Why is an idempotency-key needed if you already perform a check at the business logic level?

The idempotency key is used to prevent the repeated execution of the same operation at the infrastructure level, especially in distributed systems and during network failures.

Even if there is a duplicate check in the business logic, the idempotency key ensures:

• Guarantee of a single request execution regardless of client retries (e.g., due to timeouts or resubmissions).
• Reduction of load on business logic and the database, as a request with the same key can be quickly recognized and the stored result returned without re-executing.
• Resilience to network errors and repeated calls, which is important for financial transactions, orders, and other critical processes.

Example: a client sends a payment request with an idempotency key. If the request is repeated due to a connection failure, the server recognizes from the key that the operation has already been completed and does not create a duplicate transaction.