This was essentially thirteen questions wrapped in one.
Start by clarifying requirements (scale, consistency, latency) and then walk through the system from admission control to purchase completion, emphasizing how each component prevents overselling and handles failures. Focus on the critical path: inventory reservation with atomic operations, idempotency, and async processing, while discussing trade-offs between consistency and availability.
Pro tip: Emphasize that overselling is prevented by making inventory decrement atomic and idempotent, and that the real challenge is managing the thundering herd through admission control and queueing. Show awareness that perfect consistency at scale requires accepting some latency and eventual consistency in non-critical paths.
Ask about expected traffic (e.g., millions of concurrent users), inventory size, latency requirements, and consistency needs. Define success metrics like zero overselling and high availability.
Propose a virtual waiting room with rate limiting, CAPTCHA, and token-based admission to smooth traffic and block bots. Use a queue to serialize purchase attempts.
Use atomic decrement operations (e.g., Redis Lua scripts or database transactions) with optimistic locking. Implement idempotency keys to deduplicate requests and prevent double purchases.
Shard inventory by ticket type or event to distribute load. Cache inventory counts with write-through or write-behind strategies, ensuring cache consistency with the source of truth.
Use a message queue (e.g., Kafka) to process orders asynchronously, with retries and dead-letter queues. Implement compensation logic for failed payments and release inventory.
Estimate resource needs based on peak QPS and design for horizontal scaling. Set up monitoring for inventory levels, queue depths, error rates, and latency, with alerts for anomalies.
AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.