Design rental watchlist and notification system
Overview
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.
Airbnb
Sep 6, 2025, 12:00 AM
Software Engineer
Onsite
System Design
0
0
System Design: Rental Listing Watchlist and Availability Notifications
Context
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.
Requirements
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APIs and data model
- Users, Listings, Reservations
- Watchlist entries with date ranges
- Notification preferences
- Hosts can view watch counts
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Date-range representation and matching semantics
- Inclusive/exclusive day bounds
- Time zones and DST considerations
- Policy: full overlap vs partial overlap
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Ingestion of reservation updates/cancellations
- Eventing, pub/sub, and idempotency
- Algorithm to find affected watchers at scale (per-listing calendars, interval indexes)
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Notification pipeline
- Deduplication and suppression
- Rate limiting, retries, idempotency
- Multi-channel preferences
- Storage, sharding, and caching strategies
- Consistency model between reservations and watchlist
- Race handling between new bookings and notifications
- Privacy/abuse controls and analytics for host watch counts
Solution
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