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django-adv-cache-tag

Django advanced cache template tag:

Readable documentation on
http://documentup.com/twidi/django-adv-cache-tag

Introduction

First, notice that the arguments of the {% cache %} templatetag
provided by django-adv-cache-tag are the same as for the default cache
templatetag included in django, so it's very easy to use this new one.

With django-adv-cache-tag you can :

Installation

django-adv-cache-tag is available on PyPI:

pip install django-adv-cache-tag

Starting at version 1.0, we only support python 3.

If you upgrade from version < 1, note that the internal version
number has changed, so all cache will be reset.

If you want python 2 support, you must install by passing the version :

pip install 'django-adv-cache-tag<1.0'

Or you can find it on Github:
https://github.com/twidi/django-adv-cache-tag

(for python2 version:
https://github.com/twidi/django-adv-cache-tag/tree/python2)

When installed, just add adv_cache_tag to your INSTALLED_APPS in the
settings.py file of your django project.

See examples in the next sections to see how it works (basically the
same way as the default django cache templatetag)

Features

Versioning

Description

With the default django cache templatetag, you can add as many arguments
as you want, including a version, or date, and then the cache key will
change if this version change. So your cache is updated, as expected.

But the older key is not deleted and if you have a long expire time, it
will stay there for a very long time, consuming your precious memory.

django-adv-cache-tag provides a way to avoid this, while still
regenerating the cache when needed. For this, when activated, we use the
last argument passed to your templatetag as a "version number", and
remove it for the arguments used to generate the cache key.

This version will be used in the content of the cached template,
instead of the key, and when the cache exists and is loaded, the
cached version will be compared to the wanted one, and if the two match,
the cache is valid and returned, else it will be regenerated.

So if you like the principle of a unique key for a given template for a
given object/user or whatever, be sure to always use the same arguments,
except the last one, and activate the ADV_CACHE_VERSIONING setting.

Note that we also manage an internal version number, which will always
be compared to the cached one. This internal version number is only
updated when the internal algorithm of django-adv-cache-tag changes.
But you can update it to invalidate all cached templates by adding a
ADV_CACHE_VERSION to your settings (our internal version and the value
from this settings will be concatenated to get the internal version
really used)

Settings

ADV_CACHE_VERSIONING, default to False

ADV_CACHE_VERSION, default to ""

Example

In the following template, if ADV_CACHE_VERSIONING is set to True, the
key will always be the same, based on the string "myobj_main_template"
and the value of obj.pk, but the cached value will be regenerated each
time the obj.date_last_updated will change.

So we set a expire_time of 0, to always keep the template cached,
because we now we won't have many copies (old ones and current one) of
it.

The value to set to have no expiry may depend of your cache backend
(it's not always 0).

{% load adv_cache %}
{% cache 0 myobj_main_template obj.pk obj.date_last_updated %}
  {{ obj }}
{% endcache %}

Primary key

Description

In the default django cache templatetag, the cache keys are like this
one :

:1:template.cache.your_fragment_name.64223ccf70bbb65a3a4aceac37e21016

You may want to have more explicit cache keys, so with
django-adv-cache-tag you can add a "primary key" that will be added
between the fragment name and the hash :

:1:template.cache.your_fragment_name.your_pk.64223ccf70bbb65a3a4aceac37e21016

Although the main use of this primary key is to have one cached fragment
per object, so we can use the object primary key, you can use whatever
you want, an id, a string...

To add a primary key, simply set the ADV_CACHE_INCLUDE_PK setting to
True, and the first argument (after the fragment's name) will be used
as a pk.

If you want this only for a part of your cache templatetags, read the
Extending the default cache tag part later in this readme (it's easy,
really).

Unlike the version, the primary key will be kept as an argument to
generate the cache key hash.

