Cacheops

A slick app that supports automatic or manual queryset caching and automatic granular event-driven invalidation.

It uses redis as backend for ORM cache and redis or filesystem for simple time-invalidated one.

And there is more to it:

• decorator to cache any user function as queryset
• extension for jinja2 to cache template fragments as querysets
• a couple of hacks to make django faster

Requirements

Python 2.6, Django 1.2 and Redis 2.2.7.

Installation

Using pip:

$ pip install django-cacheops

Or you can get latest one from github:

$ git clone git://github.com/Suor/django-cacheops.git
$ ln -s `pwd`/django-cacheops/cacheops/ /somewhere/on/python/path/

Setup

Add cacheops to your INSTALLED_APPS before any apps that use it.

Setup redis connection and enable caching for desired models:

CACHEOPS_REDIS = {
    'host': 'localhost', # redis-server is on same machine
    'port': 6379,        # default redis port
    'db': 1,             # SELECT non-default redis database
                         # using separate redis db or redis instance
                         # is highly recommended
    'socket_timeout': 3,
}

CACHEOPS = {
    # Automatically cache any User.objects.get() calls for 15 minutes
    # This includes request.user or post.author access,
    # where Post.author is a foreign key to auth.User
    'auth.user': ('get', 60*15),

    # Automatically cache all gets, queryset fetches and counts
    # to other django.contrib.auth models for an hour
    'auth.*': ('all', 60*60),

    # Enable manual caching on all news models with default timeout of an hour
    # Use News.objects.cache().get(...)
    #  or Tags.objects.filter(...).order_by(...).cache()
    # to cache particular ORM request.
    # Invalidation is still automatic
    'news.*': ('just_enable', 60*60),

    # Automatically cache count requests for all other models for 15 min
    '*.*': ('count', 60*15),
}

Additionally, you can tell cacheops to degrade gracefully on redis fail with:

CACHEOPS_DEGRADE_ON_FAILURE=True

Usage

Automatic caching.

It's automatic you just need to set it up.

Manual caching.

You can force any queryset to use cache by calling it's .cache() method:

Article.objects.filter(tag=2).cache()

Here you can specify which ops should be cached for queryset, for example, this code:

qs = Article.objects.filter(tag=2).cache(ops=['count'])
paginator = Paginator(objects, ipp)
articles = list(pager.page(page_num)) # hits database

will cache .count() call in Paginator but not later in articles fetch. There are three possible actions - get, fetch and count. You can pass any subset of this ops to .cache() method even empty to turn off caching. There are, however, a shortcut for it:

qs = Article.objects.filter(visible=True).nocache()
qs1 = qs.filter(tag=2)       # hits database
qs2 = qs.filter(category=3)  # hits it once more

It is usefull when you want to disable automatic caching on particular queryset.

Function caching.

You can cache and invalidate result of a function the same way as a queryset. Cache of next function will be invalidated on any Article change, addition or deletetion:

from cacheops import cached_as

@cached_as(Article)
def article_stats():
    return {
        'tags': list( Article.objects.values('tag').annotate(count=Count('id')) )
        'categories': list( Article.objects.values('category').annotate(count=Count('id')) )
    }

Note that we are using list on both querysets here, it's because we don't want to cache queryset objects but their results.

Also note that cache key does not depend on arguments of a function, so it's result should not, either. This is done to enable caching of view functions. Instead you should use a local function:

def articles_block(category, count=5):

    @cached_as(Article.objects.filter(category=category), extra=count)
    def _articles_block():
        qs = Article.objects.filter(category=category)
        articles = list(qs.filter(photo=True)[:count])

        if len(articles) < count:
            articles += list(qs[:count-len(articles)])

        return articles

    return _articles_block()

Using local function gives additional advantage: we can filter queryset used in @cached_as() to make invalidation more granular. We also add an extra to make diffrent keys for calls with same category but diffrent count.

Invalidation

Cacheops uses both time and event-driven invalidation. The event-driven one listens on model signals and invalidates appropriate caches on Model.save() and .delete().

Invalidation tries to be granular which means it won't invalidate a queryset that cannot be influenced by added/updated/deleted object judjing by query conditions. Most time this will do what you want, if it's not you can use one of the following:

from cacheops import invalidate_obj, invalidate_model

invalidate_obj(some_article)  # invalidates queries affected by some_article
invalidate_model(Article)     # invalidates all queries for model

And last there is invalidate command:

./manage.py invalidate articles.Article.34  # same as invalidate_obj
./manage.py invalidate articles.Article     # same as invalidate_model
./manage.py invalidate articles   # invalidate all models in articles

And the one that FLUSHES cacheops redis database:

./manage.py invalidate all

Don't use that if you share redis database for both cache and something else.

