Have you ever wondered how those green online indicators (or “presence indicators”) that are so pervasive in every instant messaging application and social network actually work? How does one display them in real time for roughly 500 million members on a social network like LinkedIn?

LinkedIn provides a technical walk-through of how they used the Play Framework and the Akka Actor Model to build the massive infrastructure that keeps track of the online status of millions of members at any given moment.

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