How are downed upstream servers detected as alive again?

Rt Ibmer rtibmx at
Thu Apr 24 00:37:10 MSD 2008

Hi everyone! I am new to nginx (switching over from using Pound) and absolutely LOVE it!  What a fantastic solution.  I am using it primarily as a reverse proxy for load balancing across my Amazon EC2 instances.

I've spent the past week digging in deep and learning the ropes and its been easy to get up to speed. However I haven't been able to find out some pieces of information despite lots of research so I'm hoping you can help out.  I'll post these as a series of questions since the subjects are pretty diverse, rather than as a block of multiple questions in one message.  So let's get started!

I've searched this list, the FAQs and Wiki but couldn't find a discussion about what process nginx uses to monitor upstream servers to see if they come back up once they become skipped due to proxy_next_upstream rules.

For instance, let's say I have 3 upstream servers defined, and there is a problem with one, such that my proxy_next_upstream rules dictate it should be skipped over.  What frequency does nginx check to see if that server comes back online?  Does it check in the background at some set interval, and if so, is there a way to set the frequency (check once every x seconds)?

Or does it check the bad server on every single request nginx receives? That seems like it would not be efficient.  At any rate, in playing with it I couldn't seem to figure out what process is uses for this.  Can someone please let me know? Thank you!

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