Design a payment system that lets a user pay back an active loan

Design a payment system that lets a user pay back an active loan. The catch: the system must call an external bank API that only accepts requests during a limited window of a few hours per day.

Design how payments are accepted at any time, queued durably, and reliably submitted to the bank within its window. Cover idempotency, retries with backoff, exactly-once submission, status tracking, and reconciliation of failures.