Hard Design: Payment System (Stripe/PayPal)

Two concurrent requests with the same idempotency key arrive simultaneously. How do you handle this?

— Tests your understanding of this concept.

Answer Options

A Process both, take the first result
B Use Redis SETNX to acquire a lock — only one request processes, the other waits and returns the cached result
C Return error to both
D Process both and reconcile later

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This question is from the Design: Payment System (Stripe/PayPal) topic (System Design Cases).

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