Stream a downloaded file with Phoenix and Req

I had to proxy files from an S3 bucket hosted on Tigris because some ISPs are blocking the domain fly.storage.tigris.dev, as it was previously used to distribute malware.

The code

def show(conn, %{
      "id" => document_id
    }) do
  case Documents.get_document_by(id: document_id) do
    document when is_map(document) ->
      s3_url = Documents.get_s3_url(document)

      conn =
        conn
        # I'm serving pdf only but adapt for your use case
        |> put_resp_content_type("application/pdf")
        # Send the response in chunks
        |> send_chunked(200)

      # Perform the request to the S3 url
      # We use `into: fn ... end` to capture the streamed data
      # immediately pass it to the client with `chunk/2`
      Req.get!(s3_url,
        into: fn {:data, data}, {req, resp} ->
          chunk(conn, data)
          {:cont, {req, resp}}
        end
      )

      conn

    nil ->
      conn
      |> put_status(404)
      |> render("404.html")
  end
end

The code

Stay up to date