Design ad frequency capping

Design a frequency capping system for an advertising platform. The system must ensure that a user does not see the same advertisement more than a configured number of times within a given time window, such as: - at most 3 impressions per user per ad per day - at most 10 impressions per user per campaign per week Your design should address: - how ad-serving systems check caps in real time before showing an ad - how impressions are counted and stored - how to support multiple cap scopes, such as user-ad, user-campaign, or household-level limits - how to handle high QPS, low latency, and eventual consistency across regions - what happens when counting data is delayed, duplicated, or arrives out of order - data retention, expiration of old counts, and operational trade-offs between accuracy and latency