Inter-AZ Data Transfer Cost Structure in AWS Regions and Optimization Strategies for NLB, Valkey, and EKS

Inter-AZ Traffic Cost Structure and Basic Principles

Private traffic between Availability Zones (AZs) within an AWS region incurs a cost of $0.01/GB for each direction (totaling $0.02/GB). In contrast, communication within the same AZ is free, making it essential to keep traffic within the same AZ to reduce costs. Access via public IP is charged at $0.01/GB for both directions regardless of the AZ, so it is important to utilize free internal communication zones for internal traffic.

Load Balancer and Network Configuration Optimization

ALB does not charge for data transfer between AZs when using private IP communication within the same VPC. Unlike ALB, NLB charges $0.01/GB for both directions if the AZs differ, similar to standard EC2 traffic. To address this, you should configure dns_record.client_routing_policy to availability_zone_affinity or partial_availability_zone_affinity to prioritize NLB IPs in the same AZ as the client.

Database and Cache Service Cost Reduction

• RDS: Communication between EC2 and RDS within the same AZ is free, but for different AZs, the EC2 side is charged $0.01/GB for both sending and receiving. Transfer fees also apply during cross-region replication, making the selection of the primary location important.
• ElastiCache/Valkey: Round-robin reads generate inter-AZ traffic. Applying the AZ_Affinity read strategy in Valkey GLIDE prioritizes replicas in the same AZ, reducing costs. In a real-world case, HotelTrader reduced inter-AZ transfer costs by 95% and improved latency by 49% by applying this strategy along with request merging (HMGET).
• DynamoDB: In private subnets, it is recommended to use gateway endpoints to eliminate NAT Gateway fees.

EKS and External Traffic Optimization

EKS often involves significant traffic crossing AZ boundaries during pod-to-pod communication, so you should implement same-AZ priority routing using the IP mode and Topology Aware Routing of the AWS Load Balancer Controller, or Istio's sidecar routing. Additionally, to reduce NAT Gateway fees incurred when accessing external container registries (such as Docker Hub), you can utilize ECR Pull Through Cache. This caches external registry images in ECR, blocking traffic through the NAT Gateway, and the original example mentions a cost savings of approximately $189 per month.