> And there is no possible solution without cassandra with the last architecture update ?
It's not really the "last" architecture update, but no, it's definitely required.
> So Unscale to a tiny use case is really not possible ?
Probably not, no.
> Do you have an idea of another software like reddit, open source, but with less consumption of memory ?
pligg? It was also fashionable to "write reddit 200 lines of basic!" and "write reddit in 150 lines of C++!" a few years ago on /r/programming, which I think shows that if you don't have to scale it, reddit's fundamental operations (post link, vote on link, post comment, get comments for link #X, etc) are pretty simple. You could track down any number of those types of posts, or write your own over a weekend.