Design a Ad Serving for Postmates
Last updated: November 30, 2025
Quick Overview
Design a event-driven ad serving system that handles millions of requests. Discuss trade-offs in consistency, availability, and performance.
Postmates
System Design
Software Engineer
Postmates
November 30, 2025Software Engineer
Onsite
System Design
Medium
37
3
895 solved
Design a event-driven ad serving system that handles millions of requests. Discuss trade-offs in consistency, availability, and performance.
System design interviews at Postmates 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
- 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
Monitoring, logging, and alerting
Caching strategies (local, distributed, CDN)
API design and rate limiting
Partitioning and sharding strategies
Database selection and data modeling
Failure handling and fault tolerance
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 would the deployment pipeline look like for this system?
- What happens if one of your database nodes goes down?
- How do you ensure data consistency across multiple services?
- How would you handle a 10x increase in traffic overnight?
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Requirements
- Functional Requirements:
- Serve personalized ads to users based on their location, preferences, and order history.
- Track ad impressions, clicks, and conversions to allow for perform...
Capacity Estimation
- Estimated Scale:
- Assume Postmates has around 10 million active users monthly.
- Peak usage could reach around 100 requests per second (QPS) per user during peak hours, resulting in:
...
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