Introduction: download function

By default, download() function does some permission checks (if they are set) and sends the uploaded files to the client.

The problem: download prevents client side caching

The problem is that download() function sends the following http headers, that prevents client side caching:

• Expires: Thu, 27 May 2010 05:06:44 GMT
• Pragma: no-cache
• Cache-Control: no-store, no-cache, must-revalidate, post-check=0, pre-check=0

This may be good for some dynamic content, but, for example, a client browsing a site with several non static images, will see how each image loads every time the page is shown, slowing down navigation.

@Caching download

Caching download() with @cache will not help, it prevents permission checks to be made, and cache is done at server side, we need some at client side.

One Solution: fast_download

A best approach will be using a custom download function, allowing client side caching, so once the browser downloaded a file, that file doesn't need to be downloaded again soon.

This can be done with a custom download function (ie. fast_download), doing simple security checks and modifying http headers to favor client side caching:

• Last-Modified: Tue, 04 May 2010 19:41:16 GMT
• Expires removed
• Pragma removed
• Cache-control removed

Using Last-Modified allows If-Modified-Since http requests, whose speed up downloads preventing sending again unmodified contents. response.stream handles if-modified-since and range requests automatically.

Other Solution: let webserver handle downloads

This aproach need some webserver (apache) config files tweaking, so is not portable nor easily configurable. Using fast_download should be a better alternative, that requires no custom configuration and works quickly with almost all webservers, and the user would not notice any performance difference.

fast_download code

So, in controller, default.py add:

def fast_download():
   # very basic security (only allow fast_download on your_table.upload_field):
   if not request.args(0).startswith("yourtable.upload_field"):
       return download()
   # remove/add headers that prevent/favors client-side caching
   del response.headers['Cache-Control']
   del response.headers['Pragma']
   del response.headers['Expires']
   filename = os.path.join(request.folder,'uploads',request.args(0))
   # send last modified date/time so client browser can enable client-side caching
   response.headers['Last-Modified'] = time.strftime("%a, %d %b %Y %H:%M:%S +0000", time.localtime(os.path.getmtime(filename)))
   return response.stream(open(filename,'rb'))

In your view, remember to make URLs using fast_download instead of download function:

URL(r=request, c='default', f='fast_download', args=your_table.upload_field)

Thanks massimo for the advice and comments, for the full thread see: http://groups.google.com/group/web2py/browse_thread/thread/0c10235cb16c476f/0f2dd71668626b6d?show_docid=0f2dd71668626b6d&fwc=1