Two retries with the same idempotency key reach two servers at the same moment. What stops both from charging the card?

Options

• ✗ Each server checks a cache for the key and proceeds if it is absent
• ✓ Inserting the key under a unique constraint before charging; the second insert fails Correct
• ✗ Comparing the two requests' timestamps afterwards
• ✗ Nothing is needed; clients never retry that fast

Why

"Check, then act" is a race: both servers see no key and both charge. An atomic insert lets the database pick exactly one winner; the loser waits for the winner's result or returns 409 Conflict.

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