========================
Django's cache framework
========================

So, you got slashdotted_. Now what?

Django's cache framework gives you three methods of caching dynamic pages in
memory or in a database. You can cache the output of entire pages, you can
cache only the pieces that are difficult to produce, or you can cache your
entire site.

.. _slashdotted: http://en.wikipedia.org/wiki/Slashdot_effect

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.

.. _memcached: http://www.danga.com/memcached/

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 three 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.

.. _`middleware documentation`: http://www.djangoproject.com/documentation/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.

.. _`HTTP Vary headers`: http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.44

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.)

.. _`Cache-Control spec`: http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.9

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``