Design a Payment Service
Last updated: April 15, 2026
Quick Overview
Design a real-time payment system that handles millions of requests. Discuss trade-offs in consistency, availability, and performance.
Compass
April 15, 202669
14
368 solved
Design a real-time payment system that handles millions of requests. Discuss trade-offs in consistency, availability, and performance.
System design interviews at Compass 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
- Systematically gather requirements and estimate capacity (QPS, storage, bandwidth)
- Design a scalable architecture with clear component responsibilities
- Make well-reasoned database and caching decisions with trade-off analysis
- Address consistency vs availability trade-offs specific to the use case
- Discuss partitioning strategy, replication, and data modeling
- Cover failure handling, monitoring, and alerting strategies
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
- What monitoring and alerting would you set up on day one?
- What happens if one of your database nodes goes down?
- How would you migrate from a monolithic to a microservices architecture?
Practice a Similar Problem on Codemia
Solve a related problem with our interactive workspace, get AI feedback, and view detailed solutions.
Solve on CodemiaSample Answer
Requirements
Functional Requirements:
- Users should be able to initiate payments in real-time via web and mobile applications.
- Support for multiple payment methods (credit card, bank transfer, etc.).
- P...
Capacity Estimation
To estimate capacity for Compass's payment service:
- QPS (Queries Per Second): Assume 1 million active users, each making an average of 10 transactions per month.
- Monthly Transactions = 1,...
Submit Your Answer
