Django’s Cache Framework

A fundamental trade-off in dynamic Web sites is, well, they’re dynamic. Each time a user requests a page, the Web server makes all sorts of calculations, from database queries to template rendering to business logic to creating the page that your site’s visitors see. This is a lot more expensive, from a processing-overhead perspective, than your standard read-a-file-off-the-filesystem server arrangement.

For most Web applications, this overhead isn’t a big deal. Most Web applications aren’t washingtonpost.com or slashdot.org; they’re simply small- to medium-sized sites with so-so traffic.
But for medium- to high-traffic sites, it’s essential to cut as much overhead as possible.

That’s where caching comes in. To cache something is to save the result of an expensive calculation so that you don’t have to perform the calculation next time. Here’s some pseudocode explaining how this would work for a dynamically generated Web page:

given a URL, try finding that page in the cache if the page is in the cache:
    return the cached page
else:
    generate the page
    save the generated page in the cache (for next time)
    return the generated page 

Django comes with a robust cache system that lets you save dynamic pages so they don’t have to be calculated for each request. For convenience, Django offers different levels of cache granularity: You can cache the output of specific views, you can cache only the pieces that are difficult to produce, or you can cache your entire site.

Django also works well with downstream caches, such as Squid and browser-based caches. These are the types of caches that you don’t directly control, but to which you can provide hints (via HTTP headers) about which parts of your site should be cached, and how.

Setting Up the Cache

The cache system requires a small amount of setup. Namely, you have to tell it where your cached data should live; whether in a database, on the filesystem or directly in memory. This is an important decision that affects your cache’s performance.

Your cache preference goes in the CACHES setting in your settings file.

Memcached

The fastest, most efficient type of cache supported natively by Django, Memcached is an entirely memory-based cache server, originally developed to handle high loads at LiveJournal.com and subsequently open-sourced by Danga Interactive. It’s used by sites such as Facebook and Wikipedia to reduce database access and dramatically increase site performance.

Memcached runs as a daemon and is allotted a specified amount of RAM. All it does is provide a fast interface for adding, retrieving and deleting data in the cache. All data is stored directly in memory, so there’s no overhead of database or filesystem usage.

After installing Memcached itself, you’ll need to install a Memcached binding. There are several Python Memcached bindings available; the two most common are python-memcached and pylibmc. To use Memcached with Django:

  • Set BACKEND to django.core.cache.backends.memcached.MemcachedCache or django.core.cache.backends.memcached.PyLibMCCache (depending on your chosen memcached binding)
  • Set LOCATION to ip:port values, where ip is the IP address of the Memcached daemon and port is the port on which Memcached is running, or to a unix:path value, where path is the path to a Memcached Unix socket file.

In this example, Memcached is running on localhost (127.0.0.1) port 11211, using the python-memcached binding:

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

In this example, Memcached is available through a local Unix socket file /tmp/memcached.sock using the python-memcached binding:

CACHES = {
    'default': {
        'BACKEND': 'django.core.cache.backends.memcached.MemcachedCache',
        'LOCATION': 'unix:/tmp/memcached.sock',
    }
}

One excellent feature of Memcached is its ability to share a cache over multiple servers. This means you can run Memcached daemons on multiple machines, and the program will treat the group of machines as a single cache, without the need to duplicate cache values on each machine. To take advantage of this feature, include all server addresses in LOCATION, either separated by semicolons or as a list.

In this example, the cache is shared over Memcached instances running on IP address 172.19.26.240 and 172.19.26.242, both on port 11211:

CACHES = {
    'default': {
        'BACKEND': 'django.core.cache.backends.memcached.MemcachedCache',
        'LOCATION': [
            '172.19.26.240:11211',
            '172.19.26.242:11211',
        ]
    }
}

In the following example, the cache is shared over Memcached instances running on the IP addresses 172.19.26.240 (port 11211), 172.19.26.242 (port 11212), and 172.19.26.244 (port 11213):

CACHES = {
    'default': {
        'BACKEND': 'django.core.cache.backends.memcached.MemcachedCache',
        'LOCATION': [
            '172.19.26.240:11211',
            '172.19.26.242:11212',
            '172.19.26.244:11213',
        ]
    }
}

A final point about Memcached is that memory-based caching has a disadvantage: because the cached data is stored in memory, the data will be lost if your server crashes.

Clearly, memory isn’t intended for permanent data storage, so don’t rely on memory-based caching as your only data storage. Without a doubt, none of the Django caching backends should be used for permanent storage – they’re all intended to be solutions for caching, not storage – but we point this out here because memory-based caching is particularly temporary.

Database Caching

Django can store its cached data in your database. This works best if you’ve got a fast, well-indexed database server. To use a database table as your cache backend:

  • Set BACKEND to django.core.cache.backends.db.DatabaseCache
  • Set LOCATION to tablename, the name of the database table. This name can be whatever you want, as long as it’s a valid table name that’s not already being used in your database.