Settings

ADV_CACHE_INCLUDE_PK, default to False

Example

A common use of django-adv-cache-tag is to only use a primary key and
a version:

{% cache 0 myobj_main_template obj.pk obj.date_last_updated %}

Compression

Description

The default django cache templatetag simply saves the generated html in
the cache. Depending of your template, if may be a lot of html and your
cache memory will grow very quickly. Not to mention that we can have a
lot of spaces because of indentation in templates (two ways i know to
remove them without django-adv-cache-tag: the {% spaceless %}
templatetag, provided by django, and
django-template-preprocessor).

django-adv-cache-tag can do this for you. It is able to remove
duplicate spaces (including newlines, tabs) by replacing them by a
simple space (to keep the space behavior in html), and to compress the
html to be cached, via the zlib (and pickle) module.

Of course, this cost some time and CPU cycles, but you can save a lot of
memory in your cache backend, and a lot of bandwidth, especially if your
backend is on a distant place. I haven't done any test for this, but for
some templates, the saved data can be reduced from 2 ko to less than
one.

To activate these feature, simply set to True one or both of the
settings defined below.

WARNING : If the cache backend used use pickle and its default protocol,
compression is useless because binary is not really well handled and the
final size stored in the cache will be largely bigger than the
compressed one. So check for this before activating this option. It's ok
for the default django backends (at least in 1.4), but not for
django-redis-cache, waiting for my pull-request, but you can check my
own version:
https://github.com/twidi/django-redis-cache/tree/pickle\_version

Settings

ADV_CACHE_COMPRESS, default to False, to activate the compression
via zlib

ADV_CACHE_COMPRESS_SPACES, default to False, to activate the
reduction of blank characters.

Example

No example since you don't have to change anything to your templatetag
call to use this, just set the settings.

Choose your cache backend

Description

In django, you can define many cache backends. But with the default
cache templatetag, you cannot say which one use, it will automatically
be the default one.

django-adv-cache-tag can do this for your by providing a setting,
ADV_CACHE_BACKEND which will take the name of a cache backend defined
in your settings. And by extending the provided CacheTag object, you
can even define many backends to be used by many templatetags, say one
for heavily accessed templates, one for the others... as you want. Read
the Extending the default cache tag part to know more about this (it's
easy, really, but i already told you...)

Settings

ADV_CACHE_BACKEND, default to "default"

Example

No example since, like for the compression, you don't have to change
anything to your templatetag to use this, just set the setting.

Partial caching

With the default django cache templatetag, your templates are cached and
you can't update them before display, so you can't cache big parts of
html with a little dynamic fragment in it, for the user name, the
current date or whatever. You can cheat and save two templates
surrounding your dynamic part, but you will have more accesses to your
cache backend.

django-adv-cache-tag allow the use of one or many {% nocache %}
blocks (closed by {% endnocache %}) to put in your {% cache %}
blocks. These {% nocache %} block will be saved "as is" in the cache,
while the rest of the block will be rendered to html. It's only when the
template is finally displayed that the no-cached parts will be rendered.

You can have as many of these blocks you want.

Settings

There is no settings for this feature, which is automatically activated.

Example

{% cache 0 myobj_main_template obj.pk obj.date_last_updated %}
    <p>This is the cached part of the template for {{ obj }}, evaluated at {% now "r" %}.</p>
    {% nocache %}
        <p>This part will be evaluated each time : {% now "r" %}</p>
    {% endnocache %}
    <p>This is another cached part</p>
{% endcache %}

The fragment name

Description

The fragment name is the name to use as a base to create the cache key,
and is defined just after the expiry time.

The Django documentation states
The name will be taken as is, do not use a variable.

In django-adv-cache-tag, by setting ADV_CACHE_RESOLVE_NAME to
True, a fragment name that is not quoted will be resolved as a
variable that should be in the context.

Settings

ADV_CACHE_RESOLVE_NAME, default to False

Example

With ADV_CACHE_RESOLVE_NAME set to True, you can do this if you have
a variable named fragment_name in your context:

{% cache 0 fragment_name obj.pk obj.date_last_updated %}

And if you want to pass a name, you have to surround it by quotes:

{% cache 0 "myobj_main_template" obj.pk obj.date_last_updated %}

With ADV_CACHE_RESOLVE_NAME set to False, the default, the name is
always seen as a string, but if surrounded by quotes, they are removed.

