Design a Chat Service
Last updated: October 22, 2025
Quick Overview
Design a geo-distributed chat system that handles millions of requests. Discuss trade-offs in consistency, availability, and performance.
Doordash
System Design
Product Manager
Doordash
October 22, 2025Product Manager
System Design Round
System Design
Hard
10
10
4,302 solved
Design a geo-distributed chat system that handles millions of requests. Discuss trade-offs in consistency, availability, and performance.
Doordash asks this during the System Design Round 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
- Drive the design discussion proactively with minimal interviewer guidance
- Perform detailed capacity estimation and use it to inform design decisions
- Design for global scale with multi-region deployment and data consistency
- Deep dive into 2-3 critical components with implementation-level detail
- Address complex trade-offs: CAP theorem, eventual consistency, conflict resolution
- Discuss operational excellence: deployment strategy, chaos engineering, SLOs/SLIs
- Propose a phased rollout plan from MVP to full-scale system
Key Topics to Cover
Security and authentication
Load balancing and horizontal scaling
Partitioning and sharding strategies
Consistency models and replication
Database selection and data modeling
High-level architecture and component design
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 optimize costs as the system scales?
- How would you handle a region-wide outage?
- How would you handle schema migrations with zero downtime?
Practice a Similar Problem on Codemia
Solve a related problem with our interactive workspace, get AI feedback, and view detailed solutions.
Solve on CodemiaSample Answer
Requirements
Functional Requirements
- User Authentication: Users must securely log in using OAuth 2.0, integrating with existing DoorDash accounts.
- Real-Time Messaging: Support sending and receivi...
Capacity Estimation
Back-of-Envelope Calculations
- User Base: Assume 10 million active users.
- Message Rate: At peak, each user sends an average of 5 messages per hour. This translates to:
- 10 million us...
Submit Your Answer
Markdown supported