Nginx load balancing and authenticated users

I have a Plone setup that uses nginx on front and 2 backend servers with Varnish installed; I using a very simple configuration like this one:

upstream backend {

server {
    listen 80;

    location / {
        proxy_pass http://backend/VirtualHostBase/http/;
        proxy_http_version 1.1;
        proxy_set_header Connection "";

I need all authenticated users go to backend server 1; I read about the sticky directive on the nginx manual, but I don't understand it very well.

does anybody here has experience with that?

You don't need sticky sessions if you're putting all your logged in users on a single instance. Just switch the rewrite proxy based on the presence (or absence) of the __ac cookie in the http headers.

thanks, @gyst; at the end I think this is not a good idea at all as I will have a single point of failure for editors.