Design Ticket Booking

Ticket booking system case study (Ticketmaster, BookMyShow): browse events, select seats with TTL hold, pay, confirm. Handles flash sales (1M concurrent, 100K seats). Components: CDN, WAF/CAPTCHA with ML bot scoring, virtual waiting room (Redis sorted set), API gateway, event catalog (Cassandra), Redis SETNX hold service with 10-min TTL, sharded Postgres inventory, payment service to PSP (Stripe/Adyen), seat-map WebSocket, outbox to Kafka, notifications. Five scenarios: virtual queue under flash sale, atomic seat hold conflict, checkout to PSP and PG transaction, hold TTL expiry, anti-bot scalper detection. Two ADRs: virtual waiting room vs FCFS, Redis SETNX TTL vs Postgres row-lock.