12.30Example: payment request

Requirement:

Charge customer exactly once.

Now Salesforce Upsert is not enough.

Design:

POST /payments
Idempotency-Key = PAY-123
 ↓
atomic claim/store key
 ↓
call payment provider
 ↓
store result
 ↓
return result

Retry:

PAY-123 exists
→ return prior result

This is classic request idempotency.