Load Balancing & Reverse Proxy

Load Balancing

A load balancer distributes incoming network traffic across multiple servers to ensure no single server is overwhelmed.

                  Internet
                     │
              ┌──────┴──────┐
              │ Load Balancer│
              └──┬───┬───┬──┘
                 │   │   │
              Server Server Server
                1     2     3

Load Balancing Algorithms

Algorithm	How it works
Round Robin	Requests go to each server in rotation
Least Connections	New request goes to server with fewest active conns
IP Hash	Client IP always maps to same server (sticky)
Weighted Round Robin	Servers with more capacity get more traffic

Layer 4 vs Layer 7 Load Balancing

L4 Load Balancer (Transport layer): Routes based on IP and TCP/UDP port. Fast, doesn’t look at application content.

L7 Load Balancer (Application layer): Routes based on HTTP content — URL paths, headers, cookies. More intelligent.

Example — L7 routing:
/api/users    →  User Service servers
/api/products →  Product Service servers
/static/*     →  CDN / Storage servers

Health Checks

Load balancers continuously ping servers. If a server doesn’t respond, it’s removed from the pool. When it recovers, it’s added back. This enables zero-downtime deployments.

Reverse Proxy

A reverse proxy sits in front of backend servers and forwards client requests to them. From the client’s perspective, it’s talking directly to the server.

Client → Reverse Proxy (Nginx) → Backend Server(s)

Uses:

SSL termination (HTTPS handled at proxy, HTTP to backends)
Load balancing
Caching static content
Rate limiting
Hiding backend server details

Common reverse proxies: Nginx, HAProxy, Traefik, Caddy, AWS ALB/NLB.

Nginx reverse proxy config example:

server {
    listen 443 ssl;
    server_name example.com;

    ssl_certificate /etc/letsencrypt/live/example.com/fullchain.pem;
    ssl_certificate_key /etc/letsencrypt/live/example.com/privkey.pem;

    location / {
        proxy_pass http://localhost:3000;
        proxy_set_header Host $host;
        proxy_set_header X-Real-IP $remote_addr;
    }
}