I think maintaining a pool of 'warm' https sessions between the nginx and the app server is not a very flexible approach. What happens when all of those are occupied?
Wouldn't it be nicer to have an IPsec tunnel between the nginx and the app server and open http sessions on demand?
Backlog. That increases the latency till a new connection can be accepted. However, the number of pooled connections can be increased to a fairly large number at the expense of more memory consumption. This is something that isn't an issue with nginx by using it as a HTTPS proxy.
