Openai·Software Engineer·Onsite - System Design / Architecture·Senior

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May 2026

Summary

System design round at OpenAI, one big question about building a payment platform from scratch. The scope was massive and I'm not sure I handled the scale requirements well enough.

Questions Asked (1)

Design a payment processing system for a global platform that supports user-to-user payments, refunds, and transaction history, with high availability, security, and the ability to handle 10,000 transactions per second at peak.

System DesignTechnical Trade-offsData Modeling

Author's notes

I started with the data model and probably spent too long there.

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AI HintsAI Generated

Suggested Approach

Start by clarifying requirements and constraints, then design a high-level architecture that separates concerns into services like payment processing, ledger, and user management. Focus on scalability, consistency, and security, and discuss trade-offs for each component.

Pro tip: Emphasize idempotency and exactly-once processing to handle retries and ensure data consistency, and discuss how you would monitor and alert on key metrics like latency and error rates.

1. Clarify Requirements

Ask questions to understand scope: expected read/write ratio, consistency requirements, regulatory constraints, and integration points.

2. High-Level Design

Outline main components: API gateway, payment service, ledger service, user service, and databases. Sketch data flow for payments, refunds, and history.

3. Deep Dive into Critical Components

Detail the payment processing flow, ensuring idempotency and atomicity. Discuss database choices (e.g., ACID vs BASE) and sharding strategies for scalability.

4. Address Scalability and Availability

Explain how to handle 10k TPS: horizontal scaling, partitioning, caching, and async processing. Discuss replication, failover, and disaster recovery.

5. Security and Compliance

Cover encryption, authentication/authorization, PCI DSS, and fraud detection. Mention audit logs and monitoring.

Key Points to Mention

Idempotency keys to prevent duplicate transactions
Distributed transactions and consistency models (e.g., two-phase commit, saga pattern)
Database sharding and replication for scalability and availability
Caching strategies for transaction history and user data
Security measures: encryption at rest and in transit, tokenization, and access control
Monitoring, alerting, and logging for operational excellence

AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.