Architect a scalable Chat Engine
Last updated: January 1, 2026
Quick Overview
Design a scalable chat system that handles millions of requests. Discuss trade-offs in consistency, availability, and performance.
Notion
January 1, 202649
8
1,006 solved
Design a scalable chat 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 Notion. 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. Notion 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
- What monitoring and alerting would you set up on day one?
- How would you handle a region-wide outage?
- How would you handle schema migrations with zero downtime?
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Requirements
Functional Requirements
- Real-time Messaging: Users should be able to send and receive messages in real-time.
- User Presence: Show online/offline status of users.
- Group Chats: S...
Capacity Estimation
Assuming Notion anticipates scaling to 10 million active users:
- QPS Estimation: If each user sends an average of 5 messages per minute:
- 10 million users * 5 messages/minute = 50 million me...
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