Design an ad frequency capping system

Company: Netflix

Role: Software Engineer

Category: System Design

Difficulty: medium

Interview Round: Onsite

Quick Answer: This question evaluates a candidate's ability to design scalable, low-latency distributed systems that enforce ad frequency caps across users, devices, and profiles, testing competencies in system architecture, data modeling, consistency, and real-time enforcement; it falls under the System Design domain and requires both high-level architectural understanding and practical application-level trade-off reasoning. It is commonly asked to assess how applicants reason about scalability, availability, latency-consistency trade-offs, API and data flow design, and failure modes when enforcing multi-window caps at high QPS.