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django-fileprocessor: efficient, scalable processing of files (ie. scaling images)

This is a prototype project for something we have been wanting to make: efficient file processing - from within templates!

Right now, if you are like me, you probably use something like easy_thumbnail for scaling of images for your website. You might even have written some code to extract thumbnails from PDF files or from video files. All really great work!

BUT. As our applications relying on this stuff might actually scale up a bit, the amount of work our application servers are doing WHILE RENDERING THE TEMPLATE can quickly become excessive. At the same time, if these servers were just doing what they should be, namely data <-> view interactions, this would not be happening.

Moreover: if your server is, in principle, able to handle a mentioning on some lame but immensely popular blog - why should it not be in practice? Because, right now - if you use easy_thumbnails - your server will block while rendering templates until your thumbnails are gone.

So here is my concept solution: When we need to do something with a file, while rendering a template, we first put out a request. This request can be put out to some other server, but for small instances it can easily be the same server - just some other CPU doing the work. This 'remote tag' basically passes any contents between the {% fileprocessor %} and {% endfileprocessor %} along to this other server as a parameter (so far called instructions), telling it what to do and where to get stuff from.

The remote server writes this to the database, generates a checksum thereof and parses the instructions just well enough to generate some kind of representation (ie. and image tag) of the stuff that will be rendered later.

After the HTML has been sent out to the client, the client will start making individual requests for the URL's referred to in the representation code generate while rendering the template. Calling these URL's directly triggers processing the file in any possible way, according to the instructions in the database. When this is done, the file is written to some kind of storage, somewhere and a redirect to this file is generated.

On subsequent requests, all the 'file processing server' does is blurt out redirects to that some old file. But as these are permanent redirects, they should also be cached by the client which ought to be quite efficient. Furthermore, the template rendering results can and should also be cached: we do not have to reach out to the server all of the time to retreive the same kind of data.

So that's it. A work in progress. Feel free to join in. :)

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