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Because this is not the responsability of the nlClient to keep it, this a network configuration to set.
you should have something like
http://nlserver1
http://nlserver2

You then have to configure http://nlserver to redirect in folb to nlserver1 or nlserver2 but you need to configure the redirection server to use sticky session.
here is a doc for Apache:
Configuring Apache HTTP Server for Sticky Sessions (informatica.com)