Design a real-time Notification System
Last updated: March 18, 2026
Quick Overview
Design a real-time notification system that handles millions of requests. Discuss trade-offs in consistency, availability, and performance.
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Design a real-time notification 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 Google. 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. Google 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 10x increase in traffic overnight?
- How would you migrate from a monolithic to a microservices architecture?
- What happens if one of your database nodes goes down?
- How would you handle schema migrations with zero downtime?
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Requirements
- Functional Requirements:
- The system must support real-time notifications for various Google services (e.g., Gmail, Calendar, Drive).
- Users should be able to subscribe to different ...
Capacity Estimation
- User Base: Assume 1 billion users, with 50% active daily.
- Notifications per User: Average of 5 notifications per user per day.
- Total Daily Notifications: 500 million notification...
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