Nice post, thanks for sharing.

I recently had a similar problem - I needed to serve dynamically resized images via CloudFront (with the images being stored in S3). Images are resized on-demand - if they exist they are served directly from S3. If not, they are resized and stored in S3, then the next request will serve them directly from S3.

It works rather well, although it is quite susceptible to the thundering herd problem - putting it behind a reverse proxy that supports collapsed forwards would probably help here.

I've put the WSGI app I used for this on github - hopefully someone finds it useful! https://github.com/mikery/s3cacher