Design A/B Testing Infrastructure for real-time analytics
Last updated: April 10, 2026
Quick Overview
Design a scalable a/b testing system that handles millions of requests. Discuss trade-offs in consistency, availability, and performance.
Vercel
April 10, 2026153
6
3,778 solved
Design a scalable a/b testing system that handles millions of requests. Discuss trade-offs in consistency, availability, and performance.
System design interviews at Vercel 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
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?
- How would you optimize costs as the system scales?
- How would you migrate from a monolithic to a microservices architecture?
- How would you handle schema migrations with zero downtime?
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Requirements
Functional Requirements
- User Segmentation: Ability to segment users into different groups for A/B testing.
- Experiment Creation and Management: Admin interface for creating, managing,...
Capacity Estimation
Assuming Vercel serves approximately 1 million unique users daily with an average of 10 requests per user for A/B testing, we can estimate:
- Total Requests per Day: 1 million users × 10 request...
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