Design a low-latency Chat System
Last updated: February 20, 2026
Quick Overview
Design a low-latency chat system that handles millions of requests. Discuss trade-offs in consistency, availability, and performance.
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Design a low-latency chat system that handles millions of requests. Discuss trade-offs in consistency, availability, and performance.
This is a common system design question asked during System Design Round at Google. 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. Google 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 would the deployment pipeline look like for this system?
- What happens if one of your database nodes goes down?
- How would you optimize costs as the system scales?
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Requirements
Functional Requirements:
- Real-time Messaging: Users must be able to send and receive messages instantly.
- User Presence: Display online/offline status of users.
- Group Chats: Su...
Capacity Estimation
Back-of-Envelope Calculations:
- User Base: Assume 10 million active users.
- Message Rate: If each user sends 1 message per minute on average, that’s 10 million messages per minute, or ...
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