If I could teach a junior engineer exactly one system design skill, it would be caching. Nothing else gives you as much speed for as little effort — a well-placed cache can turn a database that's melting under load into one that's barely warm. It's also the place people make the most subtle mistakes, because a cache is a second copy of your data, and two copies of anything eventually disagree.
This is part four of my system design fundamentals series. Let's make things fast, then talk about the ways caching quietly bites back.
