Design a Feature to Support A/B Testing Infrastructure

To design a safe A/B testing infrastructure, I would propose a decoupled architecture centered around a centralized Feature Flag Service. First, we define the requirement: engineers need to toggle experiments in seconds, not hours. The solution involves a lightweight SDK embedded in our applications that caches user assignments locally. When a test is launched, the configuration—defining which user segments see which variant—is pushed to a high-availability store like Azure Cosmos DB. Crucially, the application polls this store or receives a WebSocket push for updates, allowing us to change the traffic split from 50/50 to 0% immediately upon detecting anomalies. For the rollback mechanism, I would implement a 'Kill Switch' that overrides all other logic instantly. If the monitoring system detects a spike in error rates exceeding a threshold, it automatically triggers this switch, reverting all users to the baseline version without code changes. This ensures zero-downtime recovery. Additionally, we must handle edge cases like stale cache invalidation and ensure audit logs track who changed what and when. By separating the decision logic from the codebase, we align with Microsoft's focus on reliability and speed, enabling teams to iterate rapidly while maintaining strict production stability.