Replacing a Critical Data Path Without a Flag Day

This article discusses the challenges of replacing a critical data source in a workflow, emphasizing that it's not just an API migration but a transfer of authority between different data models. The author shares a real-world example of migrating from a legacy event feed to a canonical state API, highlighting the risks of false positives/negatives and the importance of preserving business invariants. The approach includes query-only mode, shadow comparisons, discrepancy alerts, and a deliberate cutover process. The article provides practical insights for engineers facing similar data path replacements, focusing on careful validation and controlled rollout.