User Session Maintenance ....

Hello guys ! I have this task , which involves
user session maintenance . It involves deleting
somes records from 2 tables , one for user-session(which contains user id ,session id and login time) and other table contains users whose session has been locked.
My Screen displays all/specific current users of the system after a search and allows the Maintenance Person to delete any user record he wishes.
1.How do i keep the list of user currently logged in (without inserting in DB)the Web Application
2. What is the effect of Storing this list out of DB if the server is done
3.Is it possible to retrieve the list after the server is re-started.
NB : I am using a Tomcat 4.0 web server
All ideas are welcomed.