System Design: High-Concurrency Ticket Purchasing With Seat Maps

You are asked to design an end-to-end ticketing system for large on-sale events (e.g., concerts, sports) with interactive seat maps and extreme traffic spikes. The system must handle concurrent seat selection, payment, and real-time updates while preventing oversells.

Functional Requirements

• Interactive seat selection on a seat map (single and multi-seat selection, contiguous seats).
• Seat holds with TTL (e.g., 2–5 minutes) and renewal, visible across devices.
• Purchasing flow with payment authorization/capture, idempotency, and retries.
• Refunds and chargebacks handling with appropriate state transitions.
• Wait queues during spikes; fair admission and anti-bot protections.
• Real-time availability updates to clients.
• Audit logging for compliance and investigations.

Non-Functional Requirements

• Prevent oversells under massive concurrent demand.
• High write throughput and burst tolerance (tens of thousands of holds/s).
• Low-latency seat availability reads (<150 ms p95) and updates.
• Horizontally scalable (sharding, caching) and resilient to failures.
• Clear consistency model and locking approach.

Deliverables

Discuss and justify:

1. Data model and key constraints.
2. Public APIs and core flows (hold, purchase, refund, real-time updates).
3. Concurrency control and locking (pessimistic vs. optimistic).
4. Consistency model and how you prevent oversells.
5. Scaling strategy (sharding, caching, queueing, real-time fanout).
6. Failure handling, idempotency, retries, and auditability.