Design a large-scale Notification Platform
Last updated: July 15, 2025
Quick Overview
Design a high-throughput notification system that handles millions of requests. Discuss trade-offs in consistency, availability, and performance.
Reddit
System Design
Machine Learning Engineer
Machine Learning Engineer
System Design Round
System Design
Medium
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2,276 solved
Design a high-throughput notification system that handles millions of requests. Discuss trade-offs in consistency, availability, and performance.
System design interviews at Reddit typically last 45-60 minutes. You are expected to drive the conversation, starting from requirements gathering through to a detailed architecture. The interviewer will evaluate your ability to handle ambiguity and make practical engineering decisions.
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
Monitoring, logging, and alerting
Caching strategies (local, distributed, CDN)
Security and authentication
Failure handling and fault tolerance
Consistency models and replication
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 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
- User Notifications: The system must deliver notifications to users about various events (e.g., replies to posts, mentions, direct messages) in real-time.
- **Notifi...
Capacity Estimation
Back-of-Envelope Calculations
- User Base: Assume 500 million active users with an engagement rate of 20% leading to 100 million active users per day.
- Notification Volume: If each acti...
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