This article was originally published on AI Study Room . For the full version with working code examples and related articles, visit the original post. Caching Strategies for Web Apps: CDN, Redis, Browser, and API Caching Caching is the difference between a 50ms response and a 5-second timeout. But cache invalidation is famously one of the hardest problems in computer science. Here's a practical guide to caching at every layer — and when NOT to cache. The Caching Layers Layer What to Cache TTL Invalidation Browser (HTTP Cache) Static assets (JS, CSS, images, fonts) 1 year (with hash in filename) Change filename → new URL → cache miss CDN HTML, API responses, images 1 min to 1 hour Purge by URL or tag. Stale-while-revalidate. Application (Redis/Memcached) DB query results, computed values, sessions 1 second to 1 hour Delete on write. TTL-based. Cache-aside pattern. Database query cache Query results (PostgreSQL/MySQL built-in) Automatic Invalidated on table writes.…