Why load balancers exist
A load balancer sits in front of multiple servers and distributes incoming requests so no single machine gets overwhelmed.
Common algorithms
| Algorithm | Best for |
|---|---|
| Round robin | Equal-capacity servers |
| Least connections | Variable request duration |
| IP hash | Session stickiness |
Layer 4 vs Layer 7
- L4 (TCP): fast, routes by IP/port — AWS NLB
- L7 (HTTP): routes by URL, headers, cookies — AWS ALB
Health checks
Always configure health checks. Unhealthy instances should be removed automatically before users hit errors.
AWS mapping
- ALB: HTTP/HTTPS apps, path-based routing
- NLB: ultra-low latency TCP/UDP
- API Gateway: serverless APIs in front of Lambda/ECS