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How would you combine idempotency and retry logic in a payment API to prevent duplicate charges during network failures?

hard📝 Trade-off Q9 of Q15
LLD - Design — Parking Lot System
How would you combine idempotency and retry logic in a payment API to prevent duplicate charges during network failures?
ADisable retries to avoid duplicates
BAssign a unique transaction ID for each payment request and store processed IDs to ignore duplicates
CAllow multiple charges and refund duplicates manually
DRetry payment requests without any unique identifiers
Step-by-Step Solution
Solution:

  1. Step 1: Understand idempotency and retry needs

    Idempotency means repeated requests with same ID do not cause multiple charges. Retry logic resends requests on failure.

  2. Step 2: Implement unique transaction IDs and storage

    Assigning unique IDs lets system recognize duplicates and ignore repeats, preventing double charges during retries.

  3. Final Answer:

    Assign a unique transaction ID for each payment request and store processed IDs to ignore duplicates -> Option B

  4. Quick Check:

    Unique IDs + storage = safe retries without duplicates [OK]
Quick Trick: Use unique IDs to make retries safe and idempotent [OK]
Common Mistakes:
MISTAKES
  • Retrying without unique IDs causes duplicates
  • Manually refunding duplicates is error-prone
  • Disabling retries reduces reliability

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