Architect a fault-tolerant A/B Testing Engine
Last updated: December 21, 2025
Quick Overview
Design a fault-tolerant a/b testing system that handles millions of requests. Discuss trade-offs in consistency, availability, and performance.
DoorDash
December 21, 2025138
13
1,438 solved
Design a fault-tolerant 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 DoorDash. 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. DoorDash 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 implement rate limiting to protect the system?
- How would you handle a 10x increase in traffic overnight?
- How would you handle a region-wide outage?
- What would the deployment pipeline look like for this system?
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Requirements
Functional Requirements:
- Experiment Creation: Users (data scientists) can create, configure, and manage A/B tests, including defining variants and traffic allocation.
- **Experiment Tracki...
Capacity Estimation
Assuming DoorDash runs about 1,000 active A/B tests concurrently with an average of 10,000 users participating in each test daily:
- Total Users per Day: 1,000 tests * 10,000 users = 10,000,000 us...
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