All the stores have different requirements. One store requires 2500x2500 artwork, another requires FLAC audio, etc. So when users upload audio or video to DistroKid, everything gets converted to appease all the different stores.

Right now there's a cron that runs every few seconds, finds the next unprocessed file, and processes it via command line. If/when volume gets really high, I'll probably have to do this somewhat differently to make sure it scales.

DistroKid uses MediaInfo to figure out what the user uploaded:

http://mediaarea.net/en/MediaInfo

Then uses SOX for audio conversions:

http://sox.sourceforge.net/

And native Railo (the backend programming language I use) functions for image processing/resizing.

http://getrailo.com