In the following example, you see double-quotes, but it would be the
same with single quotes, or no quotes at all:

{% cache 0 "myobj_main_template" obj.pk obj.date_last_updated %}

Extending the default cache tag

If the five settings explained in the previous sections are not enough
for you, or if you want to have a templatetag with a different behavior
as the default provided ones, you will be happy to know that
django-adv-cache-tag was written with easily extending in mind.

It provides a class, CacheTag (in adv_cache_tag.tag), which has a
lot of short and simple methods, and even a Meta class (idea stolen
from the django models :D ). So it's easy to override a simple part.

All options defined in the Meta class are accessible in the class via
self.options.some_field

Below we will show many ways of extending this class.

Basic override

Imagine you don't want to change the default settings (all to False,
and using the default backend) but want a templatetag with versioning
activated :

Create a new templatetag file (myapp/templatetags/my_cache_tags.py)
with this:

from adv_cache_tag.tag import CacheTag

class VersionedCacheTag(CacheTag):
    class Meta(CacheTag.Meta):
        versioning = True

from django import template
register = template.Library()

VersionedCacheTag.register(register, 'ver_cache')

With these simple lines, you now have a new templatetag to use when you
want versioning:

{% load my_cache_tags %}
{% ver_cache 0 myobj_main_template obj.pk obj.date_last_updated %}
    obj
{% endver_cache %}

As you see, just replace {% load adv_cache %} (or the django default
{% load cache %}) by {% load my_cache_tags %} (your templatetag
module), and the {% cache %} templatetag by your new defined one,
{% ver_cache ... %}. Don't forget to replace the closing tag too:
{% endver_cache %}. But the {% nocache %} will stay the same, except
if you want a new one. For this, just add a parameter to the register
method:

MyCacheTag.register(register, 'ver_cache', 'ver_nocache')
{% ver_cache ... %}
    cached
    {% ver_nocache %}not cached{% endver_nocache %}
{% endver_cache %}

Note that you can keep the name cache for your tag if you know that
you will not load in your template another templatetag module providing
a cache tag. To do so, the simplest way is:

MyCacheTag.register(register)  # 'cache' and 'nocache' are the default values

All the django-adv-cache-tag settings have a matching variable in the
Meta class, so you can override one or many of them in your own
classes. See the "Settings" part to see them.

Internal version

When your template file is updated, the only way to invalidate all
cached versions of this template is to update the fragment name or the
arguments passed to the templatetag.

With django-adv-cache-tag you can do this with versioning, by managing
your own version as the last argument to the templatetag. But if you
want to use the power of the versioning system of
django-adv-cache-tag, it can be too verbose:

{% load adv_cache %}
{% with template_version=obj.date_last_updated|stringformat:"s"|add:"v1" %}
    {% cache 0 myobj_main_template obj.pk template_version %}
    ...
    {% endcache %}
{% endwith %}

django-adv-cache-tag provides a way to do this easily, with the
ADV_CACHE_VERSION setting. But by updating it, all cached versions
will be invalidated, not only those you updated.

To do this, simply create your own tag with a specific internal version:

class MyCacheTag(CacheTag):
    class Meta(CacheTag.Meta):
       internal_version = "v1"

MyCacheTag.register('my_cache')

And then in your template, you can simply do

{% load my_cache_tags %}
{% my_cache 0 myobj_main_template obj.pk obj.date_last_updated %}
...
{% endmy_cache %}

Each time you update the content of your template and want invalidation,
simply change the internal_version in your MyCacheTag class (or you
can use a settings for this).

Change the cache backend

If you want to change the cache backend for one templatetag, it's easy:

class MyCacheTag(CacheTag):
    class Meta:
        cache_backend = 'templates'

But you can also to this by overriding a method:

from django.core.cache import get_cache

class MyCacheTag(CacheTag):
    def get_cache_object(self):
        return get_cache('templates')

And if you want a cache backend for old objects, and another, faster,
for recent ones:

from django.core.cache import get_cache

class MyCacheTag(CacheTag):
    class Meta:
        cache_backend = 'fast_templates'

    def get_cache_object(self):
        cache_backend = self.options.cache_backend
        if self.get_pk() < 1000:
            cache_backend = 'slow_templates'
        return get_cache(cache_backend)

The value returned by the get_cache_object should be a cache backend
object, but as we only use the set and get methods on this object,
it can be what you want if it provides these two methods. And even more,
you can override the cache_set and cache_get methods of the
CacheTag class if you don't want to use the default set and get
methods of the cache backend object.

