Caching isn't hard because it's difficult to cache something. Caching is indefinitely easy; the hard part is invalidating the cache when you want to make an update. There's a well-used quote from the late Phil Karlton of Netscape, which goes as follows:
"There are only two hard things in Computer Science: cache invalidation and naming things."
There are also many humorous variants on it, as used previously throughout this book. This sentiment may be a slight exaggeration, but it highlights how complex removing your done-computer-stuff ™ from your quick-things-box 2.0 ™ is perceived to be. Naming things is genuinely very hard though.
Caching is the process of storing a temporary snapshot of some data. This temporary cache can then be used instead of regenerating the original data (or retrieving it from the canonical source) every time it is required. Doing this has obvious performance benefits, but it makes your system more complicated and harder to conceptualize. When you...