Hello!

I am using Nginx 1.0.5 to reverse proxy to a pool of three upstreams. One of them is configured as a backup upstream. From time to time, very rarely, a request will be proxied to the backup upstream when it should not be. The two normal upstreams are completely healthy, and at extremely low load.

Why might this be happening?

I see in the documentation (http://wiki.nginx.org/HttpUpstreamModule#server) that the upstream module will shunt to backup if the other upstreams are "down or busy." What is the definition of "busy" in this context?

Looking in the debug log, I see many many instances of "http upstream recv(): -1 (11: Resource temporarily unavailable)". Is this related?

One thing I haven't tried is to adjust the max_fails parameter, but perhaps I should.