Cache

You're writing a paper and need to keep referencing the same book. Option A: drive to the library every single time you need a quote. Option B: photocopy the relevant pages and keep them on your desk. Option B is caching.

Caching means storing a copy of frequently used data somewhere fast and close, so you don't have to fetch it from the original (slow, far away) source every time. Your browser caches website images so it doesn't re-download them every visit. Your phone caches app data so apps open faster. Servers cache database results so they don't re-run the same query thousands of times.

Caching is everywhere because it's one of the simplest ways to make things feel fast. The first time you visit a website, it might take 2 seconds to load. The second time? Half a second — because most of the assets were cached on your computer. The trade-off? Sometimes the cache is stale (outdated), which is why "have you tried clearing your cache?" is the first troubleshooting step for most web problems.

Because caching is why "clear your cache" fixes so many problems. When a website looks broken or shows old content, it's often serving you a stale cached version. Understanding caching also helps you understand why freshly published content sometimes takes a while to appear — the old version is cached and hasn't expired yet.

Caching stores frequently accessed data at faster-access layers in the memory hierarchy. Browser caches use HTTP headers (Cache-Control, ETag, Last-Modified) to manage freshness. Server-side caches include in-memory stores (Redis, Memcached), ORM query caches, and full-page caches. CDN edge caches distribute content globally. Cache invalidation ("one of the two hard problems in computer science") strategies include TTL-based expiration, event-driven purging, and stale-while-revalidate patterns.