Design a low-latency A/B Testing System
Last updated: November 20, 2025
Quick Overview
Design a low-latency a/b testing system that handles millions of requests. Discuss trade-offs in consistency, availability, and performance.
Vercel
November 20, 202559
12
3,512 solved
Design a low-latency a/b testing system that handles millions of requests. Discuss trade-offs in consistency, availability, and performance.
Vercel asks this during the System Design Round 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
- Drive the design discussion proactively with minimal interviewer guidance
- Perform detailed capacity estimation and use it to inform design decisions
- Design for global scale with multi-region deployment and data consistency
- Deep dive into 2-3 critical components with implementation-level detail
- Address complex trade-offs: CAP theorem, eventual consistency, conflict resolution
- Discuss operational excellence: deployment strategy, chaos engineering, SLOs/SLIs
- Propose a phased rollout plan from MVP to full-scale system
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 would the deployment pipeline look like for this system?
- How do you ensure data consistency across multiple services?
- How would you handle schema migrations with zero downtime?
- How would you migrate from a monolithic to a microservices architecture?
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Requirements
Functional Requirements
- User Segmentation: Ability to create and manage user segments for targeted experiments.
- Experiment Configuration: UI for configuring experiments with control ...
Capacity Estimation
Assuming Vercel serves around 100 million requests per day:
- Peak Requests: Estimating peak traffic at 20% of daily requests, we have approximately 20 million requests/hour or about 5,500 request...
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