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.

---
nginx for Windows http://nginx-win.ecsds.eu/