Design a large-scale Messaging Platform
Last updated: February 28, 2026
Quick Overview
Design a high-throughput messaging system that handles millions of requests. Discuss trade-offs in consistency, availability, and performance.
Reddit
System Design
Software Engineer
Software Engineer
Technical Screen
System Design
Hard
102
15
3,855 solved
Design a high-throughput messaging system that handles millions of requests. Discuss trade-offs in consistency, availability, and performance.
Reddit asks this during the Technical Screen to assess your architectural thinking. They want to see how you decompose a complex problem, choose appropriate technologies, and reason about failure modes. Strong candidates proactively discuss monitoring, alerting, and operational concerns.
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
Requirements gathering and capacity estimation
Security and authentication
Consistency models and replication
Load balancing and horizontal scaling
Monitoring, logging, and alerting
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 do you ensure data consistency across multiple services?
- How would you implement rate limiting to protect the system?
- How would you optimize costs as the system scales?
- How would you handle a region-wide outage?
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Requirements
- Functional Requirements:
- Users can send, receive, and read messages in real-time.
- Users can create group chats and manage participants.
- Search functionality for messages and threads....
Capacity Estimation
- User Base: Assuming Reddit has around 500 million active users, with 20% (100 million) engaging in messaging.
- Message Volume: If each user sends an average of 5 messages per day, the total...
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