Rule # 7: do all your image processing on upload.

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When to process uploaded images

Clients want to see their results returned to the page as visual confirmation of successful upload.

I concede that processing image files is CPU intensive - so front end processing has costs, for example it could open you up to DDOS attack. To avoid that...

That's not to rule out batch processing in off peak server time but really, you'd need to be a site the size of Facebook for that to make sense. Do your archiving later, do your processing on the front end.

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