Design a A/B Testing Service
Last updated: April 12, 2026
Quick Overview
Design a geo-distributed a/b testing system that handles millions of requests. Discuss trade-offs in consistency, availability, and performance.
Workday
April 12, 202649
11
4,235 solved
Design a geo-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 Technical Screen at Workday. 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. Workday 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 optimize costs as the system scales?
- How would you handle a 10x increase in traffic overnight?
- How would you handle schema migrations with zero downtime?
- What monitoring and alerting would you set up on day one?
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Requirements
Functional Requirements:
- Experiment Creation: Users should be able to create A/B tests specifying variants, audience segmentation, and duration.
- Traffic Allocation: The system must d...
Capacity Estimation
Back-of-Envelope Calculations:
- Estimated User Base: Assume Workday has 10 million active users.
- Traffic Estimate: If 1% of users engage with the A/B testing service daily, that equat...
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