Flack is a online messaging service, similar in spirit to Slack. It is part of the problems sets that is tasked to learners of CS50's Web Programming with Python and JavaScript back in 2019. This was my attempt of using Socket.io, with Flask as the backend and Bulma as a Frontend framework to create a real-time chat app.
Given a choice now (updated in July 2021), I would probably have went for a slightly more "modern" approach of using a JAMStack such as Nextjs with Firebase/Ably to have a more responsive, and perhaps nicer looking UI, that achieves the same effect.