Ticket booking (Ticketmaster)

Sell 50,000 concert seats to 2 million fans waiting at 10:00:00 AM without ever selling the same seat twice. The design pairs a virtual waiting room at the edge with two-phase seat reservation (an atomic bounded-TTL hold followed by payment capture) and conditional atomic updates (`decrementIfPositive` / CAS) on seat rows.

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Duration 45:00

Requirements checklist

View an event's available seats by section and price tier.Two-phase booking: place a temporary hold (e.g. 5–10 minutes) on specific seats while the user enters payment details, then confirm the ticket or automatically release the hold on expiry.Fair admission during massive on-sale drops via a virtual waiting room.Hard correctness invariant: never sell or hold the same seat for two users (`oversell` must be zero).Seat-map browsing must stay fast (< 100 ms) even at 100:1 read-to-write ratio; stale availability by a couple of seconds on the map is acceptable because the hold step is authoritative.Survive 100× flash-crowd traffic at the exact second tickets go on sale without cascading failure.

Clarifying questions

Do users pick exact individual seats on a map, or 'best available' in a section/tier?
How long is a seat held during checkout before it expires?
What is the peak concurrent user count for a major stadium on-sale drop?
How should we handle adjacent multi-seat requests (e.g. 4 seats together)?

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requirements

Suggested time: about 6 minutes. Addresses late-payment races after hold expiry and fair queuing.

Notes for requirements