Use of Variables in Upstream hash Session Persistance

Hi,

Lets say I have 1 x nginx front end servers which load balances to 2 x app server and requires session persistence.

I can set up an upstream group like this.

upstream backend {

hash $request_uri consistent;
server backend1;
server backend2;
}

If the requests always have a HTTP header called say "x-user-name", I was hoping that I could do this:

hash $http_x_user_name consistent;

but it doesn't seem to work for me..

Does anyone know if this should work? or what variables are available in the upstream context?

I've tried $sent_http_x_user_name and also $upstream_http_x_user_name also.

Would appreciate any help!

Sam