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Changed docs/cache to add docs for site-wide caching, via the cache middleware 

git-svn-id: http://code.djangoproject.com/svn/django/trunk@224 bcc190cf-cafb-0310-a4f2-bffc1f526a37
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Adrian Holovaty 2005-07-19 20:49:07 +00:00
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@ -4,81 +4,120 @@ Django's cache framework
So, you got slashdotted. Now what?
Django's cache framework gives you two methods of caching dynamic pages in
memory or in a database: you can automatically cache the entire page, or you can manually
cache only the pieces that are difficult to produce.
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.
Setting up the cache
====================
The cache framework is split into a set of "backends" that provide different
methods of caching data. There's a simple single-process memory cache
(mostly useful as a fallback), a database-backed cache, and a memcached_
backend (by far the fastest option if you've got the RAM).
methods of caching data. There's a simple single-process memory cache (mostly
useful as a fallback), a database-backed cache, and a memcached_ backend (by
far the fastest option 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.
like to use. Do this by setting the ``CACHE_BACKEND`` in your settings file.
The CACHE_BACKEND setting is a quasi-URI; examples are:
The CACHE_BACKEND setting is a quasi-URI. Examples::
============================== ===========================================
CACHE_BACKEND Explanation
============================== ===========================================
memcached://127.0.0.1:11211/ A memcached backend; the server is running
memcached://127.0.0.1:11211/ A memcached backend; the server is running
on localhost port 11211.
db://tablename/ A database backend (the db backend uses
db://tablename/ A database backend (the db backend uses
the same database/username as the rest of
the CMS, so only a table name is needed.)
simple:/// A simple single-process memory cache; you
probably don't want to use this except for
testing. Note that this cache backend is
testing. Note that this cache backend is
NOT threadsafe!
============================== ===========================================
All caches may take arguments; these are given in query-string style. Valid
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
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.
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
dumped when max_entries is reached. This makes culling *much* faster
at the expense of more cache misses.
For example::
DB_CACHE = "memcached://127.0.0.1:11211/?timeout=60"
DB_CACHE = "db://tablename/?timeout=120&max_entries=500&cull_percentage=4"
Invalid arguments are silently ignored, as are invalid values of known
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 simply
cache your entire site. Just add ``django.middleware.cache.CacheMiddleware``
to your ``MIDDLEWARE_CLASSES`` setting, as in this example::
MIDDLEWARE_CLASSES = (
"django.middleware.common.CommonMiddleware",
"django.middleware.cache.CacheMiddleware",
)
Then, add the following three required settings::
* ``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.
* ``CACHE_MIDDLEWARE_GZIP`` -- Either ``True`` or ``False``. If this is
enabled, Django will gzip all content for users whose browsers support gzip
encoding. Using gzip adds a level of overhead to page requests, but the
overhead generally is cancelled out by the fact that gzipped pages are stored
in the cache. That means subsequent requests won't have the overhead of
zipping, and the cache will hold more pages because each one is smaller.
Pages with GET or POST parameters won't be cached.
The cache middleware also makes a few more optimizations:
* Sets and deals with ``ETag`` headers.
* Sets the ``Content-Length`` header.
* Sets the ``Last-Modified`` header to the current date/time when a fresh
(uncached) version of the page is requested.
It doesn't matter where in the middleware stack you put the cache middleware.
The per-page cache
==================
Once the cache is set up, the simplest way to use the cache is to simply
cache entire view functions. ``django.views.decorators.cache`` defines
a ``cache_page`` decorator that will automatically cache the view's response
for you. Using it couldn't be easier::
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):
slashdot_this = cache_page(slashdot_this, 60 * 15)
Or, using Python 2.4's decorator syntax::
@ -86,54 +125,54 @@ Or, using Python 2.4's decorator syntax::
def slashdot_this(request):
...
This will cache the result of that view for 15 minutes (the cache timeout is
in seconds).
This will cache the result of that view for 15 minutes. (The cache timeout is
in seconds.)
The low-level cache API
=======================
There are times, however, that caching an entire rendered page doesn't gain
you very much. We often find that we only need to cache a list of object IDs
from an intensive database query, for example. In cases like these, you
can use the cache API to store objects in the cache with any level of
granularity you like.
you very much. The Django developers have found it's only necessary to cache a
list of object IDs from an intensive database query, for example. In cases like
these, you can use the cache API to store objects in the cache with any level
of granularity you like.
The cache API is almost shockingly simple::
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...)
# (Wait 30 seconds...)
>>> cache.get('my_key')
None
# get can take a default argument
# 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.
# 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}
# get_many returns a dict with all the keys you asked for that
# actually exist in the cache (and haven't expired)
# there's also a way to explicitly delete keys
# There's also a way to delete keys explicitly.
>>> cache.delete('a')
Really, that's the entire API! There's very few restrictions on what you can
Really, that's the entire API! There are very few restrictions on what you can
use the cache for; you can store any object in the cache that can be pickled
safely, although keys must be strings.
.. _memcached: http://www.danga.com/memcached/