Design a Chat Service
Last updated: December 9, 2025
Quick Overview
Design a distributed chat system that handles millions of requests. Discuss trade-offs in consistency, availability, and performance.
Plaid
System Design
Software Engineer
Plaid
December 9, 2025Software Engineer
Technical Screen
System Design
Medium
269
13
4,609 solved
Design a distributed chat system that handles millions of requests. Discuss trade-offs in consistency, availability, and performance.
Plaid asks this during the Technical Screen to assess your understanding of the full ML lifecycle. They want to see how you translate a business problem into an ML objective, design the feature pipeline, and plan for model monitoring and retraining.
What the Interviewer Expects
- Define clear ML objectives with appropriate loss functions and metrics
- Design a comprehensive feature engineering pipeline
- Discuss model selection with trade-offs (complexity vs interpretability vs latency)
- Plan online and offline evaluation strategies including A/B testing
- Address serving infrastructure: batch vs real-time, latency requirements
- Consider data quality, labeling strategy, and feedback loops
Key Topics to Cover
Data collection and labeling strategy
A/B testing and experimentation
Monitoring and model degradation detection
Feedback loops and model retraining
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 ensure fairness and reduce bias in the model?
- How would you debug a model that works well offline but poorly online?
- How would you handle a 10x increase in prediction requests?
- What would you do if model performance degrades over time?
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Requirements
Functional Requirements
- Real-time Messaging: Users can send and receive messages instantly.
- Group Chats: Support for one-on-one and group chat functionalities.
- Message History...
Capacity Estimation
Back-of-Envelope Calculations
- User Base: Assuming 5 million active users.
- Messages per User: Average of 10 messages per user per day.
- Total Messages: 5 million users * 10 mess...
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