Design a A/B Testing Service
Last updated: February 15, 2026
Quick Overview
Design a multi-tenant a/b testing system that handles millions of requests. Discuss trade-offs in consistency, availability, and performance.
Mastercard
February 15, 2026107
6
1,938 solved
Design a multi-tenant a/b testing system that handles millions of requests. Discuss trade-offs in consistency, availability, and performance.
Mastercard asks this during the Technical Screen 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
- Define clear ML objectives with appropriate loss functions and metrics
- Design a comprehensive feature engineering pipeline
- Discuss model selection with trade-offs (complexity vs interpretability vs latency)
- Plan online and offline evaluation strategies including A/B testing
- Address serving infrastructure: batch vs real-time, latency requirements
- Consider data quality, labeling strategy, and feedback loops
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 handle the cold start problem?
- How would you handle a 10x increase in prediction requests?
- What would you do if model performance degrades over time?
- What is your model retraining strategy?
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Requirements
Functional Requirements:
- Multi-Tenant Support: The service must support multiple clients (business units) running their A/B tests independently.
- Experiment Creation and Management: U...
Capacity Estimation
Assuming Mastercard runs approximately 1,000 experiments concurrently, with an average of 100,000 users per experiment:
- Total Requests: 1,000 experiments * 100,000 users = 100,000,000 requests. ...
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