Your question is Concurrent Visitors Booking System. Take a moment with it on the right.
Talk me through your thinking if you like. When you're confident, submit your answer and I'll grade it like a real screen (7/10 or better passes).
Design a system that can track the number of concurrent visitors trying to book on a hotel website.
Asked in the System design stage. Blind comment wording.
Explain how you would define an active visitor, collect and aggregate events, handle duplicate or late messages, expose counts, and detect suspicious changes. Discuss the design at realistic production scale, including reliability, latency, data quality, and operational monitoring.