Design a payment system
Company: OpenAI
Role: Software Engineer
Category: System Design
Difficulty: medium
Interview Round: Onsite
Design a payment system for an online commerce platform.
The system should support:
- Creating a payment order for a purchase
- Authorizing and capturing card or wallet payments
- Idempotent retries from clients
- Payment status tracking such as `pending`, `authorized`, `captured`, `failed`, and `refunded`
- Full and partial refunds
- An internal ledger for financial correctness
- Webhooks or callbacks to merchants when status changes
- Reconciliation with external payment processors
Explain the API design, core services, data model, end-to-end payment flow, failure handling, consistency guarantees, security considerations, and how the system scales reliably.
Quick Answer: This question evaluates a candidate's competency in designing secure, reliable payment architectures, covering API design, transaction lifecycle management (authorization, capture, refund), idempotency, internal ledgers, webhook integrations, and reconciliation with external processors.