Multiple database support

By default cacheops considers query result is same for same query, not depending on database queried. That could be changed with db_agnostic cache profile option:

CACHEOPS = {
    'some.model': ('get', TIMEOUT, {'db_agnostic': False}),
    # ...
}

Simple time-invalidated cache

To cache result of a function call for some time use:

from cacheops import cached

@cached(timeout=number_of_seconds)
def top_articles(category):
    return ... # Some costly queries

@cached() will generate separate entry for each combination of decorated function and its arguments. Also you can use extra same way as in @cached_as(), most useful for nested functions:

@property
def articles_json(self):
    @cached(timeout=10*60, extra=self.category)
    def _articles_json():
        ...
        return json.dumps(...)

    return _articles_json()

You can manually invalidate cached function result this way:

top_articles.invalidate(some_category)

Cacheops also provides get/set primitives for simple cache:

from cacheops import cache

cache.set(cache_key, data, timeout=None)
cache.get(cache_key)
cache.delete(cache_key)

cache.get will raise CacheMiss if nothing is stored for given key:

from cacheops import cache, CacheMiss

try:
    result = cache.get(key)
except CacheMiss:
    ... # deal with it

File Cache

File based cache can be used the same way as simple time-invalidated one:

from cacheops import file_cache

@file_cache.cached(timeout=number_of_seconds)
def top_articles(category):
    return ... # Some costly queries

# later, on appropriate event
top_articles.invalidate(some_category)

# primitives
file_cache.set(cache_key, data, timeout=None)
file_cache.get(cache_key)
file_cache.delete(cache_key)

It have several improvements upon django built-in file cache, both about highload. First, it is safe against concurrent writes. Second, it's invalidation is done as separate task, you'll need to call this from crontab for that to work:

/path/manage.py cleanfilecache

Jinja2 extension

Add cacheops.jinja2.cache to your extensions and use:

{% cached_as queryset [, timeout=<timeout>] [, extra=<key addition>] %}
    ... some template code ...
{% endcached_as %}

or

{% cached [timeout=<timeout>] [, extra=<key addition>] %}
    ...
{% endcached %}

Tags work the same way as corresponding decorators.

CAVEATS

1. Conditions other than __exact or __in don't provide more granularity for invalidation.
2. Conditions on related models don't provide it either.
3. Update of "selected_related" object does not invalidate cache for queryset.
4. Mass updates don't trigger invalidation.
5. ORDER BY and LIMIT/OFFSET don't affect invalidation.
6. Doesn't work with RawQuerySet.
7. Conditions on subqueries don't affect invalidation.
8. Doesn't work right with multi-table inheritance.
9. Aggregates is not implemented yet.
10. Timeout in queryset and @cached_as() cannot be larger than default.

Here 1, 3, 5, 10 are part of design compromise, trying to solve them will make things complicated and slow. 2 and 7 can be implemented if needed, but it's probably counter-productive since one can just break queries into simple ones, which cache better. 4 is a deliberate choice, making it "right" will flush cache too much when update conditions are orthogonal to most queries conditions. 6 can be cached as SomeModel.objects.all() but @cached_as() someway covers that and is more flexible. 8 is postponed until it will gain more interest or a champion willing to implement it emerge.

Performance tips

Here come some performance tips to make cacheops and Django ORM faster.

1. When you use cache you pickle and unpickle lots of django model instances, which could be slow. You can optimize django models serialization with django-pickling.

2. Constructing querysets is rather slow in django, mainly because most of QuerySet methods clone self, then change it and return a clone. Original queryset is usually thrown away. Cacheops adds .inplace() method, which makes queryset mutating, preventing useless cloning:

   items = Item.objects.inplace().filter(category=12).order_by('-date')[:20]
   

   You can revert queryset to cloning state using .cloning() call.

3. More to 2, there is a bug in django 1.4-, which sometimes make queryset cloning very slow. You can use any patch from this ticket to fix it.

4. Use template fragment caching when possible, it's way more fast because you don't need to generate anything. Also pickling/unpickling a string is much faster than list of model instances. Cacheops doesn't provide extension for django's built-in templates for now, but you can adapt django.templatetags.cache to work with cacheops fairly easily (send me a pull request if you do).

5. Run separate redis instance for cache with disabled persistence. You can manually call SAVE or BGSAVE to stay hot upon server restart.

6. If you filter queryset on many different or complex conditions cache could degrade performance (comparing to uncached db calls) in consequence of frequent cache misses. Disable cache in such cases entirely or on some heurestics which detect if this request would be probably hit. E.g. enable cache if only some primary fields are used in filter.

   Caching querysets with large amount of filters also slows down all subsequent invalidation on that model. You can disable caching if more than some amount of fields is used in filter simultaneously.

Writing a test

Writing a test for an issue you are having can speed up it's resolution a lot. Here is how you do that. I am supposing you have some application code causing it.

1. Make a fork.
2. Install all from test_requirements.txt.
3. Ensure you can run tests with ./run_tests.py.
4. Copy relevant models code to https://github.com/Suor/django-cacheops/blob/master/tests/models.py
5. Go to https://github.com/Suor/django-cacheops/blob/master/tests/tests.py and paste code causing exception to IssueTests.test_{issue_number}.
6. Execute ./run_tests.py IssueTests.test_{issue_number} and see it failing.
7. Cut down model and test code until error disappears and make a step back.
8. Commit changes and make a pull request.

TODO

• fast mode: store cache in local memory, but check in with redis if it's valid
• make a version of invalidation with scripting
• shard cache between multiple redises
• integrate with prefetch_related()