Note that we also support the django way of changing the cache backend
in the template-tag, using the using argument, to be set at the last
parameter (without any space between using and the name of the cache
backend).

{% cache 0 myobj_main_template obj.pk obj.date_last_updated using=foo %}

Change the cache key

The CacheTag class provides three classes to create the cache key:

The arguments are:

If you want to remove the "template." part at the start of the cache key
(useless if you have a cache backend dedicated to template caching), you
can do this:

class MyCacheTag(CacheTag):
    def get_base_cache_key(self):
        cache_key = super(MyCacheTag, self).get_base_cache_key()
        return cache_key[len('template:'):]  # or [9:]

Add an argument to the templatetag

By default, the templatetag provided by CacheTag takes the same
arguments as the default django cache templatetag.

If you want to add an argument, it's easy as the class provides a
get_template_node_arguments method, which will work as for normal
django templatetags, taking a list of tokens, and returning ones that
will be passed to the real templatetag, a Node class tied to the
CacheTag.

Say you want to add a foo argument between the expire time and the
fragment name:

from django import template

from adv_cache_tag.tag import CacheTag, Node

class MyNode(Node):
    def __init__(self, nodename, nodelist, expire_time, foo, fragment_name, vary_on):
        """ Save the foo variable in the node (not resolved yet) """
        super(MyNode, self).__init__(self, nodename, nodelist, expire_time, fragment_name, vary_on)
        self.foo = foo


class MyCacheTag(CacheTag):

    Node = MyNode

    def prepare_params(self):
        """ Resolve the foo variable to it's real content """
        super(MyCacheTag, self).prepare_params()
        self.foo = template.Variable(self.node.foo).resolve(self.context)

    @classmethod
    def get_template_node_arguments(cls, tokens):
        """ Check validity of tokens and return them as ready to be passed to the Node class """
        if len(tokens) < 4:
            raise template.TemplateSyntaxError(u"'%r' tag requires at least 3 arguments." % tokens[0])
        return (tokens[1], tokens[2], tokens[3], tokens[4:])

Prepare caching of templates

This one is not about overriding the class, but it can be useful. When
an object is updated, it can be better to regenerate the cached template
at this moment rather than when we need to display it.

It's easy. You can do this by catching the post_save signal of your
model, or just by overriding its save method. For this example we will
use this last solution.

The only special thing is to know the path of the template where your
templatetag is. In my case, i have a template just for this (included in
other ones for general use), so it's easier to find it and regenerate it
as in this example.

As we are not in a request, we don't have the Request object here, so
context processors are not working, we must create a context object that
will be used to render the template, with all needed variables.

from django.template import loader, Context

class MyModel(models.Model):
    # your fields

    def save(self, *args, **kwargs):
        super(MyModel, self.save(*args, **kwargs)

        template = 'path/to/my_template_file_with_my_cache_block.html'

        context = Context({
            'obj': self,

            # as you have no request, we have to add stuff from context processors manually if we need them
            'STATIC_URL': settings.STATIC_URL,

            # the line below indicates that we force regenerating the cache, even if it exists
            '__regenerate__': True,

            # the line below indicates if we only want html, without parsing the nocache parts
            '__partial__': True,

        })

        loader.get_template(template).render(context)

Load data from database before rendering

This is a special case. Say you want to display a list of objects but
you have only ids and versions retrieved from redis (with ZSET, with
id as value and updated date (which is used as a version) as score , for
example)

If you know you always have a valid version of your template in cache,
because they are regenerated every time they are saved, as seen above,
it's fine, just add the object's primary key as the pk in your
templatetag arguments, and the cached template will be loaded.

But if it's not the case, you will have a problem: when django will
render the template, the only part of the object present in the context
is the primary key, so if you need the name or whatever field to render
the cached template, it won't work.

