Design an Ad Click Tracking Service

To design an ad click tracking service capable of handling billions of events, I would first clarify that we prioritize availability and durability over strong consistency, as slight delays in reporting are acceptable. For ingestion, I'd implement a global load balancer routing traffic to a managed message queue like Azure Service Bus or Kafka. This decouples the high-velocity write path from the processing logic, absorbing traffic spikes. Next, for storage, I'd choose a wide-column store like Azure Cosmos DB or Cassandra, configured with automatic sharding based on the ad campaign ID. This ensures linear scalability as data grows. To optimize write performance, I'd employ an append-only log structure where clicks are written sequentially. Since we need eventual consistency, I would use asynchronous workers to aggregate these logs into a summary table for analytics dashboards, ensuring real-time feeds remain fast. Finally, to guarantee no data loss during region failures, I'd enable multi-region replication with active-active configuration, leveraging Microsoft's native geo-redundancy features. This architecture balances the need for high write throughput with the requirement for reliable, scalable data persistence.