Also, actual alarm drift in prod with concurrent fleet arrivals? And client reconciliation when the countdown hits zero but the alarm hasn't fired?
Clean aesthetic.
no cross DO transactions are happening. the problem won't be (yet) that fleet arrivals will be just miliseconds apart.
with alarm drifts: i havent observes that beahiuvor yet three is a drift check performed every 100 alarm cylces with self correction if necessary.
on the client sync part: countdown runs on the epoch by the server. hits zero, waits a sec for buffer, hard-reloads. if the alarm hasn't fired yet, next alarm cycle (shortly after) resolves it. safety net: any fleet 5+ minutes overdue gets cross-checked, with exponential backoff retry on failed return deliveries. no websocket push on this one as mentioned earlier.... the reload is the reconciliation.
thanks and hope you give it a try!
Edited for typos.