My last article about caching RSS feeds in a Django project generated a lot of interest. My original goal was to help other people who have tried to cache QuerySet objects and received a funky error message. Many of my visitors offered helpful advice in the comments, making it clear that I was going about caching my feeds the wrong way.

I knew my solution was wrong before I even produced it, but I couldn't get Django's site-wide caching middleware to work in my production environment. Site-wide caching worked wonderfully in my development environment, and I tried all sorts of things to make it work in my production setup. It wasn't until one "Jacob" offered a beautiful pearl of wisdom that things started to make more sense:

This doesn't pertain to feeds, but one rather large gotcha with the cache middleware is that any javascript you are running that plants a cookie will affect the cache key. Google analytics, for instance, has that effect. A workaround is to use a middleware to strip out the offending cookies from the request object before the cache middleware looks at it.

The minute I read that comment, I realized just how logical it was! If Google Analytics, or any other JavaScript used on my site, was setting a cookie, and it changed that cookie on each request, then the caching engine would effectively have a different page to cache for each request! Thank you so much, Jacob, for helping me get past the frustration of not having site-wide caching in my production environment.

import re

class StripCookieMiddleware(object):
    """Ganked from http://2ze.us/Io"""

    STRIP_RE = re.compile(r'\b(_[^=]+=.+?(?:; |$))')

    def process_request(self, request):
        cookie = self.STRIP_RE.sub('', request.META.get('HTTP_COOKIE', ''))
        request.META['HTTP_COOKIE'] = cookie

I've recently been working on some performance enhancements on my site. Apparently some of my latest articles are a little too popular for my shared hosting plan. The surge of traffic to my site took down several sites on the same server as my own.

My response to the fiasco was to, among other things, implement caching on my site. It seems like the caching has helped a lot. I've noticed that my RSS feeds are hit almost as hard as real articles on my site, and I noticed that they weren't being cached the way I had expected. I tried a couple of things that I thought would work, but nothing seemed to do the trick.

After doing some brief research into the idea of caching my RSS feeds using Django's built-in caching mechanisms, I came up empty. It occurred to me to implement caching in the feed classes themselves. I tried something like this:

from django.contrib.syndication.feeds import Feed
from django.core.cache import cache
from articles.models import Article

class LatestEntries(Feed):
    ...

    def items(self):
        articles = cache.get('latest_articles')

        if articles is None:
            articles = Article.objects.active().order_by('-publish_date')[:10]
            cache.set('latest_articles', articles)

        return articles

    ...

This code doesn't work! When I would try to retrieve one of my RSS feeds with such "caching" in place, I got the following traceback:

Traceback (most recent call last):

  File "/home/wheaties/dev/django/core/servers/basehttp.py", line 280, in run
    self.result = application(self.environ, self.start_response)

  File "/home/wheaties/dev/django/core/servers/basehttp.py", line 674, in __call__
    return self.application(environ, start_response)

  File "/home/wheaties/dev/django/core/handlers/wsgi.py", line 241, in __call__
    response = self.get_response(request)

  File "/home/wheaties/dev/django/core/handlers/base.py", line 143, in get_response
    return self.handle_uncaught_exception(request, resolver, exc_info)

  File "/home/wheaties/dev/django/core/handlers/base.py", line 101, in get_response
    response = callback(request, *callback_args, **callback_kwargs)

  File "/home/wheaties/dev/django/utils/decorators.py", line 36, in __call__
    return self.decorator(self.func)(*args, **kwargs)

  File "/home/wheaties/dev/django/utils/decorators.py", line 86, in _wrapped_view
    response = view_func(request, *args, **kwargs)

  File "/home/wheaties/dev/django/contrib/syndication/views.py", line 215, in feed
    feedgen = f(slug, request).get_feed(param)

  File "/home/wheaties/dev/django/contrib/syndication/feeds.py", line 37, in get_feed
    return super(Feed, self).get_feed(obj, self.request)

  File "/home/wheaties/dev/django/contrib/syndication/views.py", line 134, in get_feed
    for item in self.__get_dynamic_attr('items', obj):

  File "/home/wheaties/dev/django/contrib/syndication/views.py", line 69, in __get_dynamic_attr
    return attr()

  File "/home/wheaties/dev/articles/feeds.py", line 22, in items
    cache.set(key, articles)

  File "/home/wheaties/dev/django/core/cache/backends/filebased.py", line 72, in set
    pickle.dump(value, f, pickle.HIGHEST_PROTOCOL)

PicklingError: Can't pickle <class 'django.utils.functional.__proxy__'>: attribute lookup django.utils.functional.__proxy__ failed

This error took me by surprise. I didn't expect anything like this. I tried a few things to get around it, but then I actually stopped to consider what was happening to cause such an error. My Article objects are definitely serializable, which is why the error didn't make sense.

Then it hit me: the object I was actually attempting to cache was a QuerySet, not a list or tuple of Article objects. Changing the code to wrap the Article.objects.active() call with list().

from django.contrib.syndication.feeds import Feed
from django.core.cache import cache
from articles.models import Article

class LatestEntries(Feed):
    ...

    def items(self):
        articles = cache.get('latest_articles')

        if articles is None:
            articles = list(Article.objects.active().order_by('-publish_date')[:10])
            cache.set('latest_articles', articles)

        return articles

    ...

And that one worked. I would prefer to cache the actual XML version of the RSS feed, but I will settle with a few hundred fewer hits to my database each day by caching the list of articles. If anyone has better suggestions, I'd love to hear about them. Until then, I hope my experience will help others out there who are in danger of taking down other sites on their shared hosting service!