CloudFront A/B Testing Without Cache Fragmentation: The Shadow Origin Pattern

In AWS architectures, there is a silent trade-off that often goes unnoticed—until your AWS bill spikes or your latency graphs degrade: Personalization vs. Cache Efficiency The common approach to A/B testing—executing logic in a viewer-request hook and varying the cache key using cookies—seems convenient, but it is an architectural trap. By introducing variants into the cache key, you: Fragment your global edge cache Destroy your Cache Hit Ratio (CHR) Increase S3 origin load and egress costs Reintroduce cold-start latency for users There is a better way. 🧠 The Solution: The Shadow Origin Pattern The Shadow Origin Pattern is a technique where the public request URI remains unchanged, while the origin fetch path is internally rewritten after a cache miss . Instead of exposing variants to the CDN cache key, we move the decision behind the cache layer . 1. The Architecture: Performance-First Routing The goal is simple: Do not fragment the cache key at the edge. How it works Public Request (