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When a single backend server can no longer handle your traffic, or you want redundancy, Nginx can distribute incoming requests across multiple backend servers or application instances — a technique called load balancing.

Prerequisites

• Nginx installed
• Two or more backend servers/instances running the same application

Step 1 — Define the Upstream Backend Group

sudo nano /etc/nginx/nginx.conf

http {
    upstream backend_app {
        server 10.0.0.10:3000;
        server 10.0.0.11:3000;
        server 10.0.0.12:3000;
    }
}

Step 2 — Reference the Upstream in a Server Block

server {
    listen 80;
    server_name example.com;

    location / {
        proxy_pass http://backend_app;
        proxy_set_header Host $host;
        proxy_set_header X-Real-IP $remote_addr;
        proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
    }
}

Step 3 — Test and Reload

sudo nginx -t && sudo systemctl reload nginx

Load Balancing Methods

By default, Nginx uses round-robin (equal distribution). Other methods:

upstream backend_app {
    least_conn;
    server 10.0.0.10:3000;
    server 10.0.0.11:3000;
}

least_conn sends new requests to whichever backend currently has the fewest active connections — better for requests with variable processing time.

Session Persistence (Sticky Sessions)

If your app stores session data locally per-server (not in a shared store like Redis), use IP-hash so a client always reaches the same backend:

upstream backend_app {
    ip_hash;
    server 10.0.0.10:3000;
    server 10.0.0.11:3000;
}

Weighted Load Balancing

Send more traffic to a more powerful server:

upstream backend_app {
    server 10.0.0.10:3000 weight=3;
    server 10.0.0.11:3000 weight=1;
}

Marking a Server as Backup

upstream backend_app {
    server 10.0.0.10:3000;
    server 10.0.0.11:3000 backup;
}

The backup server only receives traffic if all primary servers are unavailable.

Handling a Failed Backend

upstream backend_app {
    server 10.0.0.10:3000 max_fails=3 fail_timeout=30s;
    server 10.0.0.11:3000 max_fails=3 fail_timeout=30s;
}

Nginx automatically stops sending requests to a server after 3 failures, retrying it after 30 seconds.

Common Errors

502 Bad Gateway with all upstreams marked down — verify backend servers are actually reachable from the Nginx host (check firewall rules between them, not just the public-facing firewall).

Uneven traffic distribution unexpectedly — confirm ip_hash isn't enabled if you expected even round-robin distribution; sticky sessions inherently produce uneven per-server load with a small client pool.

Best Practices

• Use a shared session store (Redis) instead of ip_hash when possible, for more even load distribution
• Set max_fails/fail_timeout so Nginx automatically routes around a down backend
• Monitor backend health independently, not just through Nginx's basic failure detection

FAQ

Do I need a separate load balancer VPS, or can Nginx run on the same server as an app instance?
Either works; a dedicated Nginx load-balancer VPS is common for larger deployments, but running it alongside one of the app instances is fine for smaller setups.

Related Articles

• How to Install Nginx on Ubuntu & Debian
• Nginx as a Reverse Proxy for Node.js/Docker Apps
• Nginx Performance Tuning: Worker Processes, Caching & Gzip