title: How can you track what terminals a user is currently logged in from?
body: <p>One great feature of Quora is that it is able to, when you log out, tell you from what other computers your login credentials (ie your session) are still valid. It enables to to see when that session began, and log that session out.</p>
  
  <p>How is it that Quora is able to do this? How would one go about replicating this functionality?</p>
  
  <p>I do most of my coding in ruby on rails, so if anyone wants to answer in that context it would be great, but I'm open to seeing answers regardless.</p>