State-Aware Mode (Preserve Mode 1) and process variables

I am building a SOAP service using Caché 2016.1.  

If I set %session.Preserve=1 in OnPreWebMethod and I set a variable in my service method, why don't I see the variable in the CSP session processes' variables after I call the method?  

The documentation says "The original motivation for the provision of a state-aware mode of operation was to make it relatively easy to migrate legacy application code from a fixed client-server environment (e.g. terminal applications) to the web. Support for transactions that spanned several HTTP requests was also a consideration in its introduction."

This is my motivation exactly.  But I don't see the variables I set in the service method.  Do I have to use the session object to store the variables I want to endure after the service method (where the variable was set) is called?  By the way, subsequent calls are going back to the same CSP session; SOAPSESSION = 1 seems to be working fine with my Caché SOAP client, I see the same CSPSESSIONID cookie going between SOAP client and CSP server.

thanks.