Design config rollout and click aggregation

The system design rounds included two prompts: 1. **Global configuration deployment** - Design a system that allows engineers to publish configuration changes and roll them out safely to services running in many regions worldwide. - Support versioning, staged rollout, rollback, validation, audit history, and fast propagation. - Explain how to balance safety, consistency, and rollout speed. 2. **Ads click event aggregation** - Design a system that ingests a very high volume of ad click events and produces aggregated metrics such as counts per campaign, ad, region, and time window. - Support near-real-time dashboards, fault tolerance, deduplication, late-arriving events, and horizontal scaling.