The problem

Sofia opens her messaging app on the train to ask a colleague about tonight’s release. Next to each of her 300 contacts is a green dot or a grey one. She writes to Karim because his dot is green.

Karim closed his laptop 40 minutes ago, and his dot stayed green because his app never said goodbye. Sofia waits for an answer that does not come.

You design the service that keeps those dots right, to within 30 seconds, for every user.