Design a Banner Ad Server

Design a banner ad server that can serve image-based banner ads to Spotify’s 400M+ monthly active users. The hot path is a GET /ad?user_id=<uuid> call that must return a JSON payload with the creative URL in <100ms p99. The system must support geo-targeting (country level), frequency capping (per-campaign 10K impressions max, and per-user “no back-to-back” rule), and real-time analytics. Ads are ingested through an internal UI and become “active” within 30s globally. The server must horizontally scale to 300K QPS and tolerate regional outages. You do not need to implement the actual image delivery—assume creatives are pushed to a CDN at upload time and the ad server only returns the signed URL.