Thanks for the answer!

I am still unable to have processes started as needed.

Let's say that one process needs 10 seconds to complete and I have one backend server in the rotation, processing two requests at the same time will take 20 seconds. Is there a way to have nginx / ngx_supervisord recognized that another backend server is needed and that the two are done in 10 seconds?

Is there a way to add backend server dynamically to the rotation?