Caching is the most universally deployed performance optimization in software engineering, yet it remains one of the most notoriously fragile. As the famous adage goes: “There are only two hard things in Computer Science: cache invalidation and naming things.”
At low scale, placing an in-memory key-value store in front of a relational database appears trivial. At high concurrency, however, distributed caches encounter subtle, catastrophic failure modes: dual-write race conditions that cause permanent cache-database data divergence, cache stampedes (thundering herds) that collapse primary database connection pools when hot keys expire, and cache avalanches that cause cascading cluster outages.
This module deconstructs the architecture of distributed caching systems: pattern topologies (Cache-Aside, Write-Through, Write-Behind), mathematical eviction algorithms (W-TinyLFU, 2Q), probabilistic early expiration (XFetch), singleflight query coalescing, and cache-consistency guarantees.