Can I find out which subscribers exist for a given topic?

Registry.lookup(Jocasta.PubSub, topic) the result can contain metadata passed to when calling subscribe (e.g. Phoenix.PubSub.subscribe(Jocasta.PubSub, topic, metadata: %{ip: ip}))

Is the assumption correct that subscribe and broadcast belong to the PubSub system?

It belongs to pubsub_server configured here

config :fset, JocastaWeb.Endpoint,
  ...
  pubsub_server: Jocasta.PubSub,
  live_view: [signing_salt: "abc"]

And that PubSub is usually (or by default, not sure since which phoenix version) started in root supervision tree.

Do I totally misunderstand here something?

Not sure! If that broadcast_from works, you would want to test it in a separate node to see if it actually works. Or use local_broadcast_from.

I have been following this same article and had some concerns about the proposed use of PubSub. The link posted above by @LostKobrakai is a much better approach.

I want to mention this:
I used phx_gen_auth in my project and that generated this line in the log_in_user function:

conn
# ...
|> put_session(:live_socket_id, "users_sessions:#{Base.url_encode64(token)}")

So for anyone else like me, you can keep this line and log users out like this:

  @doc """
  Logs a user out of any and all sessions
  """
  def force_logout_user(%User{id: user_id} = user) do
    user_tokens = UserToken
    |> where(user_id: ^user_id)
    |> Repo.all()

    Repo.delete_all(UserToken.user_and_contexts_query(user, :all))

    user_tokens
    |> Enum.each(fn %UserToken{token: token} ->
      MyAppWeb.Endpoint.broadcast(
        "users_sessions:#{Base.url_encode64(token)}",
        "disconnect",
        %{}
      )
    end)
  end

Just make sure you change mount functions in any live views you have that might assume a current_user on the assigns.

Because I didn’t knew about this. This is really helpful. Thanks for pointing this out to me.