Build a event-driven Payment Pipeline
Last updated: February 27, 2026
Quick Overview
Design a event-driven payment system that handles millions of requests. Discuss trade-offs in consistency, availability, and performance.
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Design a event-driven payment system that handles millions of requests. Discuss trade-offs in consistency, availability, and performance.
This is a common system design question asked during Onsite at Reddit. The interviewer expects you to demonstrate your ability to design large-scale distributed systems, make well-reasoned trade-offs, and communicate your thought process clearly. Reddit values engineers who can think about scalability from day one.
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?
- What happens if one of your database nodes goes down?
- How would you implement rate limiting to protect the system?
- What monitoring and alerting would you set up on day one?
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Requirements
Functional Requirements:
- Payment Processing: The system must process payments for various transactions such as subscriptions, in-app purchases, and donations.
- Event Handling: The sys...
Capacity Estimation
Assuming Reddit processes about 100 million active users with an average of 5 transactions per user per month:
- Total Transactions/Month: 100M users * 5 transactions = 500M transactions.
- **Tota...
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