Design Ticketmaster (AI mock)

Hard

A flash-sale ticketing system: sell scarce seats to a huge concurrent crowd without ever double-booking.

Interviewer

You're designing Ticketmaster for a live system-design interview. Fans browse events, hold seats during checkout, and buy tickets. The defining challenge is the flash sale: when a major tour goes on sale, ~200,000 fans contend for ~50,000 seats within the first minute.

Scale assumptions:

500M registered users; ~5M browse on a given day, ~20 page views each.
Catalog: ~10M events/year, ~2,000 seats per reserved-seating event.
Hot onsale: ~200k concurrent fans, seats sell out in minutes.
Targets: browse < 200 ms p99, 99.95% availability on browse; zero double-sells.

I'll take you through requirements, estimation, architecture, a concurrency deep dive, and trade-offs. Think out loud and state your assumptions.

The interviewer grades each stage against a hidden rubric and asks a probing follow-up, mirroring a real system design round. Sessions are capped at 30 turns.