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

A fundamental tradeoff 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 create the
page that your site's visitor sees. 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 "upstream" caches, such as Squid
(http://www.squid-cache.org/) 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; yes, some cache types are faster than others.

Your cache preference goes in the ``CACHE_BACKEND`` setting in your settings
file. Here's an explanation of all available values for CACHE_BACKEND.

Memcached
---------

By far the fastest, most efficient type of cache available to Django, Memcached
is an entirely memory-based cache framework originally developed to handle high
loads at LiveJournal.com and subsequently open-sourced by Danga Interactive.
It's used by sites such as Slashdot and Wikipedia to reduce database access and
dramatically increase site performance.

Memcached is available for free at http://danga.com/memcached/ . It runs as a
daemon and is allotted a specified amount of RAM. All it does is provide an
interface -- a *super-lightning-fast* interface -- for adding, retrieving and
deleting arbitrary 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 the Memcached Python
bindings. Two versions of this are available. Choose and install *one* of the
following modules:

    * The fastest available option is a module called ``cmemcache``, available
      at http://gijsbert.org/cmemcache/ . (This module is only compatible with
      the Django development version. Django 0.96 is only compatible with the
      second option, below.)

    * If you can't install ``cmemcache``, you can install ``python-memcached``,
      available at ftp://ftp.tummy.com/pub/python-memcached/ . If that URL is
      no longer valid, just go to the Memcached Web site
      (http://www.danga.com/memcached/) and get the Python bindings from the
      "Client APIs" section.

To use Memcached with Django, set ``CACHE_BACKEND`` to
``memcached://ip:port/``, where ``ip`` is the IP address of the Memcached
daemon and ``port`` is the port on which Memcached is running.

In this example, Memcached is running on localhost (127.0.0.1) port 11211::

    CACHE_BACKEND = 'memcached://127.0.0.1:11211/'

One excellent feature of Memcached is its ability to share cache over multiple
servers. To take advantage of this feature, include all server addresses in
``CACHE_BACKEND``, separated by semicolons. 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::

    CACHE_BACKEND = 'memcached://172.19.26.240:11211;172.19.26.242:11211/'

Memory-based caching has one 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. Actually, 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
----------------

To use a database table as your cache backend, first create a cache table in
your database by running this command::

    python manage.py createcachetable [cache_table_name]

...where ``[cache_table_name]`` is the name of the database table to create.
(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.) This command creates a single table
in your database that is in the proper format that Django's database-cache
system expects.

Once you've created that database table, set your ``CACHE_BACKEND`` setting to
``"db://tablename/"``, where ``tablename`` is the name of the database table.
In this example, the cache table's name is ``my_cache_table``:

    CACHE_BACKEND = 'db://my_cache_table'

Database caching works best if you've got a fast, well-indexed database server.

Filesystem caching
------------------

To store cached items on a filesystem, use the ``"file://"`` cache type for
``CACHE_BACKEND``. For example, to store cached data in ``/var/tmp/django_cache``,
use this setting::

    CACHE_BACKEND = 'file:///var/tmp/django_cache'

Note that there are three forward slashes toward the beginning of that example.
The first two are for ``file://``, and the third is the first character of the
directory path, ``/var/tmp/django_cache``.

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
--------------------

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. This
cache is multi-process and thread-safe. To use it, set ``CACHE_BACKEND`` to
``"locmem:///"``. For example::

    CACHE_BACKEND = 'locmem:///'

Simple caching (for development)
--------------------------------

A simple, single-process memory cache is available as ``"simple:///"``. This
merely saves cached data in-process, which means it should only be used in
development or testing environments. For example::

    CACHE_BACKEND = 'simple:///'

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 on which you don't want to
cache. As a result, your development environment won't use caching and your
production environment still will. To activate dummy caching, set
``CACHE_BACKEND`` like so::

    CACHE_BACKEND = 'dummy:///'

CACHE_BACKEND arguments
-----------------------

All caches may take arguments. They're given in query-string style on the
``CACHE_BACKEND`` setting. 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.

