The little green dot looks simple. Behind it is a distributed system tracking heartbeats from millions of devices, deciding in real time who is genuinely online and who just lost signal in an elevator.
We use a grace-period model: brief network blips don't flip you offline, but a real disconnect updates your last seen within seconds.
This post covers the presence architecture, the sweeper that catches stuck sessions, and why "last seen" is harder than it looks.
