viciu / django-cachebot

Automated caching and invalidation for the Django ORM

Django-cachebot

Django-cachebot provides automated caching and invalidation for the Django ORM.

Installation

1. easy_install django-cachebot or pip install django-cachebot

2. Add cachebot to your INSTALLED_APPS

3. Set a cache backend to one of the backends in cachebots.backends, for instance:

   CACHES = {
       'default': {
           'BACKEND': 'cachebot.backends.memcached.MemcachedCache',
           'LOCATION': '127.0.0.1:11211',
       }
   }
   

Current supported backends are:

cachebot.backends.dummy.DummyCache
cachebot.backends.memcached.MemcachedCache
cachebot.backends.memcached.PyLibMCCache

4. If you want to add caching to a model, the model's manager needs to be CacheBotManager or a subclass of it, e.g:

   from django.db import models
   from cachebot.managers import CacheBotManager
   
   class Author(models.Model):
       name = models.CharField(max_length=50)
       objects = CacheBotManager()
   
   class BookManager(CacheBotManager):
   
       def for_author(self, name):
           return self.filter(author__name=name)
   
   class Book(models.Model):
       title = models.CharField(max_length=50)
       author = models.ForeignKey(Author)
       objects = BookManager()
   

Usage

By default, all get queries for CacheBotManager will be cached:

photo = Photo.objects.get(user=user)

If you don't want this behavior, call CacheBotManager(cache_get=False) when defining the manager, or to change this globally set CACHEBOT_CACHE_GET=False in settings.

---

For more complex queries, suppose you had a query that looked like this and you wanted to cache it:

Photo.objects.filter(user=user, status=2)

Just add .cache() to the queryset chain like so:

Photo.objects.cache().filter(user=user, status=2)

This query will get invalidated if any of the following conditions are met:

1. One of the objects returned by the query is altered.
2. The user is altered.
3. A Photo is modified and has status = 2.
4. A Photo is modified and has user = user.

This invalidation criteria is probably too cautious, because we don't want to invalidate this cache every time a Photo with status = 2 is saved. To fine tune the invalidation criteria, we can specify to only invalidate on certain fields. For example:

Photo.objects.cache('user').filter(user=user, status=2)

This query will get invalidated if any of the following conditions are met:

1. One of the objects returned by the query is altered.
2. The user is altered.
3. A Photo is modified and has user = user.

django-cachebot can also handle select_related, forward relations, and reverse relations, ie:

Photo.objects.select_related().cache('user').filter(user__username="david", status=2)

Photo.objects.cache('user').filter(user__username="david", status=2)

Photo.objects.cache('message__sender').filter(message__sender=user, status=2)

Settings

• CACHEBOT_CACHE_GET
  • default: True
  • If set to True, CacheBotManager will be called with cache_get=True by default.
• CACHEBOT_TABLE_BLACKLIST
  • default: ('django_session', 'django_content_type', 'south_migrationhistory')
  • A list of tables that cachebot should ignore.

Caveats (Important!)

1. Adding/Removing objects with a ManyRelatedManager will not automatically invalidate. You'll need to manually invalidate these queries like so:

   from cachebot.signals import invalidate_object
   
   user.friends.add(friend)
   invalidate_object(user)
   invalidate_object(friend)
   

2. count() queries will not get cached.

3. If you're invalidating on a field that is in a range or exclude query, these queries will get invalidated when anything in the table changes. For example the following would get invalidated when anything on the User table changed:

   Photo.objects.cache('user').filter(user__in=users, status=2)
   
   Photo.objects.cache('user').exclude(user=user, status=2)
   

4. You should probably use a tool like django-memcache-status to check on the status of your cache. If memcache overfills and starts dropping keys, it's possible that your queries might not get invalidated.

5. .values_list() doesn't cache yet. You should do something like this instead:

   [photo['id'] for photo in Photo.objects.cache('user').filter(user=user).values('id')]
   

Dependencies

• Django 1.3

If you use Django 1.2, you can use django-cachebot version 0.3.1