nginx load balancer for node.js application

nginx.conf

 user www-data;  
 worker_processes 4;  
 pid /var/run/nginx.pid;  
 events {  
     worker_connections 768;  
     # multi_accept on;  
 }
  http {  
    server_names_hash_bucket_size 64; # multiple domains  
    upstream app_a {  
        server 127.0.0.1:3000;  # node 1
        server 127.0.0.1:3001;  # node 2
    }  
    
    server {  
        listen 81;  
        server_name localhost;  
        access_log /var/log/nginx/access.log;  
        location / {  
            proxy_set_header X-Real-IP $remote_addr;  
            proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;  
            proxy_set_header Host $http_host;  
            proxy_set_header X-NginX-Proxy true;  
            proxy_pass http://app_a/;  
            proxy_redirect off;  
        }  
    }  
 }