Nginx 502 网关错误

Question

location /app {
    proxy_pass http://localhost:8081;
    proxy_http_version 1.1;
    proxy_set_header Upgrade $http_upgrade;
    proxy_set_header Connection 'upgrade';
    proxy_set_header Host $host;
    proxy_cache_bypass $http_upgrade;
   }
}

我的节点应用程序也在监听端口 8081。想知道是什么原因吗？或者需要更多信息？