Excellent indeed! At the most basic level it would be extremely useful if nginx
could signal an external process once it declared one of the upstream servers
offline (based on the timeout configs in its main conf file).

My intention from such a thing would be to receive an email letting me know 'oh
no upstream xyz is down!!' and 'oh good, upstream xyz is back online'.

Some people have argued that such a thing should be left to external monitoring
tools like heartbeat etc. However such monitoring is no where near the same IMO.
For instance, heartbeat could see the upstream as up because it is alive and
pingable, but it wouldn't be aware if nginx declared it as offline due to
timeouts or 500 errors (again, based on settings in its conf file)!!

Thanks!