Backend · Mid-level · Theory

During a sale, an online store sold more units of a product than it had in stock. The code first checks the stock level and then decrements it. Why did this happen, and how would you fix it?

~4 min by voice

How to approach it

This checks whether the candidate recognizes the classic check-then-act race and can solve it at the database level rather than the process level: with an atomic conditional UPDATE, locks and constraints, understanding the trade-offs under high load.

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Topics

concurrency race conditions transactions

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Answer by voice

The interviewer will ask this question and listen. Think out loud and ask for a hint if you need one. When you're done, pause or tap “That's my answer”. In about a minute, your score and the points you covered show up here.

Best with headphones · ~4 min