Design Payment Infrastructure for global users
Last updated: November 19, 2025
Quick Overview
Design a real-time payment system that handles millions of requests. Discuss trade-offs in consistency, availability, and performance.
Redfin
November 19, 202544
14
3,796 solved
Design a real-time payment system that handles millions of requests. Discuss trade-offs in consistency, availability, and performance.
System design interviews at Redfin typically last 45-60 minutes. You are expected to drive the conversation, starting from requirements gathering through to a detailed architecture. The interviewer will evaluate your ability to handle ambiguity and make practical engineering decisions.
What the Interviewer Expects
- Clearly define functional and non-functional requirements
- Propose a reasonable high-level architecture with core components
- Choose appropriate data storage solutions with basic justification
- Discuss basic scaling strategies (horizontal scaling, caching)
- Identify potential bottlenecks and suggest simple solutions
Key Topics to Cover
How to Approach This
- Start by clarifying functional and non-functional requirements with the interviewer.
- Estimate the scale: QPS, storage, bandwidth. This drives your design decisions.
- Draw a high-level architecture first, then deep dive into 1-2 critical components.
- Discuss trade-offs explicitly (e.g., consistency vs availability, SQL vs NoSQL).
- Address failure scenarios, monitoring, and how the system handles 10x traffic spikes.
Possible Follow-up Questions
- How would you migrate from a monolithic to a microservices architecture?
- How would you handle a region-wide outage?
- What happens if one of your database nodes goes down?
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Requirements
Functional Requirements
- User Authentication: Secure login for users to manage payment methods and view transaction history.
- Payment Processing: Real-time processing of payments for t...
Capacity Estimation
Assuming Redfin has approximately 10 million users and estimates that 10% will make a payment request daily:
- Daily Payment Requests: 10 million users * 10% = 1 million requests per day.
- **Pe...
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