How to Minimize EC2-to-S3 Data Transfer Costs from Private Subnets?

SAA-C03 By islever Editorial Last updated

Design cost-optimized network architectures. Design secure access to AWS resources.

Short answer

A Create an S3 gateway endpoint and add it to the private subnet route tables so EC2-to-S3 traffic stays on the AWS network with no transfer cost.

Question

A solutions architect is creating an application. The application will run on Amazon EC2 instances in private subnets across multiple Availability Zones in a VPC. The EC2 instances will frequently access large files that contain confidential information. These files are stored in Amazon S3 buckets for processing. The solutions architect must optimize the network architecture to minimize data transfer costs. What should the solutions architect do to meet these requirements?
  1. Create a gateway endpoint for Amazon S3 in the VPC. In the route tables for the private subnets, add an entry for the gateway endpoint. Correct Answer
  2. Create a single NAT gateway in a public subnet. In the route tables for the private subnets, add a default route that points to the NAT gateway.
  3. Create an AWS PrivateLink interface endpoint for Amazon S3 in the VPIn the route tables for the private subnets, add an entry for the interface endpoint.
  4. Create one NAT gateway for each Availability Zone in public subnets. In each of the route tables for the private subnets, add a default route that points to the NAT gateway in the same Availability Zone.

Key Takeaways

The question tests whether you know that gateway endpoints for S3 carry no hourly or per-GB charge, while the trap is assuming a NAT gateway or PrivateLink interface endpoint is needed to reach S3 from a private subnet.

Amazon EC2 instances in private subnets can reach Amazon S3 without a NAT gateway by using an S3 gateway VPC endpoint, which keeps traffic on the AWS network. This page confirms why a gateway endpoint (option A) is the cost-minimizing choice for frequent, large confidential file transfers.

Common mistake: Many candidates pick a NAT gateway (options B or D) because it does let private instances reach the internet, but NAT gateways bill per hour plus per GB processed, which is exactly the data transfer cost the question asks you to eliminate.

Expert Analysis

Why the Answer Is Correct

A gateway VPC endpoint for Amazon S3 attaches to the route tables of the private subnets, so any traffic destined for S3 service addresses is routed privately over the AWS network instead of through a NAT gateway or the public internet. AWS does not charge for the gateway endpoint itself, nor for the data processed through it, which directly satisfies the requirement to minimize data transfer costs for frequent large-file access. It is also the more secure option, because the confidential objects never traverse a public network path. Confirming the endpoint in the private subnet route tables is the essential configuration step, and option A does exactly that.

Why the Other Options Are Wrong

Option B uses a single NAT gateway in one public subnet, which introduces per-hour and per-GB charges and creates an Availability Zone dependency and a single point of failure. Option D spreads NAT gateways across Availability Zones, improving resilience, but it multiplies the hourly cost and still bills every gigabyte of S3 traffic as NAT data processing. Option C uses an AWS PrivateLink interface endpoint for S3, which is valid for on-premises or hybrid access but bills an hourly endpoint charge plus per-GB data processing, again failing the cost-minimization goal. Gateway endpoints are the purpose-built, free path from a VPC to S3, so the interface endpoint in C is the wrong tool for this requirement.

Community Comment Notes

Every recorded voter selected A, and the explanations converge on the same reasoning: as Scheldon put it, A is the only option that makes sense because it is "cheap (no cost)" and traffic never goes to the public network. Hkayne reinforced this by noting the traffic "will stay on aws infrastructure," and AlvinC2024 captured the decisive point that a "Gateway endpoint is free," linking AWS's own comparison of interface versus gateway endpoints. The referenced AWS gateway endpoint documentation is the authoritative source here. No commenter defended a NAT gateway or PrivateLink alternative, which matches the cost analysis above.

Official Reference

https://docs.aws.amazon.com/vpc/latest/privatelink/gateway-endpoints.html

This link opens the official vendor documentation in a new tab.

Exam Strategy

When a scenario mentions frequent access to S3 from private subnets plus minimizing data transfer costs, reach for the free S3 gateway endpoint before considering NAT gateways or PrivateLink. Remember the pricing split: gateway endpoints cost nothing, while NAT gateways and interface endpoints bill hourly plus per-GB, so they are only the answer when the requirement is on-premises, cross-Region, or third-party service access.

Frequently Asked Questions

Why is a NAT gateway not sufficient for private EC2 instances accessing S3?

NAT gateways do provide outbound access, but they charge hourly plus per-GB data processing, so heavy S3 traffic becomes expensive and the data still leaves the AWS private network path.

When would an AWS PrivateLink interface endpoint for S3 be the better choice?

Interface endpoints are needed for on-premises, peered, or cross-Region clients that cannot use route-table gateway endpoints, but they add hourly and per-GB charges that fail this cost requirement.

More SAA-C03 FAQ →

Community Votes

A
100%

100% of anonymous learners picked answer A. Votes are pick records left by other test-takers — they are not the verified answer.

Discussion 6 contributions

Compiled from public community threads and reader submissions, lightly edited for clarity. Handles and vote tallies are shown as recorded, and comments reflect the views of individual contributors.

From public community threads 6

  1. Scheldon 👍 3 Answered A
    AnswerA I think only option A have any sense. It is cheap (no cost), it is secure (traffic is not going to public network). https://docs.aws.amazon.com/vpc/latest/privatelink/gateway-endpoints.html
  2. Hkayne 👍 2 Answered A
    Aws gateway will have no cost because the trafic will stay on aws infrastructure.
  3. Tanidanindo 👍 2 Answered A
    Gateway endpoint will minimize data transfer costs
  4. AlvinC2024 👍 1 Answered A
    Gateway endpoint is free https://digitalcloud.training/vpc-interface-endpoint-vs-gateway-endpoint-in-aws/.
  5. xBUGx 👍 1 Answered A
    gateway endpoint for Amazon S3
Show 1 more comment
  1. Awsbeginner87 👍 2 Answered A
    A- gateway endpoint for S3

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