Design a Messaging Service
Last updated: October 29, 2025
Quick Overview
Design a scalable messaging system that handles millions of requests. Discuss trade-offs in consistency, availability, and performance.
Grubhub
October 29, 202520
12
2,788 solved
Design a scalable messaging system that handles millions of requests. Discuss trade-offs in consistency, availability, and performance.
Grubhub asks this during the Onsite to assess your architectural thinking. They want to see how you decompose a complex problem, choose appropriate technologies, and reason about failure modes. Strong candidates proactively discuss monitoring, alerting, and operational concerns.
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
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 happens if one of your database nodes goes down?
- How do you ensure data consistency across multiple services?
- 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 Messaging: Users should be able to send and receive messages in real-time.
- Group Messaging: Support for group chats where multiple users can participate....
Capacity Estimation
Back-of-Envelope Calculations
Assuming Grubhub has 30 million active users with an average of 100 messages sent per user per day.
- Total Messages per Day: 30 million users * 100 messages = 3...
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