一个 Node.js 部署的常见错误，但我就是搜不到解决方案，搞不定，求高人指点

## 我把 log 在 google 上搜了一下，很多人描述这个问题，我也尝试了 2 天了，都搞不定，特来求助

## 环境：
- Node.js v8.9.1
- ExpressJS
- Ubuntu 16.04 LTS
- pm2

## 错误
- 描述 1：如果短时间内多次刷新首页，出现 502 Bad Gateway 错误
- 描述 2：代码里，用户注册或登陆成功后会自动跳转到首页，在本机 OK，但部署到服务器之后，跳转出现 502 Bad Gateway

## 配置：
### Nginx 配置文件：

1 upstream helloworld {
2 server <server-ip-address>:3000 max_fails=0 fail_timeout=10s weight=1;
3 ip_hash;
4 keepalive 512;
5 }
6
7 server {
8 listen 80;
9 server_name helloworld.com www.helloworld.com;
10
11 #charset koi8-r;
12 access_log /app/archives/logs/nginx/helloworld .host.access.log main;
13 error_log /app/archives/logs/nginx/helloworld .host.error.log debug;
14
15 root /app/helloworld ;
16
17 location / {
18 proxy_pass http://helloworld;
19 proxy_set_header X-Real-IP $remote_addr;
20 proxy_set_header X-Forwarded-Proto $scheme;
21 proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
22 proxy_set_header Host $host;
23 proxy_set_header X-NginX-Proxy true;
24 proxy_http_version 1.1;
25 proxy_set_header Upgrade $http_upgrade;
26 proxy_set_header Connection 'upgrade';
27 proxy_set_header Host $host;
28 proxy_cache_bypass $http_upgrade;
29
30 }
31 }

## error log
2017/12/11 09:22:23 [error] 29952#29952: *81 connect() failed (111: Connection refused) while connecting to upstream, client: 169.228.200.160, server: helloworld.com, request: "GET / HTTP/1.1", upstream: "http://<server-ip-address>:3000/", host: "www.helloworld.com", referrer: "http://www.helloworld.com/user/signin"