In this example, the cache table’s name is my_cache_table:

CACHES = {
    'default': {
        'BACKEND': 'django.core.cache.backends.db.DatabaseCache',
        'LOCATION': 'my_cache_table',
    }
}
Creating The Cache Table

Before using the database cache, you must create the cache table with this command:

python manage.py createcachetable 

This creates a table in your database that is in the proper format that Django’s database-cache system expects. The name of the table is taken from LOCATION. If you are using multiple database caches, createcachetable creates one table for each cache. If you are using multiple databases, createcachetable observes the allow_migrate() method of your database routers (see below). Like migrate, createcachetable won’t touch an existing table. It will only create missing tables.

Multiple Databases

If you use database caching with multiple databases, you’ll also need to set up routing instructions for your database cache table. For the purposes of routing, the database cache table appears as a model named CacheEntry, in an application named django_cache. This model won’t appear in the models cache, but the model details can be used for routing purposes.

For example, the following router would direct all cache read operations to cache_replica, and all write operations to cache_primary. The cache table will only be synchronized onto cache_primary:

class CacheRouter(object):
    """A router to control all database cache operations"""

    def db_for_read(self, model, **hints):
        # All cache read operations go to the replica
        if model._meta.app_label in ('django_cache',):
            return 'cache_replica'
        return None

    def db_for_write(self, model, **hints):
        # All cache write operations go to primary
        if model._meta.app_label in ('django_cache',):
            return 'cache_primary'
        return None

    def allow_migrate(self, db, model):
        # Only install the cache model on primary
        if model._meta.app_label in ('django_cache',):
            return db == 'cache_primary'
        return None 

If you don’t specify routing directions for the database cache model, the cache backend will use the default database. Of course, if you don’t use the database cache backend, you don’t need to worry about providing routing instructions for the database cache model.

Filesystem Caching

The file-based backend serializes and stores each cache value as a separate file. To use this backend set BACKEND to 'django.core.cache.backends.filebased.FileBasedCache' and LOCATION to a suitable directory.

For example, to store cached data in /var/tmp/django_cache, use this setting:

CACHES = {
    'default': {
        'BACKEND': 'django.core.cache.backends.filebased.FileBasedCache',
        'LOCATION': '/var/tmp/django_cache',
    }
}

If you’re on Windows, put the drive letter at the beginning of the path, like this:

CACHES = {
    'default': {
        'BACKEND': 'django.core.cache.backends.filebased.FileBasedCache',
        'LOCATION': 'c:/foo/bar',
    }
}

The directory path should be absolute – that is, it should start at the root of your filesystem. It doesn’t matter whether you put a slash at the end of the setting. Make sure the directory pointed to by this setting exists and is readable and writable by the system user under which your Web server runs. Continuing the above example, if your server runs as the user apache, make sure the directory /var/tmp/django_cache exists and is readable and writable by the user apache.

Local-Memory Caching

This is the default cache if another is not specified in your settings file. If you want the speed advantages of in-memory caching but don’t have the capability of running Memcached, consider the local-memory cache backend. To use it, set BACKEND to django.core.cache.backends.locmem.LocMemCache. For example:

CACHES = {
    'default': {
        'BACKEND': 'django.core.cache.backends.locmem.LocMemCache',
        'LOCATION': 'unique-snowflake'
    }
}

The cache LOCATION is used to identify individual memory stores. If you only have one locmem cache, you can omit the LOCATION; however, if you have more than one local memory cache, you will need to assign a name to at least one of them in order to keep them separate.

Note that each process will have its own private cache instance, which means no cross-process caching is possible. This obviously also means the local memory cache isn’t particularly memory-efficient, so it’s probably not a good choice for production environments. It’s nice for development.

Dummy Caching (For Development)

Finally, Django comes with a dummy cache that doesn’t actually cache – it just implements the cache interface without doing anything. This is useful if you have a production site that uses heavy-duty caching in various places but a development/test environment where you don’t want to cache and don’t want to have to change your code to special-case the latter. To activate dummy caching, set BACKEND like so:

CACHES = {
    'default': {
        'BACKEND': 'django.core.cache.backends.dummy.DummyCache',
    }
}

Using A Custom Cache Backend

While Django includes support for a number of cache backends out-of-the-box, sometimes you might want to use a customized cache backend. To use an external cache backend with Django, use the Python import path as the BACKEND of the CACHES setting, like so:

CACHES = {
    'default': {
        'BACKEND': 'path.to.backend',
    }
}

If you’re building your own backend, you can use the standard cache backends as reference implementations. You’ll find the code in the django/core/cache/backends/ directory of the Django source.

