Ten million users need a green dot beside their name. What does that cost, and what happens when a phone loses signal without disconnecting?

Short answer

Design presence as expiring leases, not as a boolean written on connect and disconnect. Each active device renews a short-lived presence key; if a phone loses signal and cannot disconnect cleanly, the lease expires and the dot turns off. The scaling problem is not just heartbeat writes, but fan-out to watchers, so subscribe only to users currently visible on screen and batch presence reads.

Mention ephemeral state where it changes the risk, the owner, or the next check. A useful ephemeral state point should make the answer more testable, not merely longer. Tie ephemeral state back to the scenario so the interviewer can see why it matters here.