| # DIST3: Payment Service Idempotency |
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| ## Goal |
| Make 4 payment operations safe to retry by implementing idempotency key deduplication. |
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| ## Requirements |
| 1. Charge creation must be idempotent: same `idempotency_key` returns same charge result |
| 2. Refund processing must be idempotent: same key returns same refund result |
| 3. Webhook delivery must be idempotent: same key sends webhook exactly once |
| 4. Ledger entry creation must be idempotent: same key creates entry exactly once |
| 5. The balance query endpoint must NOT be modified (it is already safe to retry) |
| 6. Implement the idempotency store in `payments/idempotency.py` |
| 7. All tests must pass: `pytest tests/` |
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|
| ## Supporting Documents |
| - `payments/charges.py` — Charge creation (not idempotent) |
| - `payments/refunds.py` — Refund processing (not idempotent) |
| - `payments/webhooks.py` — Webhook delivery (not idempotent) |
| - `payments/ledger.py` — Ledger entries (not idempotent) |
| - `payments/balance.py` — Balance query (ALREADY IDEMPOTENT — do not change) |
| - `payments/idempotency.py` — Empty stub — implement this |
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| ## Idempotency Key Pattern |
| Each operation receives an `idempotency_key: str` parameter. Before executing: |
| 1. Check if `idempotency_key` exists in the dedup table |
| 2. If yes: return the cached result from the first execution |
| 3. If no: execute the operation, store the result with the key, return result |
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| The idempotency key is set by the caller. Operations must never execute twice for the same key. |
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