Application Load Balancer

Distribute traffic and enable HTTPS for your application

💵 Cost: ALB has a fixed hourly cost ~$16/month plus capacity charges. This runs 24/7 once created! Delete ALB when not in use to avoid charges. See our Costs & Cleanup Guide.

What is an ALB?

An Application Load Balancer (ALB) distributes incoming HTTP/HTTPS traffic across multiple targets (like ECS tasks). It operates at Layer 7 (application layer), meaning it understands HTTP and can make routing decisions based on URLs, headers, and more.

Why Use an ALB?

Distribute traffic across multiple containers for reliability
Terminate SSL/TLS (HTTPS) at the load balancer
Health checks automatically remove unhealthy targets
Path-based routing: /api/* to one service, /* to another

ALB Components

Plain Text

Internet
    │
    ▼
┌─────────────────────────────────────┐
│  Application Load Balancer          │
│  ├── Listener (port 443, HTTPS)     │
│  │   └── SSL Certificate            │
│  │                                  │
│  └── Listener (port 80, HTTP)       │
│      └── Redirect to HTTPS          │
└─────────────────────────────────────┘
    │
    ▼
┌─────────────────────────────────────┐
│  Target Group                       │
│  ├── Health Check: /api/health      │
│  ├── Target: ECS Task 1 (10.0.1.5)  │
│  ├── Target: ECS Task 2 (10.0.1.6)  │
│  └── Target: ECS Task 3 (10.0.1.7)  │
└─────────────────────────────────────┘

Key Concepts

Component	Purpose
Listener	Checks for connection requests on a port (80, 443)
Target Group	Group of targets (ECS tasks) that receive traffic
Health Check	Periodic request to verify targets are healthy
Listener Rules	Route traffic based on path, host, headers, etc.

The ALB periodically checks if your containers are healthy. Unhealthy containers are removed from rotation until they recover.

TypeScript

// app/api/health/route.ts
export async function GET() {
  // Check database connection, external services, etc.
  try {
    // await prisma.$queryRaw`SELECT 1`
    return Response.json({ status: "healthy" })
  } catch (error) {
    return Response.json(
      { status: "unhealthy", error: "Database connection failed" },
      { status: 503 }
    )
  }
}

Health Check Settings

Default health check settings might be too aggressive. Consider:

Interval: 30 seconds (not 5 seconds)
Timeout: 10 seconds
Healthy threshold: 2 consecutive successes
Unhealthy threshold: 3 consecutive failures

ALB Pricing

Region-Specific Pricing

Prices shown are for ap-southeast-1 (Singapore). Prices vary by region. Always verify at aws.amazon.com/elasticloadbalancing/pricing. Last verified: January 2026.

Component	Cost (ap-southeast-1)
ALB per hour	~$0.0225/hour (~$16/month)
LCU (Load Balancer Capacity Unit)	~$0.008/LCU-hour

Cost Tip

For small applications, the fixed hourly cost dominates. The ALB itself costs about $16-20/month regardless of traffic. LCU charges only matter at higher traffic volumes.

Common ALB Errors

Error	Meaning	Solution
`502 Bad Gateway`	Target returned invalid response	Check container logs, verify port mapping
`503 Service Unavailable`	No healthy targets	Check health check path, container health
`504 Gateway Timeout`	Target took too long to respond	Increase timeout, optimize slow endpoints

ECS Fargate

Cognito

Application Load Balancer

What is an ALB?

ALB Components

Key Concepts

Health Checks

HTTPS with SSL Certificates

Path-Based Routing

ALB Pricing

Common ALB Errors

Security Best Practices