Every request your app serves is a chance to do less work. Caching is just the art of not redoing work you already did. Sounds simple. It isn’t — there’s a whole stack of places you can stash data, each with its own rules, and picking the wrong layer (or the wrong expiry) costs you either stale bugs or a slow site.
I’ve spent enough hours staring at a page that “should have updated” but didn’t, only to realise the answer was cached three layers deep. So let me walk through every place you can cache — from the user’s browser all the way down to compiled bytecode on your server — and where each one bites you.
“Just cache it in Redis” — you’ve probably heard that in a code review, a system design interview, or a Stack Overflow comment. It’s almost a reflex at this point. But why Redis specifically? Why not a regular database with a good index, or any other in-memory store? I dug into the history, the architecture, and the current landscape of alternatives, and the story is genuinely more interesting than the meme lets on.