Schedule and execute future payments

Q: Schedule and execute future payments

This question evaluates understanding of reliable scheduled job execution, transactional integrity, concurrency control, idempotency, and failure handling in balance-transfer and payment systems within the system design domain.

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Question

System Design: Scheduled Payments Execution

Context

You are building a balance-transfer service (e.g., an internal wallet). Users can schedule a payment to move funds from a source account to a destination account at a future timestamp.

Requirements

Design a feature that supports the following:

A user can schedule a payment at a future UTC timestamp.
The system executes the payment at the scheduled time.
On execution, the system updates the involved account balances atomically.
If the source balance is insufficient at execution time, the payment is skipped (no balance changes).

Assumptions (minimal to make the problem well-scoped)

Single region deployment; relational database available.
Timestamps are stored and compared in UTC with second-level precision.
Payments are in a single currency (multi-currency handling is out of scope).
“Execute at the scheduled time” means trigger at or immediately after the timestamp (sub-second jitter is acceptable in distributed systems).

What to Deliver

Provide a design that covers:

High-level architecture and components.
Data model and key constraints.
Execution flow (scheduling and run-time) and concurrency control.
Handling of insufficient funds, idempotency, and failure/retry.
Brief API sketch and operational considerations (monitoring, background jobs).