In my experience, Nginx returns 502 when the upstream server (Unicorn in
your case) doesn't respond or terminates the connection unexpectedly.

_Nik

On 3/19/2013 1:29 PM, fastcatch wrote:
> I have an application behind nginx (Rails on Unicorn if it matters) which
> listens on a UNIX socket.
>
> It all works nice, especially when load is low. When there is some load on
> the server though (say 70%), I randomly(?) get a bunch of 502 responses --
> usually in batches.
>
> Thus the question: when (under what conditions) does nginx send a 502
> response?
>
> Thanks in advance,
>
> fastcatch
>
> PS: The app's worker processes do not seem to be irresponsive, slow, blocked
> for IO or anything else I have been able to identify as a possible cause.
> Thus I want to understand more the nginx side so as to find the root cause
> -- which I believe is on the other side (or some config tweak needed
> soemwhere).
>
> Posted at Nginx Forum: http://forum.nginx.org/read.php?2,237573,237573#msg-237573
>
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