Ads Event Reporting System

Design an ads event reporting system for Pinterest that ingests billions of ad events per day (impressions, clicks, conversions) and serves real-time and batch analytics dashboards to advertisers. The system must support queries like "total impressions, clicks, conversions and spend for campaign X in the last 7 days broken down by day and country" with sub-second latency. Events arrive at millions per second, can be delayed up to 24 hours, and may be duplicated. Advertisers need minute-level real-time metrics, hourly roll-ups, and daily historical reports. The system should also handle late-arriving events and provide exactly-once semantics for revenue-critical data.