What a Load Balancer Does

A load balancer sits in front of your server pool and distributes incoming requests across instances. It also handles health checks — removing unhealthy instances from rotation automatically.

Routing Algorithms

Round-Robin: requests distributed sequentially. Simple, assumes all servers have equal capacity.

Least Connections: route to the server with fewest active connections. Better for variable request durations such as long uploads vs fast API calls.

IP Hash: hash the client IP to always route the same client to the same server. Use only when sticky sessions are unavoidable — it defeats horizontal scaling benefits and skews load if IPs are unevenly distributed.

Layer 4 vs Layer 7

L4 load balancers operate at TCP level — fast, no HTTP parsing. L7 load balancers read HTTP content and can route based on URL path, headers, or cookies:

# Nginx L7 routing
location /api/    { proxy_pass http://api_servers; }
location /static/ { proxy_pass http://static_servers; }
location /ws/     { proxy_pass http://websocket_servers; }

Health Checks

Always expose a /health endpoint. Load balancers poll it at regular intervals and remove a failing instance from rotation within one check interval. Without health checks, you route traffic to dead servers and users get intermittent errors.