How to ensure the integrity of a money transfer transaction between banks so that the money is not lost or duplicated?

To ensure the integrity of money transfer transactions between banks, distributed transactions with a two-phase commit (2PC) mechanism must be used. This guarantees that both operations — debiting one account and crediting another — are performed atomically: either both or none.

Main approaches:

• Two-phase commit (2PC): the transaction coordinator first requests readiness to commit from all participants, and if everyone is ready, confirms the commit. If someone is not ready — it rolls back.
• Idempotency of operations: to avoid duplication, operations should be idempotent, for example, using unique transaction identifiers.
• Logging and compensating transactions: in case of failures, changes can be rolled back or compensated.

An example of a simplified algorithm:

1. A unique transaction ID is created.
2. Money is debited from the sender, marking the operation as "pending" with this ID.
3. A request is sent to the recipient bank for crediting with the same ID.
4. If the credit is successful, the debit and credit are fixed.
5. If something goes wrong, the debit is rolled back.

In real systems, specialized protocols and middleware are often used to ensure consistency and fault tolerance.