When designing scalable and secure architectures in AWS, choosing the right load balancer is critical. AWS provides three key load balancers, each tailored for specific use cases: Application Load Balancer (ALB), Network Load Balancer (NLB), and Gateway Load Balancer (GWLB).

Let’s dive into their functionalities, routing capabilities, and use cases.

1. Application Load Balancer (ALB)

ALB is the most commonly used load balancer routing at application-layer (Layer 7). It provides advanced routing capabilities based on url and header content such as path, hostname, query parameters.

Routing Capabilities:

• Path-based Routing: Forward requests based on URL paths (e.g., /images to a specific service).

  • A request to example.com/api is routed to a backend service handling API calls, while example.com/shop is routed to the e-commerce service. ALB inspects the URL path and directs the traffic accordingly.

• Host-based Routing: Forward traffic based on domain names (e.g., api.example.com vs. shop.example.com).

• Header/Query Parameter Routing: Use HTTP headers or query parameters for granular routing.

Use Cases:

• It's most suitable for WebSocket and HTTP/HTTPS traffic handling.
• Hosting multiple applications on a single domain (e.g., /api, /shop).
• Directing traffic to different microservices based on HTTP headers or query parameters.

2. Network Load Balancer (NLB)

NLB offers high-performance routing based on network-level attributes (Layer 4). A NLB has a static IP and, hence, supports a low latency connection. Moreover, since it does not have to work on the header content and performs routing at the network level using IP and port numbers, it has a higher performance compared to the ALB.

Routing Capabilities:

• IP-based Routing: Forward traffic to specific backend servers using IP addresses.
• Port-based Routing: Direct traffic to services based on the destination port (e.g., port 443 for HTTPS).

Use Cases:

• Low-latency, high-throughput applications like gaming , real-time analytics and video streaming applications.
• TCP/UDP traffic handling.
• Load balancing traffic with static IP addresses.This is useful in establishing PrivateLinks in AWS for sharing services. We can combine NLB and ALB in such cases for multi-protocol targets.

3. Gateway Load Balancer (GWLB)

GWLB acts as a security gateway, filtering traffic before forwarding it to other load balancers like ALBs, NLBs, API Gateways , CDNs or other target groups.

As we can see in the diagram, the GWLB sits inline between the client and the backend systems, inspecting or processing bothe inccomming and outgoing traffic as it passes through. Approved traffic is then forwarded to an ALB or an NLB for processing.

Routing Capabilities:

• Traffic Filtering: All traffic passes through virtual appliances for inspection.
• Integration with ALB/NLB: Routes validated traffic to application or network layers.

Use Cases:

• Centralized traffic inspection using firewalls, Intrusion Detection Systems(IDS), or deep packet inspection tools.
• Pre-filtering traffic before routing it to ALBs, NLBs, or target groups.
• Managing security appliances with scalability and high availability.

Summary

Each load balancer serves distinct purposes, and together, they enable secure, scalable, and high-performing architectures. By combining them effectively, you can build robust systems tailored to your application’s requirements.