12.9Example where Upsert can be wrong

Suppose source sends:

{
  "transactionId": null,
  "amount": 100
}

and you attempt to identify records using:

customerId + amount

Two legitimate $100 transactions might occur.

Now your “idempotency” logic suppresses a valid transaction.

Important rule:

An idempotency key must identify the logical operation, not merely look unique most of the time.