Design a Messaging Service
Last updated: April 19, 2026
Quick Overview
Design a distributed messaging system that handles millions of requests. Discuss trade-offs in consistency, availability, and performance.
Discord
April 19, 20262
9
2,799 solved
Design a distributed messaging 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 Discord. 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. Discord values engineers who can think about scalability from day one.
What the Interviewer Expects
- Clearly define functional and non-functional requirements
- Propose a reasonable high-level architecture with core components
- Choose appropriate data storage solutions with basic justification
- Discuss basic scaling strategies (horizontal scaling, caching)
- Identify potential bottlenecks and suggest simple solutions
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 a region-wide outage?
- How would you implement rate limiting to protect the system?
- How would you handle a 10x increase in traffic overnight?
- What happens if one of your database nodes goes down?
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Requirements
- Functional Requirements:
- Users should be able to send and receive messages in real-time.
- Support for direct messages and group chats.
- Message history retrieval by users.
- Notifica...
Capacity Estimation
- User Base: Assume 100 million active users.
- Messages per User: Assume an average of 50 messages per user per day.
- Total Messages: 100 million users * 50 messages = 5 billion messages...
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