While learning AWS, I explored the Classic Load Balancer (CLB) and discovered why modern applications often use Application Load Balancer (ALB) or Network Load Balancer (NLB) instead.
Let's understand the differences! 🚀
- What Is a Classic Load Balancer (CLB)?
Classic Load Balancer is an older-generation AWS load balancing service that distributes incoming traffic across registered EC2 instances.
For example, when multiple users access a website, CLB distributes their requests across available backend instances instead of sending all traffic to a single server.
Traffic Flow:
Clients → Internet → CLB → EC2 Instances → Response to Clients
This helps improve application availability and traffic distribution.
- Why Is CLB Used Less Today?
CLB is still available in AWS, but it has fewer advanced capabilities than newer load balancers.
Here are the main reasons:
Limited routing features: CLB does not offer the advanced host-based and path-based routing available with ALB.
Modern application requirements: ALB provides features suitable for web applications, APIs, and microservices.
Layer 4 traffic needs: NLB is designed for TCP, UDP, and TLS traffic use cases.
Architecture flexibility: ALB and NLB provide different capabilities that can better match modern application requirements.
- CLB vs ALB vs NLB
Classic Load Balancer (CLB)
Older-generation load balancer.
Supports basic load balancing for applications using HTTP/HTTPS and TCP/SSL.
Useful for certain existing legacy applications.
Application Load Balancer (ALB)
Operates at Layer 7 of the OSI model.
Supports HTTP/HTTPS traffic.
Supports path-based and host-based routing.
Suitable for web applications, APIs, and microservices.
Network Load Balancer (NLB)
Operates primarily at Layer 4.
Supports TCP, UDP, and TLS traffic use cases.
Supports static IP addresses.
Suitable for applications that require high performance and low-latency network traffic handling.
- Real-World Example
Imagine an e-commerce website with three services:
/products → Product service
/payments → Payment service
/users → User service
With ALB, we can configure path-based routing to forward requests to the appropriate target groups.
For example:
Client → ALB → /products → Product EC2 instances
Client → ALB → /payments → Payment EC2 instances
This helps organize traffic routing for applications with multiple services.
- Which Load Balancer Should We Choose?
Choose ALB for HTTP/HTTPS applications that need advanced routing.
Choose NLB for suitable Layer 4 workloads that require TCP/UDP support or static IP addresses.
Consider CLB when maintaining an existing legacy application that already meets its requirements.
For new applications, AWS generally recommends considering ALB or NLB according to the workload.
- What I Learned
This exploration helped me understand that load balancing is not just about distributing requests. It is also about choosing the right architecture based on application requirements, traffic patterns, routing, and scalability.
I'm continuing to learn AWS through hands-on practice and exploring how EC2, load balancers, and Auto Scaling work together to build highly available applications.
What would you choose for a new AWS web application: ALB or NLB? Share your thoughts in the comments!
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