Cache Arguments

Each cache backend can be given additional arguments to control caching behavior. These arguments are provided as additional keys in the CACHES setting. Valid arguments are as follows:

  • TIMEOUT: The default timeout, in seconds, to use for the cache. This argument defaults to 300 seconds (5 minutes). You can set TIMEOUT to None so that, by default, cache keys never expire. A value of 0 causes keys to immediately expire (effectively don’t cache).
  • OPTIONS: Any options that should be passed to the cache backend. The list of valid options will vary with each backend, and cache backends backed by a third-party library will pass their options directly to the underlying cache library.
  • Cache backends that implement their own culling strategy (i.e., the locmem, filesystem and database backends) will honor the following options:
    • MAX_ENTRIES: The maximum number of entries allowed in the cache before old values are deleted. This argument defaults to 300.
    • CULL_FREQUENCY: The fraction of entries that are culled when MAX_ENTRIES is reached. The actual ratio is 1 / CULL_FREQUENCY, so set CULL_FREQUENCY to 2 to cull half the entries when MAX_ENTRIES is reached. This argument should be an integer and defaults to 3.A value of 0 for CULL_FREQUENCY means that the entire cache will be dumped when MAX_ENTRIES is reached. On some backends (database in particular) this makes culling much faster at the expense of more cache misses.
  • KEY_PREFIX: A string that will be automatically included (prepended by default) to all cache keys used by the Django server.
  • VERSION: The default version number for cache keys generated by the Django server.
  • KEY_FUNCTION: A string containing a dotted path to a function that defines how to compose a prefix, version and key into a final cache key.

In this example, a filesystem backend is being configured with a timeout of 60 seconds, and a maximum capacity of 1000 items:

CACHES = {
    'default': {
        'BACKEND': 'django.core.cache.backends.filebased.FileBasedCache',
        'LOCATION': '/var/tmp/django_cache',
        'TIMEOUT': 60,
        'OPTIONS': {'MAX_ENTRIES': 1000}
    }
}

The Per-Site Cache

Once the cache is set up, the simplest way to use caching is to cache your entire site. You’ll need to add 'django.middleware.cache.UpdateCacheMiddleware' and 'django.middleware.cache.FetchFromCacheMiddleware' to your MIDDLEWARE_CLASSES setting, as in this example:

MIDDLEWARE_CLASSES = [
    'django.middleware.cache.UpdateCacheMiddleware',
    'django.middleware.common.CommonMiddleware',
    'django.middleware.cache.FetchFromCacheMiddleware',
]

Then, add the following required settings to your Django settings file:

  • CACHE_MIDDLEWARE_ALIAS – The cache alias to use for storage.
  • CACHE_MIDDLEWARE_SECONDS – The number of seconds each page should be cached.
  • CACHE_MIDDLEWARE_KEY_PREFIX – If the cache is shared across multiple sites using the same Django installation, set this to the name of the site, or some other string that is unique to this Django instance, to prevent key collisions. Use an empty string if you don’t care.

FetchFromCacheMiddleware caches GET and HEAD responses with status 200, where the request and response headers allow. Responses to requests for the same URL with different query parameters are considered to be unique pages and are cached separately. This middleware expects that a HEAD request is answered with the same response headers as the corresponding GET request; in which case it can return a cached GET response for HEAD request. Additionally, UpdateCacheMiddleware automatically sets a few headers in each HttpResponse:

  • Sets the Last-Modified header to the current date/time when a fresh (not cached) version of the page is requested.
  • Sets the Expires header to the current date/time plus the defined CACHE_MIDDLEWARE_SECONDS.
  • Sets the Cache-Control header to give a max age for the page – again, from the CACHE_MIDDLEWARE_SECONDS setting.

If a view sets its own cache expiry time (i.e. it has a max-age section in its Cache-Control header) then the page will be cached until the expiry time, rather than CACHE_MIDDLEWARE_SECONDS. Using the decorators in django.views.decorators.cache you can easily set a view’s expiry time (using the cache_control decorator) or disable caching for a view (using the never_cache decorator). See the using other headers section for more on these decorators.

If USE_I18N is set to True then the generated cache key will include the name of the active language. This allows you to easily cache multilingual sites without having to create the cache key yourself.

Cache keys also include the active language when USE_L10N is set to True and the current time zone when USE_TZ is set to True.

The Per-View Cache

A more granular way to use the caching framework is by caching the output of individual views. django.views.decorators.cache defines a cache_page decorator that will automatically cache the view’s response for you. It’s easy to use:

from django.views.decorators.cache import cache_page

@cache_page(60 * 15)
def my_view(request):
    ...

cache_page takes a single argument: the cache timeout, in seconds. In the above example, the result of the my_view() view will be cached for 15 minutes. (Note that I’ve written it as 60 * 15 for the purpose of readability. 60 * 15 will be evaluated to 900 – that is, 15 minutes multiplied by 60 seconds per minute.)

