The magic in this case being "for the categories he defines." I do so much enjoy articles that re-define the general consensus on terms so that the author can try to shift entries among categories in a way that makes their product look better. It feels like watching a biologist try to create a taxonomy based on "greenness" and "whether or not this creature tastes good when served with hollandaise sauce."

I'd say Mongo fits better than Redis. Redis doesn't give you documents, just simple data structures.