In this example, ``timeout`` is set to ``60``::

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

In this example, ``timeout`` is ``30`` and ``max_entries`` is ``400``::

    CACHE_BACKEND = "memcached://127.0.0.1:11211/?timeout=30&max_entries=400"

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 caching 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. Optionally, if the ``CACHE_MIDDLEWARE_ANONYMOUS_ONLY`` setting is
``True``, only anonymous requests (i.e., not those made by a logged-in user)
will be cached. This is a simple and effective way of disabling caching for any
user-specific pages (include Django's admin interface). Note that if you use
``CACHE_MIDDLEWARE_ANONYMOUS_ONLY``, you should make sure you've activated
``AuthenticationMiddleware`` and that ``AuthenticationMiddleware`` appears
before ``CacheMiddleware`` in your ``MIDDLEWARE_CLASSES``.

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`: ../middleware/

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

    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 query. 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, ``django.core.cache``, 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!'

The ``timeout_seconds`` argument is optional and defaults to the ``timeout``
argument in the ``CACHE_BACKEND`` setting (explained above).

If the object doesn't exist in the cache, ``cache.get()`` returns ``None``::

    >>> cache.get('some_other_key')
    None

    # Wait 30 seconds for 'my_key' to expire...

    >>> 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. get_many()
returns a dictionary with all the keys you asked for that actually exist in the
cache (and haven't expired)::

    >>> cache.set('a', 1)
    >>> cache.set('b', 2)
    >>> cache.set('c', 3)
    >>> cache.get_many(['a', 'b', 'c'])
    {'a': 1, 'b': 2, 'c': 3}

Finally, you can delete keys explicitly with ``delete()``. This is an easy way
of clearing the cache for a particular object::

    >>> 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.

Upstream caches
===============

So far, this document has focused on caching your *own* data. But another type
of caching is relevant to Web development, too: caching performed by "upstream"
caches. These are systems that cache pages for users even before the request
reaches your Web site.

Here are a few examples of upstream caches:

    * Your ISP may cache certain pages, so if you requested a page from
      somedomain.com, your ISP would send you the page without having to access
      somedomain.com directly.

    * Your Django Web site may sit behind a Squid Web proxy
      (http://www.squid-cache.org/) that caches pages for performance. In this
      case, each request first would be handled by Squid, and it'd only be
      passed to your application if needed.

    * Your Web browser caches pages, too. If a Web page sends out the right
      headers, your browser will use the local (cached) copy for subsequent
      requests to that page.

Upstream caching is a nice efficiency boost, but there's a danger to it:
Many Web pages' contents differ based on authentication and a host of other
variables, and cache systems that blindly save pages based purely on URLs could
expose incorrect or sensitive data to subsequent visitors to those pages.

For example, say you operate a Web e-mail system, and the contents of the
"inbox" page obviously depend on which user is logged in. If an ISP blindly
cached your site, then the first user who logged in through that ISP would have
his user-specific inbox page cached for subsequent visitors to the site. That's
not cool.

Fortunately, HTTP provides a solution to this problem: A set of HTTP headers
exist to instruct caching mechanisms to differ their cache contents depending
on designated variables, and to tell caching mechanisms not to cache particular
pages.

Using Vary headers
==================

One of these headers is ``Vary``. It defines which request headers a cache
mechanism should take into account when building its cache key. For example, if
the contents of a Web page depend on a user's language preference, the page is
said to "vary on language."

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.

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, ``django.utils.cache.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.

For more on Vary headers, see the `official Vary spec`_.

.. _`official Vary spec`: 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`` HTTP directive is valid in ``cache_control()``.
Here's a full list:

    * ``public=True``
    * ``private=True``
    * ``no_cache=True``
    * ``no_transform=True``
    * ``must_revalidate=True``
    * ``proxy_revalidate=True``
    * ``max_age=num_seconds``
    * ``s_maxage=num_seconds``

For explanation of Cache-Control HTTP directives, see the `Cache-Control spec`_.

(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.)

If you want to use headers to disable caching altogether, 
``django.views.decorators.never_cache`` is a view decorator that adds
headers to ensure the response won't be cached by browsers or other caches. Example::

    from django.views.decorators.cache import never_cache
    @never_cache
    def myview(request):
        ...

.. _`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 to 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``
