I can see useful feedback during the photo-upload process

Summary: Not trivial, since we don't have all the needed infrastructure, but it would be hugely helpful to show a true progress bar, showing all the steps we are going through.

Ideally, we would upload the photo in discrete chunks of, say, 10k, so that we can measure the streaming progress as it loads.

Then, we should show updates as the photo is post-processed, and uploaded to Amazon.

To make this work, we probably need the photo-upload API to be providing back a stream of events, instead of a single shot. How do we do this from the Javascript side?

Probably implies that the actual photo processing and upload needs to be encapsulated in a worker Actor. The ID of this Actor gets passed to my UserSession, which can coordinate. (The worker should not be owned by the UserSession: we want to avoid having to wrap the huge photo in a message if we can avoid it, so the worker should live on the node where the request was originally received.) We can then have Comet-style requests to the UserSession, which passes them on to the worker, to get the current status of the process.