Nginx didn't try the next available backend server

[allenhe]

Hi,

My Nginx is configured with:
proxy_next_upstream error timeout http_429 http_503;

But I find it won't try the next available upstream server with the
following error returned:

2019/04/05 20:11:41 [error] 85#85: *4903418 recv() failed (104: Connection
reset by peer) while reading response header from upstream....

The "error" part for the proxy_next_upstream states:
error
an error occurred while establishing a connection with the server, passing a
request to it, or reading the response header;

I understand the error above would fall into the "or reading the response
header;", but why it didn't work? 

BR,
Allen

Posted at Nginx Forum: https://forum.nginx.org/read.php?2,283701,283701#msg-283701