[plt-scheme] share variable in servlet programs

Hello plt-scheme,

In PLT v20x, sharing variable in servlet programs is easy:
I just write a program that defines all the shared variables,
and call serve in that program to start the server, then
all variables defined in that program become available 
in all servlets.

But when I try the same program in v300, this trick can't
work. Could anyone suggest a way to share variables in
v300 of servlets? Many thanks.

Sincerely,  
Chongkai Zhu