nginx reverse proxy - try upstream A, then B, then A again

I'm trying to set up nginx as a reverse proxy, with a large number of backend servers. I'd like to start up the backends on-demand (on the first request that comes in), so I have a control process (controlled by HTTP requests) which starts up the backend depending on the request it receives. My problem is configuring nginx to do it. Here's what I have so far: server { listen 80; server_name $DOMAI

[kazeburo]

maji..