Ok, So I got official answer from Support team:
For HTTP API -> CloudMap integration, API Gateway service calls "DiscoverInstances" API call. This call returns healthy instances. API Gateway then selects any one of the instances randomly to route the actual request to.
Hence we can understand that traffic distribution differs in HTTP API -> CloudMap integration from that of HTTP API -> Application Load Balancer. Because ALB uses round robin to distribute the traffic, where in the traffic is distributed randomly in case of integration with CloudMap.
Tim here with the Premium Support Team and I’ve been reading through your notes and checked out the blog post you linked.
If you can could you open a AWS support case with us to further explore this topic? We’d like to understand the use case you have a bit more in depth and possibly gather some ARNs which you can privately share with us then.
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