/django-ultracache

Drop-in replacement for Django's template fragment caching. Provides automatic cache invalidation.

Primary LanguagePythonBSD 3-Clause "New" or "Revised" LicenseBSD-3-Clause

Note

https://github.com/hedleyroos/django-ultracache is the official home of the project. It has moved from https://github.com/praekelt/django-ultracache.

Django Ultracache

Cache views, template fragments and arbitrary Python code. Monitor Django object changes to perform automatic fine-grained cache invalidation from Django level, through proxies, to the browser.

Travis

Contents

Overview

Cache views, template fragments and arbitrary Python code. Once cached we either avoid database queries and expensive computations, depending on the use case. In all cases affected caches are automatically expired when objects "red" or "blue" are modified, without us having to add "red" or "blue" to the cache.

View:

from ultracache.decorators import cached_get, ultracache

@ultracache(300)
class MyView(TemplateView):
    template_name = "my_view.html"

    def get_context_data(self, **kwargs):
        context = super().get_context_data(**kwargs)
        # Note we never use red
        red = Color.objects.get(slug="red")
        # For our example color_slug is blue
        context["color"] = Color.objects.get(slug=kwargs["color_slug"])
        return context

Template:

{# variable "color" is the object "blue" #}
{% load ultracache_tags %}
{% ultracache 300 "color-info" color.pk %}
    {# expensive properties #}
    {{ color.compute_valid_hex_codes }}
    {{ color.name_in_all_languages }}
{% endultracache %}

Python:

from ultracache.utils import Ultracache

...

color_slug = request.GET["color_slug"]
uc = Ultracache(300, "another-identifier", color_slug)
if uc:
    codes = uc.cached
else:
    color = Color.objects.get(slug=color_slug)
    codes = color.compute_valid_hex_codes()
    uc.cache(codes)
print(codes)

Installation

  1. Install or add django-ultracache to your Python path.
  2. Add ultracache to your INSTALLED_APPS setting.
  3. Add ultracache.middleware.UltraCacheMiddleware to your MIDDLEWARE setting. It is recommended to add it as one of the first entries.
  4. Ensure django.template.context_processors.request is in the context processors setting.

Features

  1. Caches template fragments, views, Django Rest Framework viewsets.
  2. It takes the sites framework into consideration, allowing different caching per site.
  3. Crucially, it is aware of model objects that are subjected to its caching. When an object is modified all affected cache key are automatically expired. This allows the user to set longer expiry times without having to worry about stale content.
  4. The cache invalidation can be extended to issue purge commands to Varnish, Nginx or other reverse caching proxies.

Usage

The cached_get and ultracache view decorators

django-ultracache also provides decorators cached_get and ultracache to cache your views. The parameters follow the same rules as the ultracache template tag except they must all resolve. request.get_full_path() is always implicitly added to the cache key. The ultracache decorator is newer and cleaner, so use that where possible:

from ultracache.decorators import cached_get, ultracache


class CachedView(TemplateView):
    template_name = "cached_view.html"

    @cached_get(300, "request.is_secure()", 456)
    def get(self, *args, **kwargs):
        return super(CachedView, self).get(*args, **kwargs)

@ultracache(300, "request.is_secure()", 456)
class AnotherCachedView(TemplateView):
    template_name = "cached_view.html"

The cached_get decorator can be used in an URL pattern:

from ultracache.decorators import cached_get

url(
    r"^cached-view/$",
    cached_get(3600)(TemplateView.as_view(
        template_name="myproduct/template.html"
    )),
    name="cached-view"
)

Do not indiscriminately use the decorators. They only ever operate on GET requests but cannot know if the code being wrapped retrieves data from eg. the session. In such a case they will cache things they are not supposed to cache.

If your view is used by more than one URL pattern then it is highly recommended to apply the cached_get decorator in the URL pattern. Applying it directly to the get method may lead to cache collisions, especially if get_template_names is overridden.

The ultracache template tag

django-ultracache provides a template tag {% ultracache %} that functions much like Django's standard cache template tag; however, it takes the sites framework into consideration, allowing different caching per site, and it handles undefined variables.

