System design interview question
"Design a flash sale. At exactly noon, 10,000 units of a product go on sale, and 5 million people are waiting with the page open. They all click buy in the same second. Nobody may buy more than one, you must not oversell, and the site must stay up."
extreme write contention on a single resource, plus the realization that the fair and scalable answer is to reject most requests as early and as cheaply as possible. It is the sharpest contention question in this list because the numbers leave no room for a general-purpose design.
On Practick another engineer interviews you live for 60 minutes on a question from this bank. They have the full interviewer guide beside the call: the answers to your clarifying questions, the numbers to hand out, a ladder of follow-ups and a scoring rubric. You get the model solution once the session settles. It is free: interview someone for an hour and you have earned your own.
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