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I have a Multi-AZ ECS cluster with EC2 instances and an Aurora RDS cluster with cross-az read replicas. I use the cluster URL for selecting data in RDS. Does the cluster automatically forward my request to the RDS in the same AZ, if it is possible? If not, how can I prevent unnecessary data transfer between availability zones? (I want to use RDS in different AZ, if and only if the RDS in the same AZ is out of service)

According to AWS documentation:

"Data transferred between Amazon RDS and Amazon EC2 Instances in the same Availability Zone is free."

"Amazon RDS DB Instances inside VPC: For data transferred between an Amazon EC2 instance and Amazon RDS DB Instance in different Availability Zones of the same Region, Amazon EC2 Regional Data Transfer charges apply on both sides of transfer."

user1552545
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  • Since you haven't got an answer in a week: I think, but I'm not completely sure, that you'll end up in a random AZ. I'm not sure for Aurora but for MySQL it's not like this as far as I know, since the DNS is usually just round robin. My question would be: Why do you care? The traffic between EC2 and RDS is free, regardless of the AZ. The connection between AZs is also extremely good, the latency difference is hardly measurable. What exactly is the problem you're trying to solve? – Pampy Jul 30 '18 at 09:45
  • I have updated the description. – user1552545 Aug 11 '18 at 09:08

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