upstream, next node when node returns busy

[itpp2012]

Any ideas how to implement the following?

Upstream pool of 4 nodes
Node1
Node2
Node3
Node4

Proxy_pass upstream

If node in upstream returns a 501 move request to next node in upstream but
don’t mark it as down or failed and keep client waiting while trying other
nodes.

If all nodes return a 501 return a 503 to client, and still don’t mark any
node as down or failed.

nb. any node can process 2 – 500 requests, depending on what its doing this
can be sometimes 2, or 40 or 300 or anything between 2 and 500 and obviously
varies per second per node.

Posted at Nginx Forum: http://forum.nginx.org/read.php?2,261986,261986#msg-261986