Design a real-time A/B Testing System
Last updated: July 30, 2025
Quick Overview
Design a real-time a/b testing system that handles millions of requests. Discuss trade-offs in consistency, availability, and performance.
HRT
July 30, 202550
5
2,119 solved
Design a real-time a/b testing system that handles millions of requests. Discuss trade-offs in consistency, availability, and performance.
HRT asks this during the System Design Round to assess your understanding of the full ML lifecycle. They want to see how you translate a business problem into an ML objective, design the feature pipeline, and plan for model monitoring and retraining.
What the Interviewer Expects
- Map the business problem to a concrete ML objective
- Propose reasonable features and a baseline model
- Discuss basic model evaluation metrics
- Outline a simple serving architecture
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 debug a model that works well offline but poorly online?
- How would you handle a 10x increase in prediction requests?
- How would you ensure fairness and reduce bias in the model?
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Requirements
Functional Requirements:
- Real-time Data Collection: Capture user interactions and A/B test results in real-time.
- Experiment Management: Allow users to create, modify, and monitor A/B...
Capacity Estimation
Assuming HRT handles approximately 10 million users per month, with an average of 5 interactions per user per day:
- Daily Requests: 10M users * 5 interactions = 50 million requests/day.
- **Re...
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