Fragment cache invalidation by using a per model version token to prefix the cache keys. The version token can either be an internal memcached counter or a timestamped attribute from the model, such as updated_at.
Install with pip or with python setup.py install and add 'cachesweeper' to your settings.INSTALLED_APPS
An example setup; an article has many comments, each comment is cached, a single vote should invalidate the comment's specific cached fragment as well as the total article's page.
Template fragment caching
{% cachesweeper %} takes a Django ORM model as its first argument, the expiry time as its second and any following arguments are used to construct the rest of the cache key
{% load markup %}
{% load cachesweeper_tags %}
{% cachesweeper comment 500 "comment.xml" %}
<p>
<strong>{{comment.user}}</strong> said at {{comment.created_at}}:<br/>
{{comment.content|markdown}}
<br/>
<a href={% url like article_id=article.pk,comment_id=comment.pk %}>Like ({{comment.likes.count}})</a>
<a href={% url dislike article_id=article.pk,comment_id=comment.pk %}>Dislike ({{comment.dislikes.count}})</a>
</p>
{% endcachesweeper %}Invalidating the fragment when the model changes
On a post_save invalidate the cache for the given model. There are two options, either have Memcached keep an internal version counter for each model or using the keyword using as a means of versioning the cache.
from cachesweeper.utils import invalidate_cache_for
# using Memcached's internal counter
def invalidate_vote_cache(sender, **kwargs):
vote = kwargs.get('instance')
invalidate_cache_for(vote.comment)
post_save.connect(invalidate_vote_cache, sender=Vote)
# using a model's attribute
def invalidate_article_cache(sender, **kwargs):
article = kwargs.get('instance')
invalidate_cache_for(article, using='updated_at')
post_save.connect(invalidate_article_cache, sender=Article)