System design interview question
"Design an inventory system for a large e-commerce marketplace. It must track how much of each product is available across many warehouses, prevent overselling, and support reservations during checkout. Millions of SKUs, thousands of updates per second."
a correctness invariant under concurrency and distribution — closely related to the wallet question, but with extra dimensions: inventory lives in multiple physical locations, "available" is a derived quantity (on-hand minus reserved minus damaged), and the system must reconcile with the physical world, which does not obey your database.
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