Design a frequency-capping service for an advertising platform. The ad platform receives ad requests for users and must decide whether a candidate ad, ad group, or campaign can be shown. Each cap has the form: a user may receive at most `N` impressions for a given ad scope during the last `W` units of time. The window is rolling, not calendar-based. Your design should cover: - the read path used when deciding whether to show an ad, - the write path used after an ad impression is served, - data models for cap definitions and impression history, - how to enforce caps correctly under high request volume, - how to handle distributed-system issues such as race conditions, duplicate requests, failures, and data retention.