> It returns no owner if the document is deleted or just does not exist, which is accurate.

"I don't know" is not the same as "definitely no owner". You may handle the two answers the same for a particular scenario, but then again you may not.

Again, consider the case where get_owner is working on some kind of cache (in which case sure, the function name could stand to be improved). If the document is no longer in the cache it could be because it was actually deleted or because it was simply evicted to make room for other entries. In general you cannot assume that a missing cache entry is equivalent to the document existing and having no owner.

Or similarly (and also yet again), consider the case where get_owner is working on a versioned filesystem, in which case the outer optional could indicate that the (generational) document ID is out of date. In such a case treating "I don't know" and "no owner" the same is incorrect because the owner could have been updated from "no owner" to something else (such a case would likely be more realistically handled by returning a Result<Option<UserId>, SomeErrorKind>, though, given the myriad other ways a file system handle can go stale)

> Why do you need get_owner to tell you if the document exists or not?

Because sometimes there is no good alternative. Asking a cache whether it has an entry and then trying to get the entry value in a separate operation (particularly in a concurrent environment) is a classic setup for TOCTOU errors. You can say something similar for the versioned file system example.

> If anything it should be an error if the document doesn’t exist because you’ve violated a precondition.

I mean, enabling that kind of handling is precisely the point of the outer optional?