Master Nginx Load Balancing: Round Robin, Weighted, IP Hash & Least Connections Explained

Round Robin (default)

Nginx uses Round Robin as its default load‑balancing method, distributing incoming requests to backend servers sequentially like a circular queue.

Working principle: Requests are assigned to servers in time order, one after another.

Advantages: Simple and fair, ensuring each server receives an equal number of requests.

Disadvantages: It cannot detect the actual processing capacity of servers; a slower server may become overloaded.

Suitable scenario: When backend servers have similar performance and request processing times.

upstream myapp {
    server 192.168.1.10:8080;
    server 192.168.1.11:8080;
    server 192.168.1.12:8080;
}

Weighted Round Robin

Weighted Round Robin enhances the basic algorithm by assigning a weight to each server, reflecting its processing capability.

Working principle: Requests are distributed according to the weight ratio; higher weight means more requests.

Advantages: Smart distribution based on server performance, making better use of hardware resources.

Disadvantages: Still sequential; cannot sense real‑time load.

Suitable scenario: When backend servers have unequal performance or when gradually increasing traffic to new servers.

upstream myapp {
    # Best server, receives 10 requests
    server 192.168.1.10:8080 weight=10;
    # Average server, receives 5 requests
    server 192.168.1.11:8080 weight=5;
    # Backup server, receives 1 request
    server 192.168.1.12:8080 weight=1;
}

IP Hash

IP Hash hashes the client’s IP address and consistently routes requests from the same IP to the same backend server.

Working principle: Nginx uses the ip_hash directive to map the client IP hash to a specific server.

Advantages: Session persistence – the same client always reaches the same server, which is ideal for stateful applications like shopping carts.

Disadvantages: Uneven distribution if some IPs generate much more traffic; a single server failure can affect all its bound clients.

Suitable scenario: When session affinity is required without implementing shared session storage.

upstream myapp {
    ip_hash;
    server 192.168.1.10:8080;
    server 192.168.1.11:8080;
    server 192.168.1.12:8080;
}

Least Connections

Least Connections dynamically routes each request to the backend server with the fewest active connections.

Working principle: Nginx monitors current connection counts and selects the server with the smallest number.

Advantages: Real‑time load awareness, fairer distribution, and better handling of long‑lived connections such as WebSocket.

Disadvantages: Slight overhead to maintain connection state, though generally negligible.

Suitable scenario: Environments where request processing times vary widely or many long‑running connections exist.

upstream myapp {
    least_conn;
    server 192.168.1.10:8080;
    server 192.168.1.11:8080;
    server 192.168.1.12:8080;
}