Most of the CDN's that I've seen require you to upload your content in advance. I'm looking for a CDN that, upon receiving a request for a resource it hasn't seen, will contact my application server. If the application server returns something, it should be sent to the user and then cached in the CDN. If not, it should just return a 404. If the user requests an unexpired item, the CDN should just serve it without bothering my app server.
Does anything like this exist? Is there a way to get Cloudfront to work like this?