Ask HN: How to implement realtime tracking in a webapp

I recently started to help out a startup which already had some software in place.

The front-end is written in Angular.

We wish to track activity of the users, especially how much people are currently active on a page(actually thats a route).

Currently we have a polling mechanism in place to update the data on the front-end. So I attached additional logic which creates a REDIS key with with the IP and a cookie, to track each client. Keys are structured in a similar manner:

viewing:resource:ip:cookie

And there is a TTL on each key. Consecutive call refresh that key. Now I only count the viewing:resource:* keys and I have a number of people on this resource.

But the solution is done in an urge of drunken 'brilliance'. How do the big players track concurrent online visitors on webaps?