Does lichess use pgn or fen internally to track board state during the game?

[theAnuragMishra]

Hi all!
I m making my own chess website (for the purposes of learning and having fun). I'm confused whether to save fen or pgn for games in the database. I m currently using fen because that's simpler and faster. But for stuff like fifty move rule and threefold repitition, i think ill need to track the pgn or manually have to check for all of them each move.
On the other hand, using pgn will make it easier to check for three fold repetition and 50 move rule, but then ill have to udpate and read the pgn every move which looks like will be heavy on the server and the browser.