cache a request conditionally based on cookies?

Hi,

Using NginX as a reverse proxy, I want to cache the response ONLY if the user has NOT logged in, which can be checked by testing the existence of a cookie.

First I tried to use "if", but NginX complained that I cannot put a proxy_cache_valid inside an "if":

location / {
proxy_pass http://www.xyz.com;
if ($cookie_login = '') {
proxy_cache my-cache;
proxy_cache_valid 200 301 302 60m;
}
}


Then I tried to set the cache-duration to 0s by using a variable, but then NginX complained the time value is incorrect.
set $cacheDuration 60m;
if ($cookie_login != '') {
set $cacheDuration 0s;
}

proxy_cache my-cache;
proxy_cache_valid 200 301 302 $cacheDuration


So is there a way to cache conditionally based on the existence of a cookie?


Thanks.

- Patrick