Design a high-throughput A/B Testing System
Last updated: November 27, 2025
Quick Overview
Design a high-throughput a/b testing system that handles millions of requests. Discuss trade-offs in consistency, availability, and performance.
Discord
System Design
Software Engineer
Discord
November 27, 2025Software Engineer
Onsite
System Design
Medium
11
8
4,470 solved
Design a high-throughput a/b testing system that handles millions of requests. Discuss trade-offs in consistency, availability, and performance.
System design interviews at Discord 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
Monitoring, logging, and alerting
Partitioning and sharding strategies
High-level architecture and component design
Load balancing and horizontal scaling
Consistency models and replication
Caching strategies (local, distributed, CDN)
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 handle a region-wide outage?
- How would you migrate from a monolithic to a microservices architecture?
- How do you ensure data consistency across multiple services?
- How would you handle schema migrations with zero downtime?
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Requirements
Functional Requirements:
- User Segmentation: The system must support segmenting users into different groups for A/B testing.
- Experiment Creation: Users should be able to create, modif...
Capacity Estimation
Back-of-Envelope Calculations:
- User Base: Assume Discord has 200 million active users.
- Concurrent A/B Tests: Assume 1% of users are participating in A/B tests at any time, leading to 2...
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