Architect a low-latency Ad Serving Engine
Last updated: September 11, 2025
Quick Overview
Design a low-latency ad serving system that handles millions of requests. Discuss trade-offs in consistency, availability, and performance.
Citadel
September 11, 20255
5
310 solved
Design a low-latency ad serving system that handles millions of requests. Discuss trade-offs in consistency, availability, and performance.
Citadel asks this during the Technical Screen to assess your depth in software engineering. They want to see understanding of design patterns, system architecture, and the trade-offs involved in different technical approaches.
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 this design change if the team size doubled?
- How would you document this for other engineers?
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Core Design Principles
For the low-latency ad serving engine, three core design principles are crucial: Separation of Concerns, High Cohesion, and Scalability.
- Separation of Concerns ensures that differ...
Architecture
The architecture will follow a microservices approach, where distinct services handle various aspects of ad serving.
- Ad Request Service: Handles incoming requests and routes them to appropria...
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