Setting up the cache

The cache framework allows for different "backends" -- different methods of caching data. There's a simple single-process memory cache (mostly useful as a fallback) and a memcached backend (the fastest option, by far, if you've got the RAM).

Before using the cache, you'll need to tell Django which cache backend you'd like to use. Do this by setting the CACHE_BACKEND in your settings file.

The CACHE_BACKEND setting is a "fake" URI (really an unregistered scheme). Examples:

CACHE_BACKEND	Explanation
memcached://127.0.0.1:11211/	A memcached backend; the server is running on localhost port 11211. You can use multiple memcached servers by separating them with semicolons.
db://tablename/	A database backend in a table named "tablename". This table should be created with "django-admin createcachetable".
file:///var/tmp/django_cache/	A file-based cache stored in the directory /var/tmp/django_cache/.
simple:///	A simple single-process memory cache; you probably don't want to use this except for testing. Note that this cache backend is NOT thread-safe!
locmem:///	A more sophisticated local memory cache; this is multi-process- and thread-safe.

All caches may take arguments -- they're given in query-string style. Valid arguments are:

timeout

Default timeout, in seconds, to use for the cache. Defaults to 5 minutes (300 seconds).

max_entries

For the simple and database backends, the maximum number of entries allowed in the cache before it is cleaned. Defaults to 300.

cull_percentage

The percentage of entries that are culled when max_entries is reached. The actual percentage is 1/cull_percentage, so set cull_percentage=3 to cull 1/3 of the entries when max_entries is reached.

A value of 0 for cull_percentage means that the entire cache will be dumped when max_entries is reached. This makes culling much faster at the expense of more cache misses.

For example:

CACHE_BACKEND = "memcached://127.0.0.1:11211/?timeout=60"

Invalid arguments are silently ignored, as are invalid values of known arguments.

The per-site cache

Once the cache is set up, the simplest way to use the cache is to cache your entire site. Just add django.middleware.cache.CacheMiddleware to your MIDDLEWARE_CLASSES setting, as in this example:

MIDDLEWARE_CLASSES = (
    "django.middleware.cache.CacheMiddleware",
    "django.middleware.common.CommonMiddleware",
)

(The order of MIDDLEWARE_CLASSES matters. See "Order of MIDDLEWARE_CLASSES" below.)

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

• 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.

The cache middleware caches every page that doesn't have GET or POST parameters. Additionally, CacheMiddleware automatically sets a few headers in each HttpResponse:

• Sets the Last-Modified header to the current date/time when a fresh (uncached) 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.

See the middleware documentation for more on middleware.

The per-page 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

def slashdot_this(request):
    ...

slashdot_this = cache_page(slashdot_this, 60 * 15)

Or, using Python 2.4's decorator syntax:

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

cache_page takes a single argument: the cache timeout, in seconds. In the above example, the result of the slashdot_this() view will be cached for 15 minutes.

The low-level cache API

Sometimes, however, caching an entire rendered page doesn't gain you very much. For example, you may find it's only necessary to cache the result of an intensive database. In cases like this, you can use the low-level cache API to store objects in the cache with any level of granularity you like.

The cache API is simple:

# The cache module exports a cache object that's automatically
# created from the CACHE_BACKEND setting.
>>> from django.core.cache import cache

# The basic interface is set(key, value, timeout_seconds) and get(key).
>>> cache.set('my_key', 'hello, world!', 30)
>>> cache.get('my_key')
'hello, world!'

# (Wait 30 seconds...)
>>> cache.get('my_key')
None

# get() can take a default argument.
>>> cache.get('my_key', 'has_expired')
'has_expired'

# There's also a get_many() interface that only hits the cache once.
# Also, note that the timeout argument is optional and defaults to what
# you've given in the settings file.
>>> cache.set('a', 1)
>>> cache.set('b', 2)
>>> cache.set('c', 3)

# get_many() returns a dictionary with all the keys you asked for that
# actually exist in the cache (and haven't expired).
>>> cache.get_many(['a', 'b', 'c'])
{'a': 1, 'b': 2, 'c': 3}

# There's also a way to delete keys explicitly.
>>> cache.delete('a')

That's it. The cache has very few restrictions: You can cache any object that can be pickled safely, although keys must be strings.

Controlling cache: Using Vary headers

The Django cache framework works with HTTP Vary headers to allow developers to instruct caching mechanisms to differ their cache contents depending on request HTTP headers.

Essentially, the Vary response HTTP header defines which request headers a cache mechanism should take into account when building its cache key.

