Hello!

On Fri, May 12, 2017 at 01:24:14PM +0800, Wu Bingzheng wrote:

>
> The last request before this 502 request is almost 20 minutes ago and its response code is 200.
>
> The proxy_next_upstream conf:
> proxy_next_upstream error timeout invalid_header http_502 http_503 http_504;
>
> Here is the access log. The upstream server 192.168.0.6 is DOWN. The line-10 is the 502 request:
>
> 1 [03/May/2017:14:35:38 -0400] "POST /x/y HTTP/1.1" 200 "192.168.0.6:8181, 192.168.0.5:8181" 0.012 0.001, 0.011
> 2 [03/May/2017:14:35:38 -0400] "POST /x/y HTTP/1.1" 200 "192.168.0.5:8181" 0.013 0.013
> 3 [03/May/2017:14:54:30 -0400] "POST /x/y HTTP/1.1" 200 "192.168.0.5:8181" 0.206 0.206
> 4 [03/May/2017:15:03:08 -0400] "POST /x/y HTTP/1.1" 200 "192.168.0.5:8181" 0.154 0.154
> 5 [03/May/2017:15:40:51 -0400] "POST /x/y HTTP/1.1" 200 "192.168.0.6:8181, 192.168.0.5:8181" 0.012 0.000, 0.012
> 6 [03/May/2017:15:40:51 -0400] "POST /x/y HTTP/1.1" 200 "192.168.0.5:8181" 0.014 0.014
> 7 [03/May/2017:15:40:51 -0400] "POST /x/y HTTP/1.1" 200 "192.168.0.5:8181" 0.016 0.016
> 8 [03/May/2017:15:40:51 -0400] "POST /x/y HTTP/1.1" 200 "192.168.0.5:8181" 0.017 0.017
> 9 [03/May/2017:15:40:51 -0400] "POST /x/y HTTP/1.1" 200 "192.168.0.5:8181" 0.011 0.011
> 10 [03/May/2017:15:59:06 -0400] "POST /x/y HTTP/1.1" 502 "192.168.0.6:8181, test_backend" 0.000 0.000, 0.000
> 11 [03/May/2017:15:59:07 -0400] "POST /x/y HTTP/1.1" 200 "10.255.222.206:8181" 0.260 0.260

Looking into response status code in access logs is not enough to
understand if a server is up or down. For at least the following
reasons:

- there might be over requests currently in flight which are not
yet logged;

- errors may occur while sending response body, and hence status
code will not show if there was an error.

It is usually a good idea to look into error logs instead.

--
Maxim Dounin
http://nginx.org/
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