Design a low-latency Chat System
Last updated: February 2, 2026
Quick Overview
Design a low-latency chat system that handles millions of requests. Discuss trade-offs in consistency, availability, and performance.
Apple
February 2, 2026265
13
613 solved
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 Apple. 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. Apple values engineers who can think about scalability from day one.
What the Interviewer Expects
- Drive the design discussion proactively with minimal interviewer guidance
- Perform detailed capacity estimation and use it to inform design decisions
- Design for global scale with multi-region deployment and data consistency
- Deep dive into 2-3 critical components with implementation-level detail
- Address complex trade-offs: CAP theorem, eventual consistency, conflict resolution
- Discuss operational excellence: deployment strategy, chaos engineering, SLOs/SLIs
- Propose a phased rollout plan from MVP to full-scale system
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
- How would you handle schema migrations with zero downtime?
- What happens if one of your database nodes goes down?
- What monitoring and alerting would you set up on day one?
- How would you migrate from a monolithic to a microservices architecture?
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Requirements
Functional Requirements:
- Real-time Messaging: Users should be able to send and receive messages instantly.
- User Presence: Show online/offline status of users.
- Group Chats: Sup...
Capacity Estimation
User Base Assumptions:
- Active Users: 10 million concurrent users.
- Messages per User: Average of 50 messages per user per day.
Calculations:
- Total Messages per Day: 10 milli...
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