Conditional balancing

[Nulik Nol]

I am developing a webmail service where the user's inbox and all
related session  variables will be loaded to one of many application
servers which will be behind the balancer (running nginx). So, I need
nginx to direct all requests of a particular user, to a particular
application server. How can it be done?
One way, I think, is to integrate zookeeper into nginx as module, this
way nginx will communicate with the application daemon and always know
which appserver is holding user's session and data. But this is a lot
of work, is there any other solution to do this sort of conditional
balancing ?

Thanks in advance
Nulik