This question evaluates system architecture and scalability competencies, including data modeling for date-range watchlists, event-driven notification pipelines, time-zone and DST handling, consistency and concurrency semantics, idempotency, sharding/caching strategies, and privacy/analytics considerations.
You are designing a feature for a vacation-rental marketplace where guests can "watch" a listing for a specific date range (check-in to check-out). If the listing becomes available for that exact range (per a defined matching policy), the system should notify the watcher. Hosts can see how many users are watching their listing.
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