Nolen Royalty made a website (now turned off) with a million checkboxes on it which was realtime for everyone viewing it. Users can check (or uncheck) a checkbox and that is persisted to a datastore and propagated to all visitors instantly. This, as it turns out, is a non-trivial task if you want it to run well, handle scale, and not cost a ton of money.
I’m posting this because if reading this kind of thing is interesting and kind of thrilling for you, a career in DevOps is probably not a bad idea for you.