In a flash sale, how do you decrement stock so it never goes below zero?

Options

• ✗ Read the stock in the app, check it is above 0, then write stock − 1
• ✓ UPDATE items SET stock = stock - 1 WHERE id = ? AND stock > 0, and check the rows affected Correct
• ✗ Keep the count in each app server's memory
• ✗ Decrement a Redis counter and copy it to the database every minute

Why

Read-then-write lets two buyers read 1 and both write 0. A conditional update is atomic in the database: one buyer's update affects a row, the other's affects none and gets "sold out".

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