Simplest use case:

{% load ultracache_tags %}
{% ultracache 3600 "my_identifier" object 123 undefined "string" %}
    {{ object.title }}
{% endultracache %}

The tag can be nested. ultracache is aware of all model objects that are subjected to its caching. In this example cache keys outer and inner_one are expired when object one is changed but cache key inner_two remains unaffected:

{% load ultracache_tags %}
{% ultracache 1200 "outer" %}
    {% ultracache 1200 "inner_one" %}
        title = {{ one.title }}
    {% endultracache %}
    {% ultracache 1200 "inner_two" %}
        title = {{ two.title }}
    {% endultracache %}
{% endultracache %}

Specifying a good cache key

The cache key decides whether a piece of code or template is going to be evaluated further. The cache key must therefore accurately and minimally describe what is being subjected to caching.

todo

Django Rest Framework viewset caching

Cache list and retrieve actions on viewsets:

# Cache all viewsets
ULTRACACHE = {
    "drf": {"viewsets": {"*": {}}}

}

# Cache a specific viewset by name
ULTRACACHE = {
    "drf": {"viewsets": {"my.app.MyViewset": {}}}

}

# Cache a specific viewset by class
ULTRACACHE = {
    "drf": {"viewsets": {MyViewset: {}}}

}

# Timeouts default to 300 seconds
ULTRACACHE = {
    "drf": {"viewsets": {"*": {"timeout": 1200}}}

}

# Evaluate code to append to the cache key. This example caches differently
# depending on whether the user is logged in or not.
ULTRACACHE = {
    "drf": {"viewsets": {"*": {"evaluate": "request.user.is_anonymous"}}}

}

# Evaluate code to append to the cache key via a callable.
def mycallable(viewset, request):
    if viewset.__class__.__name__ == "foo":
        return request.user.id

ULTRACACHE = {
    "drf": {"viewsets": {"*": {"evaluate": mycallable}}}

}

Purgers

You can create custom reverse caching proxy purgers. See purgers.py for examples:

ULTRACACHE = {
    "purge": {"method": "myproduct.purgers.squid"}
}

The most useful purger is broadcast. As the name implies it broadcasts purge instructions to a queue. Note that you need celery running and configured to write to a RabbitMQ instance for this to work correctly.

The purge instructions are consumed by the cache-purge-consumer.py script. The script reads a purge instruction from the queue and then sends a purge instruction to an associated reverse caching proxy. To run the script:

virtualenv ve
./ve/bin/pip install -e .
./ve/bin/python bin/cache-purge-consumer.py -c config.yaml

The config file has these options:

  1. rabbit-url
    Specify RabbitMQ connection parameters in the AMQP URL format amqp://username:password@host:port/<virtual_host>[?query-string].
    Optional. Defaults to amqp://guest:guest@127.0.0.1:5672/%2F. Note the URL encoding for the path.
  2. host
    A reverse caching proxy may be responsible for many domains (hosts), and ultracache will keep track of the host that is involved in a purge request; however, if you have a use case that does not supply a hostname, eg. doing a PURGE request via curl, then forcing a hostname solves the use case.
    Optional.
  3. proxy-address
    The IP address or hostname of the reverse caching proxy.
    Optional. Defaults to 127.0.0.1.
  4. logfile
    Set to a file to log all purge instructions. Specify stdout to log to standard out.
    Optional.

Other settings

Automatic invalidation defaults to true. To disable automatic invalidation set:

ULTRACACHE = {
    "invalidate": False
}

django-ultracache maintains a registry in Django's caching backend (see How does it work). This registry can"t be allowed to grow unchecked, thus a limit is imposed on the registry size. It would be inefficient to impose a size limit on the entire registry so a maximum size is set per cached value. It defaults to 1000000 bytes:

ULTRACACHE = {
    "max-registry-value-size": 10000
}

It is highly recommended to use a backend that supports compression because a larger size improves cache coherency.

If you make use of a reverse caching proxy then you need the original set of request headers (or a relevant subset) to purge paths from the proxy correctly. The problem with the modern web is the sheer amount of request headers present on every request would lead to a large number of entries having to be stored by django-ultracache in Django's caching backend. Your proxy probably has a custom hash computation rule that considers only the request path (always implied) and Django's sessionid cookie, so define a setting to also consider only the cookie on the Django side:

ULTRACACHE = {
    "consider-headers": ["cookie"]
}

If you only need to consider some cookies then set:

ULTRACACHE = {
    "consider-cookies": ["sessionid", "some-other-cookie"]
}

How does it work?

django-ultracache monkey patches django.template.base.Variable._resolve_lookup and django.db.models.Model.__getattribute__ to make a record of model objects as they are resolved. The ultracache template tag, ultracache decorator and ultracache context manager inspect the list of objects contained within them and keep a registry in Django's caching backend. A post_save signal handler monitors objects for changes and expires the appropriate cache keys.

Tips

  1. If you are running a cluster of Django nodes then ensure that they use a shared caching backend.