Architect a fault-tolerant A/B Testing Engine
Last updated: November 4, 2025
Quick Overview
Design a fault-tolerant a/b testing system that handles millions of requests. Discuss trade-offs in consistency, availability, and performance.
Jane Street
System Design
Software Engineer
Jane Street
November 4, 2025Software Engineer
Technical Screen
System Design
Medium
10
10
4,703 solved
Design a fault-tolerant a/b testing system that handles millions of requests. Discuss trade-offs in consistency, availability, and performance.
System design interviews at Jane Street 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
Monitoring, logging, and alerting
Consistency models and replication
Message queues and async processing
Failure handling and fault tolerance
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 do you ensure data consistency across multiple services?
- What would the deployment pipeline look like for this system?
- How would you optimize costs as the system scales?
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Requirements
- Functional Requirements:
- Ability to create and manage A/B tests, including defining control and variant groups.
- Real-time tracking of user interactions and conversion metrics for each va...
Capacity Estimation
- User Base: Assume 10 million active users.
- Requests: Each user interacts with the A/B testing system approximately 5 times a day, leading to:
- Total Requests per Day = 10,000,000 users ...
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