Session expiry on closing browser window

Ever wondered what occurs when a user closes his browser window. For the time, the user was interacting with a site, his session was active and everything was going smooth.But how does our player ("Server" , here) come to know that a user has closed his window and the session is no longer active? Read below to unfold the mystery..

Whenever a browser requests a server for a page/resource, that request is served with a particular SessionID and the Session timer starts. It is this ID which identifies the future requests coming from the same user. Now when the user remains idle for a duration equaling the Session timeout (default 20 minutes), the Session expires at the server-side and if a request is made after this, a new SessionID is generated.

However, if the user abruptly or mistakenly closes his window, the SessionID remains active at the server-side, thinking that the user is idle. Until the Session expires for this particular SessionID, it cannot be used for another request. Once the Session is time-out, the SessionID is free again to be used for serving another new web-requests.

 Thanks for reading !!