Building a BitTorrent tracker out of GenServers makes zero sense. ETS is the correct choice to make here 100% (for a single server configuration). Obviously it doesn't scale horizontally but it will scale vertically to the moon and chances are, yagni anyway
This is really good, my dude. I took a peruse through the code and I'm definitely impressed, this is like, code I would expect my senior engineers with elixir experience to turn in. Great job!