A junior monk said to master Kaimu: One of the Patriarchs claimed that there are “only two hard things” in our craft. What are these “two hard things”?

Kaimu answered: You only need to remember that the first hard thing is called “cache invalidation”.

The junior monk asked: Does that mean that a whole cache is made invalid, or just some of its elements? And why call it “invalidation” when the only error is that the data is stale? Wouldn’t “cache element expiration” be a better name?

Kaimu answered: Now you know the second hard thing.