I don't know about libraries, but you'll basically need to create the
concept of a job. As soon as the user finishes uploading the file, save what
tasks need to be performed on that file, and immediately return to the an
"OK, we'll get to it as soon as possible" message to the user.

   Potentially, thousands or millions of users might be uploading files at
the same time, but maybe only 1 or 2 thread is actually going around doing
whatever processing needs to be done.

   Users can log in and query the status of the job. Depending on how much
detail you want, you might simply say something like "Job is done", "Job is
running" or "There are 248 jobs in queue before yours", for example.

   You can optionally have a notification system (perhaps via e-mail) to
let a user know when their job is done.

   - Oliver