Since you’re not getting any errors it’s hard to say what’s wrong. I’ve never tried to do it that way, but I can suggest an alternative solution. You can cache rendered content by using Plug.Conn.register_before_send/2. Here’s some untested code

def post(conn, _param) do
  conn
  |> Plug.Conn.register_before_send(cache/1)
  |> render("post.html")
end

def cache(conn) do
  content = :erlang.iolist_to_binary(conn.resp_body)
  :ets.insert(:table, {you_pick_your_key, content})
  conn
end

The reason you want to coerce the iodata to binary is that based on experience the ETS memory usage will balloon if you don’t.

This solution also means you can create a Plug responsible for checking the cache and caching responses, and add it to a pipeline. I have an example of that here https://github.com/joladev/cache_example/blob/master/lib/cache_example_web/plugs/item_cache2.ex

Note that the example is specific to my use case, you’d want to adjust it to yours