Architect a fault-tolerant Notification Engine
Last updated: September 26, 2025
Quick Overview
Design a fault-tolerant notification system that handles millions of requests. Discuss trade-offs in consistency, availability, and performance.
Waymo
System Design
Product Manager
Waymo
September 26, 2025Product Manager
Onsite
System Design
Medium
33
10
1,855 solved
Design a fault-tolerant notification system that handles millions of requests. Discuss trade-offs in consistency, availability, and performance.
System design interviews at Waymo 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
Load balancing and horizontal scaling
Failure handling and fault tolerance
Message queues and async processing
Requirements gathering and capacity estimation
Partitioning and sharding strategies
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
- How would you migrate from a monolithic to a microservices architecture?
- How do you ensure data consistency across multiple services?
- How would you implement rate limiting to protect the system?
- How would you handle schema migrations with zero downtime?
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Requirements
Functional Requirements
- Notification Types: The system must support various types of notifications (e.g., alerts, reminders, updates).
- Multi-Channel Delivery: Notifications should be...
Capacity Estimation
Back-of-Envelope Calculations
- Estimated Users: 10 million active users.
- Notifications per User: Average of 10 notifications per day.
- Total Notifications: 10 million users * 10 no...
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