Route 53 : Simple Routing Policy vs Multivalued Answer

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If I have a Simple Routing Policy for www.example.com with an A record pointing to 5 IP addresses, this is what the A record would look like:

A - 1.2.3.4, 1.2.3.5, 1.2.3.6, 1.2.3.7, 1.2.3.8

Now, if a client that makes a request to "www.example.com", he gets "all the 5" IP addresses regardless of the health status of the 5 IP addresses in any random order.

If I have to configure the above using a Multivalued Answer, then we create 5 A records each with an individual IP with health checks in place.

A - 1.2.3.4

A - 1.2.3.5

A - 1.2.3.6

A - 1.2.3.7

A - 1.2.3.8

Now, if a client that makes a request to "www.example.com", then, based on the health checks, the client gets up to 8 healthy responses. In our case we have 5 IPs if all 5 are healthy. We assume all 5 are healthy.

Now, as per https://aws.amazon.com/premiumsupport/knowledge-center/multivalue-versus-simple-policies/, in case of Simple Routing Policy, the 5 IPs are not health checked. If the client receives an unhealthy IP and it queries this IP, the client experiences downtime. The Multivalued Answer scores by already having the assurance in the form of 8 healthy records (in our case 5 records). If one of the IP addresses goes down, while the DNS entry is cached in the browser, then in case of Multivalued Answer, the client software can always try for another one from the rest of the healthy IP addresses.

My question is, if availability of multiple IPs in the DNS response is the thing that makes the client software try for other healthy IPs , then so is the case for Simple Routing policy also. You still get the entire list of IPs in the A record as the response to the DNS query. Yes, they are not health checked unlike Multivalued Answer. But, just like, in the case of Multivalued Answer, the client software can still try other IPs. But the link says that the user experiences downtime if it tries the unhealthy IP in case of Simple Routing Policy. It does not talk about the presence of other IPs in the DNS Response which the client software can still try out.

Am I missing something?

Edited by: sheelstera on Apr 7, 2019 11:34 AM

asked 5 years ago607 views
3 Answers
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Accepted Answer

You're totally right, the document is misleading. Giving clients the opportunity to try different addresses is an advantage of any multi-value response, regardless of the routing logic used by Route 53 to select that answer.

The referenced downtime would occur if the client software isn't smart enough to failover between addresses, and for smarter clients there would still be a latency penalty because it takes time for the client to try different addresses. So Multivalue Answer is an additional improvement on top of the merits of having multiple addresses in the responses, by fixing the latency penalty or downtime once the resolver refreshes the answer.

AWS
answered 5 years ago
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Thank you so much Matt for clearing the confusion.

answered 5 years ago
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I feel the Document is still correct. Assume we have 9 IP with status (5 healty and 3 Unhealty) as below as below

A - 1.2.3.1 (Healty)

A - 1.2.3.2 (Healty)

A - 1.2.3.3 (Unhealty)

A - 1.2.3.4 (Healty)

A - 1.2.3.5 (Unhealty)

A - 1.2.3.6 (Unhealty)

A - 1.2.3.7 (Healty)

A - 1.2.3.8 (Healty)

A - 1.2.3.9 (Healty)

If we choose simple routing policy then Client receives all the IPs, and Client will choose the random on. If it chooses unhealty IP then you expirience downtime else you can access the resource.

Whereas with Multi-value, Client receives only Healty IPs, so we will not expirience any downtime eventhough 3 IPs are unhealty.

Ibrahim
answered a year ago

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