How to Use Weighted Load Balancing in Nginx

Use the weight parameter inside the upstream block in Nginx to assign different traffic shares to servers. Higher weights mean more requests go to that server, enabling weighted load balancing.

Syntax

The upstream block defines a group of backend servers. Each server directive inside it can have an optional weight parameter. The syntax is:

upstream backend_name {
    server server1 weight=number;
    server server2 weight=number;
    ...
}

The weight is a positive integer that controls how much traffic each server receives relative to others.

nginx

upstream backend {
    server backend1.example.com weight=3;
    server backend2.example.com weight=1;
}

Example

This example shows how to send 75% of traffic to backend1 and 25% to backend2 using weights 3 and 1 respectively.

nginx

http {
    upstream backend {
        server backend1.example.com weight=3;
        server backend2.example.com weight=1;
    }

    server {
        listen 80;

        location / {
            proxy_pass http://backend;
        }
    }
}

Common Pitfalls

• Forgetting to set weights: If weights are omitted, all servers get equal traffic.
• Using zero or negative weights: Weights must be positive integers; zero or negative values are invalid.
• Not reloading Nginx: Changes to the config require nginx -s reload to apply.
• Ignoring server health: Weighted load balancing does not replace health checks; unhealthy servers may still receive traffic unless configured.

nginx

## Wrong way (weight=0 disables server):
upstream backend {
    server backend1.example.com weight=0;
    server backend2.example.com weight=2;
}

## Right way:
upstream backend {
    server backend1.example.com weight=1;
    server backend2.example.com weight=2;
}

Quick Reference

Directive	Description	Example
upstream	Defines a group of backend servers	upstream backend { ... }
server	Specifies a backend server with optional weight	server backend1.example.com weight=3;
weight	Assigns relative traffic share to a server	weight=2
proxy_pass	Sends client requests to the upstream group	proxy_pass http://backend;

Key Takeaways

Use the weight parameter inside upstream server blocks to control traffic distribution.

Weights must be positive integers; higher weights get more requests.

Reload Nginx after config changes to apply weighted load balancing.

Weighted load balancing does not replace health checks for backend servers.

If weights are omitted, traffic is distributed evenly among servers.