Design a Chat Service
Last updated: October 17, 2025
Quick Overview
Design a low-latency chat system that handles millions of requests. Discuss trade-offs in consistency, availability, and performance.
DoorDash
October 17, 2025540
6
987 solved
Design a low-latency chat system that handles millions of requests. Discuss trade-offs in consistency, availability, and performance.
ML system design at DoorDash goes beyond model selection. This Onsite question evaluates your ability to design end-to-end ML pipelines, from data collection to model serving, while considering production constraints like latency and reliability.
What the Interviewer Expects
- Design the full ML lifecycle from data collection to model monitoring
- Address cold start, exploration/exploitation, and model freshness
- Discuss multi-objective optimization and ranking systems
- Plan for model debugging, fairness, and bias mitigation
- Design the feature store and training pipeline for scale
- Address model versioning, canary deployments, and rollback strategies
- Discuss the data flywheel and long-term system evolution
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
- What is your model retraining strategy?
- How would you run A/B tests on different model versions?
- How would you handle a 10x increase in prediction requests?
- How would you debug a model that works well offline but poorly online?
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Requirements
Functional Requirements
- Real-time Messaging: Users can send and receive messages instantly within the app.
- Group Chats: Support for multiple users in a single chat thread.
- **Messa...
Capacity Estimation
Assuming DoorDash has 100 million active users and averages 5 messages per user per day:
- Total Messages per Day: 100 million users * 5 messages = 500 million messages/day.
- **Messages per Secon...
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