Apache stickiness based on query string

[Noam]

Hi,

I've got an Apache as a load balancer and a couple of Tomacts as the
application servers.
I want to implement sticky sessions based on a manipulation on a query
string/header value.
In the query string or the header I have a user Id. Based on this
value I want that the same user will always reach the same Tomcat.
For instance, by performing hash(user-id) mod 2 (2 is the number of
Tomcats)..

Is it possible at all, and if yes, how can I do it?

Many thanks,
Noam