Architect a real-time Chat Engine
Last updated: December 30, 2025
Quick Overview
Design a real-time chat system that handles millions of requests. Discuss trade-offs in consistency, availability, and performance.
DoorDash
December 30, 202524
8
2,372 solved
Design a real-time chat system that handles millions of requests. Discuss trade-offs in consistency, availability, and performance.
Software engineering fundamentals questions at DoorDash test your understanding of core CS concepts and their practical application. This Onsite question evaluates how you apply engineering principles to build maintainable, scalable software.
What the Interviewer Expects
- Apply engineering principles to a realistic design scenario
- Discuss trade-offs between different approaches with concrete examples
- Demonstrate understanding of testability, maintainability, and extensibility
- Connect theoretical concepts to production engineering practices
- Discuss how the approach scales with team and codebase size
Key Topics to Cover
How to Approach This
- Apply SOLID principles. Single Responsibility makes code testable, Open/Closed makes it extensible.
- Choose data structures based on access patterns, not familiarity.
- Prefer immutable data and message passing over shared mutable state for concurrency.
- Design APIs with RESTful conventions, versioning, meaningful errors, and pagination from day one.
Possible Follow-up Questions
- What testing strategy would you use for this component?
- How would you handle backward compatibility?
- How would this design change if the team size doubled?
- How would you measure the performance of this component in production?
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Core Design Principles
For designing a real-time chat engine at DoorDash, the following core design principles apply:
- SOLID Principles: Specifically, the Single Responsibility Principle (SRP) and Open/Closed Princip...
Architecture
The chat engine can be designed using a microservices architecture:
- Microservices: Each service (e.g., User Service, Message Service, Notification Service) can be independently deployed and sc...
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