Kozmos saves your bookmarks into your browser first, then syncs with its server. And the database it creates in your browser is open source; https://github.com/kozmos/likedb
You can hack it the way you'd like.
You can hack it the way you'd like.
I'd be much more interested in running my own backend, than frontend db. Yeah, I'm a control freak, I guess. But at least my data stays mine.