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I have an ECS cluster of services running on a few EC2 machines. Each service is internet facing and forms a target group, all these target groups are on the same load balancer.

Architecture:
Service 1 -> TG1 -> ELB
Service 2 -> TG2 -> ELB

ELB Rules:
If request from 1.domain.com -> route to Service 1
If request from 2.domain.com -> route to Service 2

The services communicate with one another via their public CNAMEs.

I'm trying to figure out data-out transfer costs when Service 1 communicates with Service 2 by calling an endpoint on 2.domain.com. I think we'd be charged at the same rate as data-out to public internet even though the services are in the same region (could literally be hosted on the same machine), as the communication doesn't happen through a private IP but through the public internet. I've consulted the AWS documentation and can't find anything related to confirm this. Can someone help with this please?

Edited to Add: My AWS bill reflects data out charges because we use several public services and we provide information to clients outside the network. I want to know if this specific case is charged as well.

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    What does your AWS bill say when you try it? – womble Apr 17 '19 at 6:52
  • OpenGuide may help, specifically this picture. – Tim Apr 17 '19 at 7:34
  • Updated my question to include billing output. – svetha.cvl Apr 17 '19 at 15:37
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Even though you are accessing a service within the same server or VPC, if you access that service through the public IP address, then the connection will exit your VPC and re-enter. This will incur outgoing data transfer costs.

To access a service within the same VPC (or within the same server) without incurring data transfer costs, use the private IP address or localhost/127.0.0.1 (if the service is on the same server).

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  • That makes sense, thanks. I have multiple tasks to a service, all of them behind a load balancer, plus dynamic port management by ECS. So I don't think I can access using the private IP or localhost. – svetha.cvl Apr 18 '19 at 5:16
  • If your load balancer was internal only, then you could access it via the internal IP addresses and that wouldn't incur any data transfer costs. – Matt Houser Apr 18 '19 at 11:08

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