The Phoenix Framework (web framework for Elixir) already solves this for you.[1] It's called Phoenix.Presence. They used a combination of CRDTs with heartbeats

The Phoenix Framework (web framework for Elixir) already solves this for you.[1] It's called Phoenix.Presence. They used a combination of CRDTs with heartbeats to implement it. You are right, it is a hard problem because of the distributed nature of Erlang/Elixir. That's why Chris provided a framework level solution for it. [1] https://youtu.be/XJ9ckqCMiKk?t=921 https://youtu.be/XJ9ckqCMiKk?t=921 (Erlang Factory SF 2016 Keynote Phoenix and Elm – Making the Web Functional)