You have 90 seconds to verbally explain how you would design a proximity service that shows users their friends within 1km radius in real-time.
Your system should: - Update locations every 30 seconds when app is active - Show nearby friends on a map - Send notifications when friends come within range - Handle 50 million active users - Respect privacy settings
Explain your location storage, query optimization, and notification system.
How to approach it
- Hint 1
Store user locations with geohash in Redis for fast radius queries
- Hint 2
Use change detection - only broadcast when user moves to different geohash cell
- Hint 3
Fan-out notifications only to users who have you in their friend list
Ready to answer it out loud?
Record your answer in 1:30 and Preptile scores it 1–10 with specifics — what landed, what you skipped, and what to say next time.
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