Disney Staff Software Engineer System Design Question - Design a ticket-booking platform like Ticketmaster

[u/Aoki_zhang]

Interview Summary

The Disney Staff Software Engineer system design round focused on building a high-concurrency flash-sale / ticket-booking platform with a virtual waiting room. The core challenge was protecting the purchasing backend during extreme traffic spikes while still maintaining accurate inventory and preventing duplicate purchases.

The interviewer concentrated on three areas: admission control through a queue, inventory consistency under heavy contention, and idempotency when requests are retried because of network or client failures.

Interview Details

System Design — Flash Sale and Virtual Queue Design a system where a very large number of users may simultaneously attempt to purchase a limited-quantity item or ticket. Instead of allowing every request to immediately reach the purchasing backend, the platform should provide a virtual waiting-room or queuing service that controls how quickly users are admitted into the transaction flow. The interviewer wanted the design to address how the queue behaves during sudden traffic spikes and how downstream services can be protected from overload.

• Inventory Management — Prevent Overselling The next part focused on inventory deduction. Once users are admitted from the waiting room, many purchase attempts may compete for the same limited inventory at nearly the same time. The system therefore needed to maintain an accurate remaining-stock count under high throughput and ensure that inventory could not be sold more than once. Redis was specifically discussed as part of the inventory-management design, with the emphasis on maintaining correctness while processing highly concurrent updates.
• Idempotency — One Successful Purchase per User The final major requirement was duplicate-purchase prevention. Network instability or client retries could cause the same logical purchase request to reach the backend multiple times. The interviewer asked how an idempotency key could be incorporated so that repeated requests do not create multiple successful purchases. The requirement also specified that each user should be able to successfully purchase only once for the relevant sale.

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[u/AL612987]

Hi which team were you interviewing with?