在nginx日志中看到这个：
"upstream": "52.86.112.192:443, 52.86.78.197:443",
"upstream_response_time": "7.005, 7.016",
nginx的文档说：
If several servers were contacted during request processing,
their addresses are separated by commas,
e.g. “192.168.1.1:80, 192.168.1.2:80, unix:/tmp/sock”.
不幸的是，我不清楚在处理1个请求时如何调用两个服务器？这是没有内部重定向？在第一次尝试失败后重试了吗？
DNS查询返回两条A记录：
xxx.us-east-1.elb.amazonaws.com. 60 IN A 54.84.139.107
xxx.us-east-1.elb.amazonaws.com. 60 IN A 52.71.207.21
是否意味着，nginx会自动为每个失败的请求重试？或者它可以配置？ （这是AWS，因此负载平衡器的IP不断变化）