Design a fault-tolerant A/B Testing System
Last updated: February 9, 2026
Quick Overview
Design a fault-tolerant a/b testing system that handles millions of requests. Discuss trade-offs in consistency, availability, and performance.
Coinbase
System Design
Machine Learning Engineer
Coinbase
February 9, 2026Machine Learning Engineer
Technical Screen
System Design
Medium
184
12
1,133 solved
Design a fault-tolerant a/b testing system that handles millions of requests. Discuss trade-offs in consistency, availability, and performance.
Coinbase asks this during the Technical Screen to assess your architectural thinking. They want to see how you decompose a complex problem, choose appropriate technologies, and reason about failure modes. Strong candidates proactively discuss monitoring, alerting, and operational concerns.
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
Monitoring, logging, and alerting
Requirements gathering and capacity estimation
Failure handling and fault tolerance
Security and authentication
Load balancing and horizontal scaling
Caching strategies (local, distributed, CDN)
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 would the deployment pipeline look like for this system?
- How do you ensure data consistency across multiple services?
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Requirements
Functional Requirements
- User Segmentation: The system must allow defining user segments for A/B testing based on demographics, behavior, and historical data.
- Experiment Management: U...
Capacity Estimation
- User Base: Assume 10 million active users on Coinbase.
- A/B Testing Frequency: Estimate that 5% of users are involved in A/B tests at any moment, resulting in 500,000 concurrent requests....
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