I've also tried this using the userid module I set a custom cookie like so

userid on;
userid_name "testid";
userid_expires 100s;

and set the fastcgi_cache_key like so

fastcgi_cache_key $cookie_testid$request_method$scheme$host$request_uri

the problem with this setup is when I login or logout, since the key is based on only $cookie_testid without $http_cookie, then the user will still see the logged in page even after logging out. Alternatively how would I user the cache purge module to purge the key when the user hits the login or logout button? this is the sample config

http {
proxy_cache_path /tmp/cache keys_zone=tmpcache:10m;

server {
location / {
proxy_pass http://127.0.0.1:8000;
proxy_cache tmpcache;
proxy_cache_key $uri$is_args$args;
}

location ~ /purge(/.*) {
allow 127.0.0.1;
deny all;
proxy_cache_purge tmpcache $1$is_args$args;
}
}
}

not sure how to rewrite the url.