Basic concept for List management :: Psudo Code need

I have in my Program a limited number of Resources(Boxes ) available let say 200. Every Request which is sent with a SessionID will get a Free(Available) box and as long as the request with the same ID comes in will use the same BOX. If the request with an existing SessionID do not comes in for 30 Min the Box will be available( Free) again Or if the Server sees that the process for a specific SessionID is finished then the Box will also be available/free. Every request with new SessionID will get next available BOX and if no Box is available then the Oldest assigned Box will be freed as assigned to new SessionID.