⚖️ Load Balancing

What Is Load Balancing?

Imagine a supermarket checkout — with only one checkout counter (server), there's a long queue. After opening multiple counters, customers are distributed across them and things move fast. Load balancing is the dispatcher that assigns customers.

Round Robin — Default Strategy

# Define a group of backend servers
upstream backend_servers {
    server 192.168.1.101:3000;
    server 192.168.1.102:3000;
    server 192.168.1.103:3000;
}

server {
    listen 80;
    server_name example.com;

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

Requests are distributed in order: 1st → server 1, 2nd → server 2, 3rd → server 3, 4th → server 1, and so on.

Weighted Round Robin

upstream backend_servers {
    server 192.168.1.101:3000 weight=5;   # High-spec server, gets more traffic
    server 192.168.1.102:3000 weight=3;   # Mid-spec
    server 192.168.1.103:3000 weight=1;   # Low-spec, gets less traffic
}

IP Hash — Session Affinity

upstream backend_servers {
    ip_hash;                              # Same IP always goes to the same server
    server 192.168.1.101:3000;
    server 192.168.1.102:3000;
}

> 💡 Tip: > 💡 When should you use ip_hash? When the backend uses Sessions (not JWT) for login state management, every request from a user must go to the same server.

Least Connections

upstream backend_servers {
    least_conn;                           # Prioritize the server with fewest connections
    server 192.168.1.101:3000;
    server 192.168.1.102:3000;
    server 192.168.1.103:3000;
}

Health Checks and Failover

upstream backend_servers {
    server 192.168.1.101:3000 max_fails=3 fail_timeout=30s;
    server 192.168.1.102:3000 max_fails=3 fail_timeout=30s;
    server 192.168.1.103:3000 backup;     # Backup server, not used normally
}

> 📝 Note: > 📝 max_fails=3 fail_timeout=30s means: if 3 consecutive requests fail, mark that server as unavailable, then retry after 30 seconds.