By default, Django's cache system creates its cache keys using the requested path -- e.g., "/stories/2005/jun/23/bank_robbed/". This means every request to that URL will use the same cached version, regardless of user-agent differences such as cookies or language preferences.

That's where Vary comes in.

If your Django-powered page outputs different content based on some difference in request headers -- such as a cookie, or language, or user-agent -- you'll need to use the Vary header to tell caching mechanisms that the page output depends on those things.

To do this in Django, use the convenient vary_on_headers view decorator, like so:

from django.views.decorators.vary import vary_on_headers

# Python 2.3 syntax.
def my_view(request):
    ...
my_view = vary_on_headers(my_view, 'User-Agent')

# Python 2.4 decorator syntax.
@vary_on_headers('User-Agent')
def my_view(request):
    ...

In this case, a caching mechanism (such as Django's own cache middleware) will cache a separate version of the page for each unique user-agent.

The advantage to using the vary_on_headers decorator rather than manually setting the Vary header (using something like response['Vary'] = 'user-agent') is that the decorator adds to the Vary header (which may already exist) rather than setting it from scratch.

Note that you can pass multiple headers to vary_on_headers():

@vary_on_headers('User-Agent', 'Cookie')
def my_view(request):
    ...

Because varying on cookie is such a common case, there's a vary_on_cookie decorator. These two views are equivalent:

@vary_on_cookie
def my_view(request):
    ...

@vary_on_headers('Cookie')
def my_view(request):
    ...

Also note that the headers you pass to vary_on_headers are not case sensitive. "User-Agent" is the same thing as "user-agent".

You can also use a helper function, patch_vary_headers(), directly:

from django.utils.cache import patch_vary_headers
def my_view(request):
    ...
    response = render_to_response('template_name', context)
    patch_vary_headers(response, ['Cookie'])
    return response

patch_vary_headers takes an HttpResponse instance as its first argument and a list/tuple of header names as its second argument.

Controlling cache: Using other headers

Another problem with caching is the privacy of data and the question of where data should be stored in a cascade of caches.

A user usually faces two kinds of caches: his own browser cache (a private cache) and his provider's cache (a public cache). A public cache is used by multiple users and controlled by someone else. This poses problems with sensitive data: You don't want, say, your banking-account number stored in a public cache. So Web applications need a way to tell caches which data is private and which is public.

The solution is to indicate a page's cache should be "private." To do this in Django, use the cache_control view decorator. Example:

from django.views.decorators.cache import cache_control
@cache_control(private=True)
def my_view(request):
    ...

This decorator takes care of sending out the appropriate HTTP header behind the scenes.

There are a few other ways to control cache parameters. For example, HTTP allows applications to do the following:

• Define the maximum time a page should be cached.
• Specify whether a cache should always check for newer versions, only delivering the cached content when there are no changes. (Some caches might deliver cached content even if the server page changed -- simply because the cache copy isn't yet expired.)

In Django, use the cache_control view decorator to specify these cache parameters. In this example, cache_control tells caches to revalidate the cache on every access and to store cached versions for, at most, 3600 seconds:

from django.views.decorators.cache import cache_control
@cache_control(must_revalidate=True, max_age=3600)
def my_view(request):
    ...

Any valid Cache-Control directive is valid in cache_control(). For a full list, see the Cache-Control spec. Just pass the directives as keyword arguments to cache_control(), substituting underscores for hyphens. For directives that don't take an argument, set the argument to True.

Examples:

• @cache_control(max_age=3600) turns into max-age=3600.
• @cache_control(public=True) turns into public.

(Note that the caching middleware already sets the cache header's max-age with the value of the CACHE_MIDDLEWARE_SETTINGS setting. If you use a custom max_age in a cache_control decorator, the decorator will take precedence, and the header values will be merged correctly.)

Other optimizations

Django comes with a few other pieces of middleware that can help optimize your apps' performance:

• django.middleware.http.ConditionalGetMiddleware adds support for conditional GET. This makes use of ETag and Last-Modified headers.
• django.middleware.gzip.GZipMiddleware compresses content for browsers that understand gzip compression (all modern browsers).

Order of MIDDLEWARE_CLASSES

If you use CacheMiddleware, it's important to put it in the right place within the MIDDLEWARE_CLASSES setting, because the cache middleware needs to know which headers by which to vary the cache storage. Middleware always adds something the Vary response header when it can.

Put the CacheMiddleware after any middlewares that might add something to the Vary header. The following middlewares do so:

• SessionMiddleware adds Cookie
• GZipMiddleware adds Accept-Encoding