Design a Notification Service
Last updated: March 6, 2026
Quick Overview
Design a real-time notification system that handles millions of requests. Discuss trade-offs in consistency, availability, and performance.
Slack
March 6, 202655
15
4,714 solved
Design a real-time notification system that handles millions of requests. Discuss trade-offs in consistency, availability, and performance.
System design interviews at Slack 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
- 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
- What would the deployment pipeline look like for this system?
- How would you implement rate limiting to protect the system?
- How would you migrate from a monolithic to a microservices architecture?
- How would you handle a 10x increase in traffic overnight?
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Requirements
Functional Requirements
- Real-time Notifications: Users should receive notifications immediately when events occur (e.g., messages, mentions, reactions).
- User Preferences: Users can c...
Capacity Estimation
To estimate capacity, we need to consider:
- User Base: Assume 10 million active users.
- Notification Rate: On average, each user generates 1 notification per minute, leading to 10 million ...
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