checking status of session with session id

checking status of session with session id

I am trying to develop a chat application and I need to know whether the
other user is active or not.I already went through "timestamp" example( if
the last activity is more than specified time consider the user as offline
) and the problem with this method is that this code need to executed
repeatedly in regular intervals through javascript.
So, I was thinking that if there is any way to check whether a sessions is
active or not in server by knowing the session ID.If this is possible I am
making an extra column for "last session id" in MYSQL table and I will
check if it is active or closed the every time I send a message and I can
know for sure if the person on the other side is active or not.