How Multi-Layer Caching Complicates Data Consistency in Production
For a long time, I thought of caching as a performance feature. You put a cache in front of something slow, and the thing gets faster. That mental model holds until the system grows enough independent caching layers that “how fast is it?” stops being the most interesting question. A harder question appears instead: when the underlying data changes, which layers may still contain an older version of it? I work on KOBOR, a production e-commerce platform built around Nginx, Next.js, a Symfony API, and MySQL. Over time, caching appeared at several layers, each for a legitimate reason. Static assets got long-lived immutable caching. Images got their own policy. Public HTML got a short shared cache. The Next.js application got an in-memory cache for frequently reused data. The PHP backend has its own FastCGI caching layer. None of these mechanisms is unusual on its own. Together, however, they changed the...
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