Schedule and execute future payments
Overview
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.
HubSpot
Aug 4, 2025, 10:55 AM
Software Engineer
Take-home Project
System Design
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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).
Solution
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