i checked out your github project. It works correctly, the thing what happens is the following:
during the controller action you broadcast a message and then render the page, the page connects to your websocket and misses the broadcast because it is too ‘slow’ (better said the broadcast is too fast as the page renders in 250µs during development 
).
So it’s more the fact that you broadcast inside the controller action and expect to see it during it’s page render what’s now going wrong.
If you open another tab to the same page and you refresh the other one you’ll see that the broadcast fires fine because that other tab doesn’t have to do a full http page render (and let javascript connect to the websocket again.
If you want to actually see the broadcast that you’re sending rendered from the same controller action you have to introduce some delay in the broadcast itself (to make sure the page is already rendered and connected to your websocket before the broadcast is sent out) example code for this could be something like this:
defmodule RealTimeWeb.PageController do
use RealTimeWeb, :controller
def index(conn, _params) do
Task.start(fn ->
:timer.sleep(100)
RealTimeWeb.Endpoint.broadcast!("PAGE", "WELCOME", %{data: "Welcome to Real Time Page"})
end)
render(conn, "index.html")
end
end
(i wouldn’t do this in any production code, it’s just to show what is happening)
Hope this helps!
Redmar
