upstream backend {
    least_conn;

    server backend1:8080 weight=2 max_fails=3 fail_timeout=30s;
    server backend2:8080 weight=2 max_fails=3 fail_timeout=30s;
    server backend3:8080 weight=1 max_fails=2 fail_timeout=30s;

    server backup:8080 backup;
}

server {
    location / {
        proxy_pass http://backend;
        proxy_set_header X-Real-IP $remote_addr;
    }
}