The per-view cache, like the per-site cache, is keyed off of the URL. If multiple URLs point at the same view, each URL will be cached separately. Continuing the my_view
example, if your URLconf looks like this:

urlpatterns = [
    url(r'^foo/([0-9]{1,2})/$', my_view),
]

then requests to /foo/1/ and /foo/23/ will be cached separately, as you may expect. But once a particular URL (e.g., /foo/23/) has been requested, subsequent requests to that URL will use the cache.

cache_page can also take an optional keyword argument, cache, which directs the decorator to use a specific cache (from your CACHES setting) when caching view results.

By default, the default cache will be used, but you can specify any cache you want:

@cache_page(60 * 15, cache="special_cache")
def my_view(request):
    ...

You can also override the cache prefix on a per-view basis. cache_page takes an optional keyword argument, key_prefix, which works in the same way as the CACHE_MIDDLEWARE_KEY_PREFIX setting for the middleware. It can be used like this:

@cache_page(60 * 15, key_prefix="site1")
def my_view(request):
    ...

The key_prefix and cache arguments may be specified together. The key_prefix argument and the KEY_PREFIX specified under CACHES will be concatenated.

Specifying Per-View Cache in the Urlconf

The examples in the previous section have hard-coded the fact that the view is cached, because cache_page alters the my_view function in place. This approach couples your view to the cache system, which is not ideal for several reasons. For instance, you might want to reuse the view functions on another, cache-less site, or you might want to distribute the views to people who might want to use them without being cached.

The solution to these problems is to specify the per-view cache in the URLconf rather than next to the view functions themselves. Doing so is easy: simply wrap the view function with cache_page when you refer to it in the URLconf.

Here’s the old URLconf from earlier:

urlpatterns = [
    url(r'^foo/([0-9]{1,2})/$', my_view),
]

Here’s the same thing, with my_view wrapped in cache_page:

from django.views.decorators.cache import cache_page

urlpatterns = [
    url(r'^foo/([0-9]{1,2})/$', cache_page(60 * 15)(my_view)),
]

Template Fragment Caching

If you’re after even more control, you can also cache template fragments using the cache template tag. To give your template access to this tag, put {% load cache %} near the top of your template. The {% cache %} template tag caches the contents of the block for a given amount of time.

It takes at least two arguments: the cache timeout, in seconds, and the name to give the cache fragment. The name will be taken as is, do not use a variable.

For example:

{% load cache %}
{% cache 500 sidebar %}
    .. sidebar ..
{% endcache %}

Sometimes you might want to cache multiple copies of a fragment depending on some dynamic data that appears inside the fragment.

For example, you might want a separate cached copy of the sidebar used in the previous example for every user of your site. Do this by passing additional arguments to the {% cache %} template tag to uniquely identify the cache fragment:

{% load cache %}
{% cache 500 sidebar request.user.username %}
    .. sidebar for logged in user ..
{% endcache %}

It’s perfectly fine to specify more than one argument to identify the fragment. Simply pass as many arguments to {% cache %} as you need. If USE_I18N is set to True the per-site middleware cache will respect the active language.

For the cache template tag you could use one of the translation-specific variables available in templates to achieve the same result:

{% load i18n %}
{% load cache %}

{% get_current_language as LANGUAGE_CODE %}

{% cache 600 welcome LANGUAGE_CODE %}
    {% trans "Welcome to example.com" %}
{% endcache %}

The cache timeout can be a template variable, as long as the template variable resolves to an integer value.

For example, if the template variable my_timeout is set to the value 600, then the following two examples are equivalent:

{% cache 600 sidebar %} ... {% endcache %}
{% cache my_timeout sidebar %} ... {% endcache %}

This feature is useful in avoiding repetition in templates. You can set the timeout in a variable, in one place, and just reuse that value. By default, the cache tag will try to use the cache called template_fragments. If no such cache exists, it will fall back to using the default cache. You may select an alternate cache backend to use with the using keyword argument, which must be the last argument to the tag.

{% cache 300 local-thing ...  using="localcache" %}

It is considered an error to specify a cache name that is not configured.

If you want to obtain the cache key used for a cached fragment, you can use make_template_fragment_key. fragment_name is the same as second argument to the cache template tag; vary_on is a list of all additional arguments passed to the tag. This function can be useful for invalidating or overwriting a cached item, for example:

>>> from django.core.cache import cache
>>> from django.core.cache.utils import make_template_fragment_key
# cache key for {% cache 500 sidebar username %}
>>> key = make_template_fragment_key('sidebar', [username])
>>> cache.delete(key) # invalidates cached template fragment