SQLite backend Because comments are not Big Data. Yep, you got it. Also MIT licensed. Please go ahead and kill Facebook comments. I strongly dislike realname policies in this day and time where a single remark can cost you your job.
https://github.com/skx/e-comments/
Mine uses Redis or SQLite for storage, and uses markdown. The biggest difference is mine is flat. I keep thinking I should rework it to allow nested comments, and avatars, but at the same time it is nice and simple as-is..