subrequest failover

[James Lyons]

I am trying to use nginx as reverse proxy to perform failover from 1
critical service to another.

When processing a request, it sends a request (http GET) to a backend
(custom http server) using SubRequest methods.  If it is for some
reason down, and there is no response, i'd like it to fail over to a
second server.  Is there a way to do this in conf settings or do I
have to make code modifications to accomplish this?

Any help appreciated.

-James-