tasks/DIST3_idempotency/spec.md

DIST3: Payment Service Idempotency

Goal

Make 4 payment operations safe to retry by implementing idempotency key deduplication.

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/

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

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

The idempotency key is set by the caller. Operations must never execute twice for the same key.