amazon and Netflix imply that they are using DNS for round-robin load-balancing?

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As the top answer in the link says, load balancing based DNS is not reliable; it also is known for not being able to distribute load evenly.

When using DNS load-balancing techniques you are dependent both in caching choices done by intermediate DNS servers and by the client decisions.

In case of a failure, if only using DNS load balancing alone, you might have clients which do not move on to the healthy nodes.

You are also assuming servers/CDN IP addresses may map 1:1 to individual machines, which is not usually the case for the players you are naming; in fact with anycast technologies the same group of requests to "the same" IP addresses of the service will be directed to different data centers and different machines or even different technologies based on the geographical location from where the service request is made.

  • I find that misleading. If it wasn't reliable Netflix/Amazon/Stackoverflow wouldn't be using it. You're saying that each IP may resolve to an additional layer of load-balancing. It is still the case that DNS load-balancing is being used in addition to the lower level balancing... is it not? – Philip Kirkbride Sep 25 '17 at 2:03
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    It may be argued some technologies are on top of it to compensate for the shortcomings, including clustering and BGP routing. Point in case, if you are using DNS to distribute load for 1:1 servers, or without having several geographic locations, it wont work in a failure; it also wont distribute load evenly. – Rui F Ribeiro Sep 25 '17 at 12:39

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