Design a fault-tolerant Monitoring System
Last updated: April 8, 2026
Quick Overview
Design a fault-tolerant monitoring system that handles millions of requests. Discuss trade-offs in consistency, availability, and performance.
Reddit
System Design
Software Engineer
Software Engineer
Onsite
System Design
Medium
36
12
3,380 solved
Design a fault-tolerant monitoring 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
Database selection and data modeling
Failure handling and fault tolerance
Security and authentication
Partitioning and sharding strategies
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?
- What would the deployment pipeline look like for this system?
- What happens if one of your database nodes goes down?
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Requirements
Functional Requirements:
- Real-time Monitoring: The system should track the health and performance of various services across Reddit, including API endpoints, database queries, and external ...
Capacity Estimation
Back-of-Envelope Calculations:
- User Base: Assume Reddit has about 50 million daily active users.
- Requests per Second (QPS): If each user generates an average of 0.5 requests per minute...
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