AWS ALB vs. AWS NLB: Which one to use?

In the rapidly evolving world of cloud computing, selecting the right load balancer is crucial for building scalable, reliable, and high-performing applications. Amazon Web Services (AWS) offers multiple load balancing solutions, with the Application Load Balancer (ALB) and Network Load Balancer (NLB) being two of the most prominent options. This comprehensive guide will help you understand the key differences between ALB and NLB, their respective features and limitations, and how to decide which one is the best fit for your needs.

Understanding Load Balancing

Load balancing is a critical component in modern cloud architectures. It distributes incoming network traffic across multiple targets—such as Amazon EC2 instances, containers, IP addresses, and even AWS Lambda functions—to ensure high availability and reliability. AWS offers several load balancing options, but ALB and NLB are the most commonly used for application and network layers, respectively.

AWS Application Load Balancer (ALB)

Features

The AWS Application Load Balancer operates at the Application Layer (Layer 7) of the OSI model. It is designed to handle HTTP and HTTPS traffic, making it ideal for web applications.

• Content-Based Routing: ALB supports advanced request routing based on the content of the request. You can route traffic to different backend services based on URL paths, host headers, HTTP headers, HTTP methods, query string parameters, and source IP addresses.

• Microservices and Microfrontends Support: ALB is excellent for microservices architectures. You can use a single ALB to route requests to multiple services based on path or host. This is particularly beneficial for applications using microfrontends, where different parts of the UI are served by different services.

• AWS Lambda Integration: ALB can directly invoke AWS Lambda functions as targets. This allows you to build serverless applications that can be accessed over HTTP(S) without the need for an API Gateway, simplifying your architecture.

• Integration with AWS WAF: ALB integrates seamlessly with AWS Web Application Firewall (WAF), allowing you to protect your web applications from common web exploits and bots.

• WebSockets and HTTP/2 Support: ALB supports WebSockets and HTTP/2 protocols, enabling real-time communication between clients and servers.

• Authentication and SSL Termination: ALB can authenticate users using Amazon Cognito or OIDC-compliant identity providers and terminate SSL connections, reducing the load on backend instances.

Limitations

• Maximum Request Body Size: ALB has a maximum request body size limit of 1 MB. This means it cannot process requests (such as file uploads) larger than this limit.

• Performance Considerations: While ALB is sufficient for most web applications, it may introduce higher latency compared to NLB due to operating at Layer 7 and performing content-based routing.

Use Cases

• Web Applications: Ideal for applications that use HTTP/HTTPS protocols and require advanced routing features.

• Microservices Architectures: Suitable for routing traffic to multiple backend services based on request content.

• Serverless Applications: Great for invoking AWS Lambda functions directly, enabling serverless web applications without additional infrastructure.

• Web Application Security: When you need to protect your application using AWS WAF.

AWS Network Load Balancer (NLB)

Features

The AWS Network Load Balancer operates at the Transport Layer (Layer 4) of the OSI model. It is designed for extreme performance and can handle millions of requests per second while maintaining ultra-low latencies.

• Protocol Support: NLB supports TCP, UDP, TCP_UDP, and TLS protocols, making it suitable for both HTTP/HTTPS and non-HTTP applications.

• TLS Termination: NLB can terminate TLS (formerly SSL) connections. This allows you to offload the encryption and decryption workload from your backend servers, improving their performance.

• Preservation of Source IP Address: NLB preserves the client’s source IP address, allowing your backend applications to see the original IP address of the client. This is essential for logging, security policies, and other IP-based functions.

• Static IP Addresses: NLB provides a single static IP address per Availability Zone, which can be associated with Elastic IP addresses.

• High Performance and Low Latency: NLB is capable of handling volatile traffic patterns and sudden spikes, making it ideal for performance-critical applications.

• Zonal Isolation: Designed for high resiliency, NLB can handle failures in a single zone without affecting the overall service.

Limitations

• No AWS WAF Support: NLB does not integrate with AWS WAF because it operates at Layer 4, and WAF is a Layer 7 service.

• Limited Content-Based Routing: NLB does not support advanced content-based routing like ALB. It routes traffic based on protocol data rather than request content.

Use Cases

• Non-HTTP/HTTPS Traffic: Ideal for applications that use protocols other than HTTP/HTTPS, such as SMTP, FTP, or custom TCP/UDP protocols.

• Performance-Critical Applications: Suitable for applications that require ultra-low latency and can handle millions of requests per second.

• Static IP Requirements: When you need a fixed IP address for your load balancer.

• Applications Requiring Source IP Preservation: Useful when backend servers need to know the client's IP address for security or logging purposes.

Comparing ALB and NLB

Decision Checklist

To help you decide between ALB and NLB, consider the following questions:

1. What protocols does your application use?

   • If you use HTTP/HTTPS/WebSockets, consider ALB.
   • If you use TCP/UDP, TLS, or need support for non-HTTP protocols, consider NLB.

2. Do you need content-based routing?

   • For advanced path-based or host-based routing, ALB is the right choice.
   • If basic protocol-based routing suffices, NLB may be adequate.

3. Is integration with AWS WAF important for your application?

   • ALB supports AWS WAF.
   • NLB does not support AWS WAF.

4. Do you require static IP addresses or Elastic IPs?

   • NLB provides static IP addresses.
   • ALB does not offer static IP addresses natively.

5. Are you dealing with large request payloads (over 1 MB)?

   • ALB cannot handle request bodies larger than 1 MB.
   • NLB has no such limitation.

6. What are your performance requirements?

   • For ultra-low latency and high throughput, NLB is preferable.
   • ALB offers adequate performance for standard web applications.

7. Are you implementing microservices or microfrontends?

   • ALB is well-suited for microservices architectures with its content-based routing capabilities.

8. Do you need to invoke AWS Lambda functions directly from the load balancer?

   • ALB can route traffic directly to Lambda functions.
   • NLB does not support invoking Lambda functions.

9. Do you need TLS termination?

   • Both ALB and NLB support TLS termination.
   • Choose based on other factors like protocol support and routing needs.

10. Is preserving the client's source IP address important?

    • NLB preserves the source IP address to the backend targets.
    • ALB does not preserve the source IP but includes it in the X-Forwarded-For header.

11. Do you need SSL/TLS offloading and user authentication?

    • ALB supports SSL termination and integrates with authentication services.
    • NLB supports TLS termination but does not handle user authentication.

Conclusion

If you need advanced routing features, integration with AWS WAF, support for HTTP/HTTPS protocols, and the ability to invoke AWS Lambda functions directly, ALB is the way to go. On the other hand, if your application demands high performance, supports TCP/UDP protocols, requires static IP addresses, needs TLS termination at Layer 4, and must preserve the client's source IP address, NLB is the better choice.

By carefully evaluating your application's needs against the features and limitations of each load balancer, you can make an informed decision that ensures scalability, reliability, and optimal performance for your applications.