We direct a user’s traffic to our services via a combination of UltraDNS and Route53 entries, our Denominator project provides a single client library and command line that controls multiple DNS providers. There are several reasons why we ended up using a combination of two:

UltraDNS provides us ability to directionally route customers from different parts of North America to different regional endpoints. This feature is supported by other vendors including Dyn, but is not supported in Route53. We didn’t want to use a latency based routing mechanism because it could cause unpredictable traffic migration effects.

By using Route53 layer between UltraDNS and ELBs, we have an additional ability to switch user traffic, and the Route53 API provides reliable and fast configuration changes that are not a strong point for other DNS vendor APIs.

Switching traffic using a separate Route53 layer makes such change much more straightforward. Instead of moving territories with directional groups, we just move Route53 CNAMEs.