caching.rst

Cache Machine

Settings

Before we start, you'll have to update your settings.py to use one of the caching backends provided by CacheMachine. Django's built-in caching backends don't allow infinite cache timeouts, which are critical for doing invalidation (see below). CacheMachine extends the locmem and memcached backends provided by Django to enable indefinite caching when a timeout of 0 is passed. If you were already using one of these backends, you can probably go on using them just as you were. If you were caching things with a timeout of 0, there will be problems with those entities now getting cached forever. You shouldn't have been doing that anyways.

For memcached:

CACHE_BACKEND = 'caching.backends.memcached://localhost:11211'

For locmem (only recommended for testing):

CACHE_BACKEND = 'caching.backends.locmem://'

CacheMachine will not work properly with the file or database cache backends.

Cache Manager

To enable caching for a model, add the :class:`~caching.base.CachingManager` to that class and inherit from the :class:`~caching.base.CachingMixin`. If you want related lookups (foreign keys) to hit the cache, CachingManager must be the default manager. If you have multiple managers that should be cached, return a :class:`~caching.base.CachingQuerySet` from the other manager's get_query_set method instead of subclassing CachingManager, since that would hook up the post_save and post_delete signals multiple times.

Here's what a minimal cached model looks like:

from django.db import models

import caching.base

class Zomg(caching.base.CachingMixin, models.Model):
    val = models.IntegerField()

    objects = caching.base.CachingManager()

Whenever you run a query, CachingQuerySet will try to find that query in the cache. Queries are keyed by {locale}:{sql}. If it's there, we return the cached result set and everyone is happy. If the query isn't in the cache, the normal codepath to run a database query is executed. As the objects in the result set are iterated over, they are added to a list that will get cached once iteration is done.

Note

Nothing will be cached if the QuerySet is not iterated through completely.

Caching is supported for normal :class:`QuerySets <django.db.models.QuerySet>` and for :meth:`django.db.models.Manager.raw`. At this time, caching has not been implemented for QuerySet.count, QuerySet.values, or QuerySet.values_list.

To support easy cache invalidation, we use "flush lists" to mark the cached queries an object belongs to. That way, all queries where an object was found will be invalidated when that object changes. Flush lists map an object key to a list of query keys.

When an object is saved or deleted, all query keys in its flush list will be deleted. In addition, the flush lists of its foreign key relations will be cleared. To avoid stale foreign key relations, any cached objects will be flushed when the object their foreign key points to is invalidated.

During cache invalidation, we explicitly set a None value instead of just deleting so we don't have any race condtions where:

• Thread 1 -> Cache miss, get object from DB
• Thread 2 -> Object saved, deleted from cache
• Thread 1 -> Store (stale) object fetched from DB in cache

The foundations of this module were derived from Mike Malone's django-caching.

Classes that May Interest You

.. autoclass:: caching.base.CacheMachine

.. autoclass:: caching.base.CachingManager
    :members:

    This :class:`manager <django.db.models.Manager>` always returns a
    :class:`~caching.CachingQuerySet`, and hooks up ``post_save`` and
    ``post_delete`` signals to invalidate caches.

.. autoclass:: caching.base.CachingMixin
    :members:

Overrides the default :class:`~django.db.models.QuerySet` to fetch objects from cache before hitting the database.