Discovering ECS Fargate Fixed Costs (ALB + NAT Gateway) and Rethinking the Architecture

Introduction

I thought ECS Fargate was “pay only for what you use,” but when I actually ran it, I discovered fixed monthly costs of several thousand yen or more.

The culprits were ALB (Application Load Balancer) and NAT Gateway.

Understanding the Cost Structure

ECS Fargate costs fall into two main categories.

Variable Costs (pay for what you use)

Item	Billing Unit
Fargate container	vCPU & memory × uptime
RDS	DB instance uptime

Fixed Costs (charged constantly while running)

Item	Tokyo region estimate (2024)	Monthly estimate
ALB	$0.0243/hr + LCU	~$18+ / month
NAT Gateway (per AZ)	$0.062/hr + data transfer	~$45+ / month

NAT Gateway is charged per AZ. With multi-AZ configuration (2 AZs) for redundancy, NAT Gateway alone costs $90+/month.

Why I Didn’t Notice

I had the impression that “ECS Fargate is nearly free when no containers are running.” However, ALB and NAT Gateway are charged just by existing, regardless of container instance count.

Even if you scale tasks down to 0 thinking you’re saving money, fixed costs continue as long as ALB and NAT Gateway exist.

Architecture Revision

Before revision

Internet → ALB → ECS Fargate (Private Subnet)
                              ↓
                         NAT Gateway → Internet (ECR/SSM outbound)
                              ↓
                            RDS

After revision (minimum cost)

Internet → ALB → ECS Fargate (Private Subnet)
                              ↓
                    VPC Endpoint (ECR/S3/SSM) ← NAT Gateway replacement
                              ↓
                            RDS

Replace NAT Gateway with VPC Endpoints

Resource	Purpose	Alternative
NAT Gateway	Pull images from ECR	VPC Endpoint for ECR
NAT Gateway	Access to S3	VPC Gateway Endpoint for S3 (free)
NAT Gateway	SSM Parameter Store	VPC Endpoint for SSM

VPC Endpoint Costs

VPC Endpoint (Interface type) costs $0.014/hr × number of AZs.

Using 3 endpoints in a single AZ:

$0.014 × 3 = $0.042/hr
Monthly: ~$30

This is cheaper than NAT Gateway ($0.062/hr × 1 AZ = $45/month).

ALB Alternatives (for dev/test environments)

For development cost reduction, removing ALB and accessing ECS directly is also an option.

Method	Cost	Downside
With ALB	$18+/month	—
Without ALB (direct public IP)	Free	No HTTPS (requires self-signed cert, etc.)
Without ALB (via CloudFront)	Low cost	Complex configuration

ALB is required for production, but we reconsidered whether ALB was needed for dev/test environments.

Revised Monthly Cost Estimate

Resource	Configuration	Monthly estimate
ECS Fargate	0.25vCPU × 0.5GB, 1 task, 24 hrs	~$10
RDS PostgreSQL	db.t4g.micro, single AZ	~$15
ALB	1 unit	~$18
VPC Endpoint	Interface × 3 (single AZ)	~$30
S3	Static file delivery	~$1
Total		~$74/month

Compared to before (NAT Gateway × 2 AZs), this saves about $30–40 per month.

Saving with Task Scale-Down

In dev environments, scaling ECS tasks to 0 overnight reduces Fargate compute costs.

# Scale down to 0 tasks (nights/weekends)
aws ecs update-service \
    --cluster dvd-rental-cluster \
    --service dvd-rental-service \
    --desired-count 0

# Scale back to 1 when starting development
aws ecs update-service \
    --cluster dvd-rental-cluster \
    --service dvd-rental-service \
    --desired-count 1

Note that fixed costs for ALB and NAT Gateway (or VPC Endpoint) remain unchanged.

Summary

Easy-to-miss ECS Fargate cost points:

ALB has a $18+/month fixed cost — charged even when tasks = 0
NAT Gateway costs $45+/month (per AZ) — doubles with multi-AZ
Switching NAT Gateway → VPC Endpoint can save $15–30/month
Use different configurations for dev vs. production — production needs multi-AZ, single-AZ is enough for dev

“Serverless is cheap” is half true — understanding the fixed costs of ALB and NAT Gateway upfront is critical.

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