Pick a system. Work through the problem. Compare your approach.
Company tags are community-reported. Counts on cards show how many people reported that design.
Scarce inventory — a seat, a room, the last unit — with far more buyers than units. Holding a database row for the minutes someone takes to pay does not scale, and checking availability before writing is a race that sells the same seat twice.
Google Calendar-style events with recurrence, RSVPs and on-time reminders, using CDC for change notifications and a delay queue for reminders.
Search 10 million businesses by place, words and category, keep every average rating exact, and keep fake reviews out.