Elastic Load Balancing (ALB/NLB)
Automatically distributes incoming traffic across multiple targets
A load balancer is like a traffic cop at a busy intersection, directing cars (requests) to different lanes (servers) to prevent any one lane from getting overwhelmed. Imagine a restaurant with 5 waiters; if all customers went to one waiter, they'd be swamped while others stand idle. A load balancer distributes customers evenly across all waiters. AWS offers three types: Application Load Balancer (ALB) for HTTP/HTTPS traffic with smart routing, Network Load Balancer (NLB) for ultra-high performance TCP/UDP traffic, and Classic Load Balancer (legacy). Load balancers also do health checks; if a server fails, they stop sending traffic to it automatically, ensuring users always reach healthy servers.
ELB distributes traffic across multiple targets (EC2 instances, containers, IP addresses, Lambda functions) in one or more AZs. ALB operates at Layer 7 (application), supporting path-based routing (/api → backend, /images → image server), host-based routing (api.example.com → API servers), and HTTP/2, WebSocket. NLB operates at Layer 4 (transport), providing ultra-low latency (microseconds), static IPs, and handling millions of requests per second.
Key Capabilities
Key features: SSL/TLS termination, sticky sessions (route user to same target), connection draining (graceful shutdown), and cross-zone load balancing (distribute evenly across AZs).
Gotchas & Constraints
Gotcha #1: ALB adds latency (milliseconds) due to Layer 7 processing; use NLB for latency-sensitive workloads. Gotcha #2: Load balancers are regional; for global load balancing, use Route 53 with health checks. Constraints: ALB supports HTTP/HTTPS only (no TCP/UDP), NLB doesn't support path-based routing, and load balancers require at least 2 subnets in different AZs.
An e-commerce site runs 20 EC2 instances across 3 AZs. Without a load balancer, they'd need to manually distribute traffic or use DNS round-robin (no health checks, no SSL termination). They deploy an ALB in front of instances, configuring target groups for different services: /api → API servers, /checkout → payment servers, /images → image servers. ALB performs health checks every 30 seconds; when an instance fails, ALB stops routing traffic to it within 1 minute. For SSL, they attach an ACM certificate to the ALB, which handles SSL termination (instances receive plain HTTP, reducing CPU load). During Black Friday, traffic spikes 10x; Auto Scaling launches 100 more instances, and ALB automatically includes them in the target group. ALB distributes traffic evenly, preventing any instance from being overwhelmed. They enable access logs to S3 for analytics and use ALB's built-in WAF integration to block malicious traffic.
The Result
99.99% availability, automatic failover, and simplified SSL management.