RSS和ATOM如何通知客户更新？长期民意调查或民意调查还是其他什么？

I read that RSS and ATOM are HTTP-based using XML format. So how an RSS or Atom reader get informed about content updates if HTTP doesn't support connections? Do they need to poll the server periodically? I'm interested in using it in a real time web application but I heard that "polling doesn't scale". I read that RSS and ATOM are HTTP-based using X