title: Chat Architecture: Real time presence notification
body: <p>I would like to better understand how does the presence notification mechanism works in a large scale chat, like gtalk and facebook chat.</p>
  
  <p>I saw some videos and read articles about these two chats, but I didn't understand how the presence notification is working:
  If I am entering the chat - how does the server knows who should be notified? </p>
  
  <p>Thx!</p>