Application Load Balancer (ALB) vs Network Load Balancer (NLB)

When building on AWS, managing traffic efficiently is a core requirement. Whether you're deploying a web application, a streaming service, or a high-frequency trading system, load balancing ensures high availability, fault tolerance, and performance.

Two of the most commonly used AWS Load Balancer types are:

• 🔷 Application Load Balancer (ALB)

• 🔶 Network Load Balancer (NLB)

But what really sets them apart? Let's break it down in detail — from OSI layers to latency, use cases, and configurations.

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What is a Load Balancer?

A Load Balancer distributes incoming network traffic across multiple targets (EC2 instances, containers, IP addresses, Lambda, etc.) to:

• Prevent server overload

• Improve responsiveness

• Ensure fault tolerance

• Scale horizontally

AWS Elastic Load Balancing (ELB) offers:

• Application Load Balancer (ALB) – Layer 7

• Network Load Balancer (NLB) – Layer 4

• Gateway Load Balancer (GLB) – Layer 3/Transparent

• (Classic Load Balancer is legacy)

Today, we focus on ALB and NLB.

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1. Application Load Balancer (ALB) – The Smart HTTP Router

ALB is Layer 7 in the OSI model — it understands HTTP and HTTPS traffic. This means it can inspect headers, cookies, methods, paths, hostnames, and even query strings.

📌 Features of ALB:

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🔄 Real-world Use Cases for ALB:

1. Microservices-based apps: Route /users to User service, /orders to Order service.

2. Multi-tenant apps: Different domains for different customers.

3. Single-page apps (React/Angular): Handle deep linking and fallback routes.

4. Serverless apps: ALB can directly invoke Lambda functions.

5. Kubernetes on AWS (EKS): ALB Ingress Controller is widely used.

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🧰 Configuration Tips:

• Set up Target Groups for each microservice or task.

• Use Listener Rules to define how traffic gets routed.

• Attach an SSL certificate from ACM for HTTPS traffic.

• Ensure your app exposes a /health endpoint (important for Target Group health checks!).

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⚡️ 2. Network Load Balancer (NLB) – The Ultra-fast Packet Dispatcher

NLB works at Layer 4 – the Transport layer. It doesn't inspect HTTP headers — it simply forwards TCP or UDP packets to targets. It's built for extremely high performance and low latency scenarios.

Features of NLB:

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🚀 When to Use NLB:

1. Gaming or Streaming apps: Where latency and throughput are critical.

2. IoT or custom TCP apps: That don’t use HTTP.

3. Financial trading apps: Require static IPs and super-low latency.

4. Internal load balancing: Between services inside a VPC.

5. Lift-and-shift legacy apps: That aren’t web-based.

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🧰 Configuration Tips:

• Choose TCP or UDP listeners.

• You can assign static IPs or Elastic IPs per AZ.

• Health checks can be just a simple TCP handshake (much faster).

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🧾 Comparison Table: ALB vs NLB

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Interview Insights

• ALB Target Group Health Checks expect a valid HTTP status (200–399). If you don’t expose a /health endpoint, EC2 will be marked unhealthy.

• NLB Target Group Health Checks are TCP handshakes unless HTTP is configured.

• NLB is ideal for hybrid or on-premise integrations via PrivateLink.

• ALB supports redirects, fixed responses, and even OIDC authentication flows – extremely useful for SaaS platforms.

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🧩 Final Thoughts

Both ALB and NLB are powerful, but they solve very different problems.

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