Design a distributed A/B Testing System
Last updated: February 11, 2026
Quick Overview
Design a distributed a/b testing system that handles millions of requests. Discuss trade-offs in consistency, availability, and performance.
Plaid
February 11, 202611
14
1,946 solved
Design a distributed a/b testing system that handles millions of requests. Discuss trade-offs in consistency, availability, and performance.
This is a common system design question asked during Onsite at Plaid. The interviewer expects you to demonstrate your ability to design large-scale distributed systems, make well-reasoned trade-offs, and communicate your thought process clearly. Plaid values engineers who can think about scalability from day one.
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
- How would you migrate from a monolithic to a microservices architecture?
- How would you handle schema migrations with zero downtime?
- How would you handle a region-wide outage?
- What happens if one of your database nodes goes down?
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Requirements
Functional Requirements
- User Segmentation: Ability to define user segments for A/B testing based on various attributes (e.g., region, device type).
- Experiment Creation: Users can cre...
Capacity Estimation
Capacity Estimation
- Daily Active Users (DAU): Assume Plaid has 1 million DAUs.
- Requests Per Second (QPS): If each user makes an average of 5 requests per day, that translates to around...
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