With django-adv-cache-tag it's easy to resolve this, as we can load
the object from the database and adding it to the context.

View

def my_view(request):
    objects = [
        dict(
            pk=val[0],
            date_last_updated=val[1]
        )
        for val in
            redis.zrevrange('my_objects', 0, 19, withscores=True)
    ]
    return render(request, "my_results.html", dict(objects=objects))

Template "my_results.html"

{% for obj in objects %}
    {% include "my_result.html" %}
{% endfor %}

Template "my_result.html"

{% load my_cache_tags %}
{% my_cache 0 myobj_main_template obj.pk obj.date_last_updated %}
    {{ obj }}
{% endmy_cache %}

Templatetag

In myapp/templatetags/my_cache_tags

from my_app.models import MyModel

class MyCacheTag(CacheTag):

    class Meta(CacheTag.Meta):
        """ Force options """
        include_pk = True
        versioning = True

    def create_content(self):
        """ If the object in context is not a real model, load it from db """
        if not isinstance(context['obj'], MyObject):
            context['obj'] = MyModel.objects.get(id=self.get_pk())
        super(MyCacheTag, self).create_content()

MyCacheTag.register('my_cache')

Careful with this, it generates as database requests as objects to be
loaded.

And more...

If you want to do more, feel free to look at the source code of the
CacheTag class (in tag.py), all methods are documented.

Settings

django-adv-cache-tag provide 5 settings you can change. Here is the
list, with descriptions, default values, and corresponding fields in the
Meta class (accessible via self.options.some_field in the CacheTag
object)

How it works

Here is a quick overview on how things work in django-adv-cache-tag

Partial caching

Your template :

{% load adv_cache %}
{% cache ... %}
    foo
    {% nocache %}
        bar
    {% endnocache %}
    baz
{% endcache %}

Cached version (we ignore versioning and compress here, just to see how
it works):

foo
{% endRAW_xyz %}
    bar
{% RAW_xyz %}
baz

When cached version is loaded, we parse :

{% RAW_xyz %}
foo
{% endRAW_xyz %}
    bar
{% RAW_xyz %}
baz
{% endRAW_xyz %}

The first {% RAW_xyz %} and the last {% endRAW_xyz %} are not
included in the cached version and added before parsing, only to save
some bytes.

Parts between {% RAW_xyz %} and {% endRAW_xyz %} are not parsed at
all (seen as a TextNode by django)

The xyz part of the RAW and endRAW templatetags depends on the
SECRET_KEY and so is unique for a given site.

It allows to avoid at max the possible collisions with parsed content in
the cached version.

We could have used {% nocache %} and {% endnocache %} instead of
{% RAW_xyz %} and {% endRAW_xyz %} but in the parsed template,
stored in the cache, if the html includes one of these strings, our
final template would be broken, so we use long ones with a hash (but we
can not be sure at 100% these strings could not be in the cached html,
but for common usages it should suffice)

License

django-adv-cache-tag is published under the MIT License (see the
LICENSE file)

Running tests

If adv_cache_tag is in the INSTALLE_APPS of your project, simply
run:

django-admin test adv_cache_tag

(you may want to use django-admin or ./manage.py depending on your
installation)

If you are in a fresh virtualenv to work on adv_cache_tag, install the
django version you want:

pip install django

Then make the adv_cache_tag module available in your python path. For
example, with virtualenv-wrapper, considering you are at the root of
the django-adv-cache-tag repository, simply do:

add2virtualenv .

Or simply:

pip install -e .

Then to run the tests, this library provides a test project, so you can
launch them this way:

DJANGO_SETTINGS_MODULE=adv_cache_tag.tests.testproject.settings django-admin.py test adv_cache_tag

Or simply launch the runtests.sh script (it will run this exact
command):

./runtests.sh

Supported versions

Django version Python version Library version
1.7 to 1.11 2.7 0.4
1.7 3.4 1.1.1
1.8 to 1.10 3.4, 3.5 1.1.1
1.11 to 2.0 3.4, 3.5, 3.6 1.1.1

Support for Python 2 is dropped since version 1 of
django-adv-cache-tag