How I Rebuilt a Three-Layer Cache System in Java — Redis, L1, and MongoDB Done Right

I've been working on Nexus , a backend infrastructure project, and recently hit a point where the data synchronization layer needed a serious rethink. What looked like a working cache system turned out to have a broken hierarchy, silent data loss paths, race conditions, and a latent deadlock waiting to happen. This post walks through every problem I found in the original code and exactly how I fixed each one. Code comparisons included throughout. The Architecture: What We're Building The system