The insufficient funds check tripped me up more than I expected.
Start by clarifying requirements and scale, then design a system with a scheduler that triggers payment execution, a transaction processor that checks balances and performs transfers atomically, and a data model that tracks scheduled payments and account balances. Discuss trade-offs around consistency, idempotency, and failure handling.
Pro tip: Emphasize idempotency and exactly-once execution: scheduled jobs can be retried, so ensure each payment is processed only once even if the scheduler fires multiple times. Also, consider using a database transaction with row-level locking to prevent race conditions on balance checks.
Ask about expected volume, latency requirements, and whether payments can be cancelled or modified. Understand if the system needs to handle time zones and recurring payments.
Propose a scheduler service that polls for due payments and enqueues them, and a payment processor that executes transfers. Consider using a distributed job scheduler like Quartz or a cloud service like AWS EventBridge.
Design tables for scheduled_payments (id, source_account, destination_account, amount, scheduled_time, status) and accounts (id, balance). Use a relational database with ACID transactions for consistency.
Describe how the processor locks the source account row, checks balance, and if sufficient, debits and credits atomically. If insufficient, mark payment as failed/skipped. Use idempotency keys to avoid double processing.
Discuss retries with exponential backoff, dead-letter queues for failed payments, and how to scale horizontally by partitioning payments. Mention monitoring and alerting for failed executions.
AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.