5. Payment reliability: "How was the issue of idempotency solved in gRPC?"
Interviewer’s goal: Payments — the most dangerous area for duplicates (Double Spending).
Possible question:
"The payment request went through gRPC but failed due to timeout (DEADLINE_EXCEEDED). Did the money get deducted — unknown. What does the payment module do?"
How to answer:
"In a financial transactional context, any repeated request must have an Idempotency Key:
1. In the metadata of the gRPC request (metadata=((‘x-idempotency-key’, key_uuid),)) we always transmitted a unique transaction key.
2. When receiving DEADLINE_EXCEEDED, the client could safely Retry with the same key.
3. The acquiring service checked the presence of this key in its database / Redis and either returned the status of the already processed transaction or processed it for the first time."
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