The Drawbacks of Using NAT Gateways: A Costly Experience

In today’s cloud-native world, serverless services and managed container orchestration platforms like AWS ECS Fargate promise ease of use, scalability, and reduced operational overhead. However, these benefits come with their own set of challenges, especially when combined with Network Address Translation (NAT) Gateways. In this article, I’ll share my personal experience with NAT Gateways and highlight the potential pitfalls that you should be aware of.

Our Setup: AWS ECS Fargate with Private Subnets

In one of our projects, we’ve been using AWS ECS Fargate for over two years to run our containerized applications. Our ECS Fargate tasks run in private subnets for enhanced security, which means they require a NAT Gateway for outbound internet access. This setup worked well for us until we encountered an unexpected issue.

The Incident: Restarting ECS Tasks and Data Transfer Costs

Recently, some features with potential issues were released in our test and staging environments. Due to these issues, some of our ECS Fargate tasks were repeatedly restarting. Since ECS Fargate is a serverless service, there is no guarantee that a task will be placed on the same machine it was previously on. Our Docker images were configured to pull Always, which meant every task restart resulted in a fresh image pull from the Docker registry.

Here’s where the NAT Gateway came into play: every time an ECS task restarted, it had to download the Docker image through the NAT Gateway. Our region was Singapore, where data transfer costs through the NAT Gateway are approximately $0.06 per GB. With tasks restarting frequently, we saw data transfers spike to 179GB per day.

The Cost: Unexpected and Unplanned Expenses

This unexpected data transfer resulted in a cost of nearly $70 for just 5 days. We had set up alerts for daily costs, and fortunately, we noticed the incident quickly, but not before incurring an extra $70 in costs. Had we not had these alerts, the situation could have been much worse, potentially leading to an extra $450 in costs for a month.

Lessons Learned: Be Cautious with Serverless and NAT Gateways

This experience has taught us some valuable lessons that we want to share:

1. Monitor and Alert for Costs

Set up detailed monitoring and alerts for your cloud costs. This helped us catch the issue early and mitigate further expenses. Without these alerts, we could have faced significant unexpected costs.

2. Understand the Cost Implications of NAT Gateways

NAT Gateways can quickly become expensive, especially when dealing with large amounts of data transfer. Be aware of the costs in your specific AWS region and consider how frequent task restarts or deployments might impact these costs.

3. Review and Optimize Image Pull Policies

n our case, the Always image pull policy led to frequent and unnecessary downloads, significantly driving up data transfer costs. While using the IfNotPresent policy can mitigate this issue by only pulling the image if it is not already present locally, this approach has limitations in a serverless environment like AWS ECS Fargate. In serverless setups, tasks may be placed on different underlying hosts each time they start, leading to image downloads with every restart regardless of the pull policy.

To optimize and reduce data transfer costs in such environments:

• Use Smaller, Optimized Images: Ensure your Docker images are as small and optimized as possible. Remove unnecessary layers, use multi-stage builds, and leverage lightweight base images to minimize the amount of data that needs to be transferred.
• Leverage Regional Container Registries: Use regional container registries such as Amazon ECR (Elastic Container Registry) to host your images. This can reduce latency and data transfer costs compared to pulling images from non-regional or public registries.
• Implement Efficient Caching Strategies: While serverless environments make local caching difficult, explore options for caching layers or using shared storage solutions to minimize repeated downloads.

4. Test Thoroughly Before Releasing Features

Ensure that new features are thoroughly tested before release, especially in environments that use serverless platforms and NAT Gateways. This can help avoid unexpected behavior that might lead to increased costs or other issues.

5. Explore Alternatives and Optimizations

Consider alternatives to NAT Gateways, such as using VPC endpoints for services like Amazon S3 or DynamoDB, which can reduce data transfer costs. Additionally, review your architecture for potential optimizations.

Conclusion

Using NAT Gateways in conjunction with serverless platforms like AWS ECS Fargate can offer many benefits, but they also come with potential risks and costs. Our experience underscores the importance of vigilant cost monitoring, thorough testing, and careful consideration of network architecture choices. By being aware of these potential pitfalls, you can better manage your cloud infrastructure and avoid unexpected expenses.

So, before releasing any feature on a serverless platform, especially when using NAT Gateways, take the necessary precautions to ensure you don’t encounter the same costly surprises we did.