Design a large-scale Ad Serving Platform
Last updated: August 28, 2025
Quick Overview
Design a low-latency ad serving system that handles millions of requests. Discuss trade-offs in consistency, availability, and performance.
Slack
System Design
Software Engineer
Slack
August 28, 2025Software Engineer
Technical Screen
System Design
Hard
4
10
652 solved
Design a low-latency ad serving system that handles millions of requests. Discuss trade-offs in consistency, availability, and performance.
System design interviews at Slack 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
- 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
Monitoring, logging, and alerting
Message queues and async processing
Load balancing and horizontal scaling
Requirements gathering and capacity estimation
API design and rate limiting
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
- What happens if one of your database nodes goes down?
- How would you migrate from a monolithic to a microservices architecture?
- How would you handle schema migrations with zero downtime?
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Requirements
Functional Requirements:
- Ad Request Handling: The system must handle millions of ad requests per second, returning relevant ads based on user profile and context.
- Targeting Mechanism...
Capacity Estimation
Capacity Estimation:
- Traffic Estimation: Assuming an average of 1 million users active per minute, with each user generating 10 ad requests per minute, we expect 10 billion requests per min...
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