Design a A/B Testing Service
Last updated: May 15, 2026
Quick Overview
Design a distributed a/b testing system that handles millions of requests. Discuss trade-offs in consistency, availability, and performance.
Brex
System Design
Software Engineer
Brex
May 15, 2026Software Engineer
Technical Screen
System Design
Medium
106
13
1,286 solved
Design a distributed a/b testing system that handles millions of requests. Discuss trade-offs in consistency, availability, and performance.
System design interviews at Brex 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
Security and authentication
High-level architecture and component design
Monitoring, logging, and alerting
Failure handling and fault tolerance
Load balancing and horizontal scaling
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 a region-wide outage?
- How would you optimize costs as the system scales?
- How would you implement rate limiting to protect the system?
- How do you ensure data consistency across multiple services?
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Requirements
Functional Requirements:
- User Management: Users must be able to create and manage A/B tests through a user-friendly interface.
- Experiment Configuration: Support for different experim...
Capacity Estimation
- User Base: Assume Brex has 1 million users. If each user interacts with 5 A/B tests a month, we expect:
- Daily Active Users (DAU): 100,000 users (10% of user base)
- Total interactions p...
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