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180 lines
6.8 KiB
Plaintext
180 lines
6.8 KiB
Plaintext
========================
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Django's cache framework
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========================
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So, you got slashdotted. Now what?
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Django's cache framework gives you three methods of caching dynamic pages in
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memory or in a database. You can cache the output of entire pages, you can
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cache only the pieces that are difficult to produce, or you can cache your
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entire site.
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Setting up the cache
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====================
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The cache framework is split into a set of "backends" that provide different
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methods of caching data. There's a simple single-process memory cache (mostly
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useful as a fallback), a database-backed cache, and a memcached_ backend (by
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far the fastest option if you've got the RAM).
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Before using the cache, you'll need to tell Django which cache backend you'd
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like to use. Do this by setting the ``CACHE_BACKEND`` in your settings file.
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The CACHE_BACKEND setting is a "fake" URI (really an unregistered scheme).
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Examples:
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============================== ===========================================
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CACHE_BACKEND Explanation
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============================== ===========================================
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memcached://127.0.0.1:11211/ A memcached backend; the server is running
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on localhost port 11211.
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db://tablename/ A database backend (the db backend uses
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the same database/username as the rest of
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the CMS, so only a table name is needed.)
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simple:/// A simple single-process memory cache; you
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probably don't want to use this except for
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testing. Note that this cache backend is
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NOT threadsafe!
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============================== ===========================================
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All caches may take arguments -- they're given in query-string style. Valid
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arguments are:
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timeout
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Default timeout, in seconds, to use for the cache. Defaults to 5
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minutes (300 seconds).
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max_entries
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For the simple and database backends, the maximum number of entries
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allowed in the cache before it is cleaned. Defaults to 300.
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cull_percentage
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The percentage of entries that are culled when max_entries is reached.
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The actual percentage is 1/cull_percentage, so set cull_percentage=3 to
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cull 1/3 of the entries when max_entries is reached.
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A value of 0 for cull_percentage means that the entire cache will be
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dumped when max_entries is reached. This makes culling *much* faster
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at the expense of more cache misses.
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For example::
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DB_CACHE = "memcached://127.0.0.1:11211/?timeout=60"
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DB_CACHE = "db://tablename/?timeout=120&max_entries=500&cull_percentage=4"
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Invalid arguments are silently ignored, as are invalid values of known
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arguments.
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The per-site cache
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==================
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Once the cache is set up, the simplest way to use the cache is to simply
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cache your entire site. Just add ``django.middleware.cache.CacheMiddleware``
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to your ``MIDDLEWARE_CLASSES`` setting, as in this example::
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MIDDLEWARE_CLASSES = (
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"django.middleware.common.CommonMiddleware",
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"django.middleware.cache.CacheMiddleware",
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)
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Then, add the following three required settings:
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* ``CACHE_MIDDLEWARE_SECONDS`` -- The number of seconds each page should be
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cached.
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* ``CACHE_MIDDLEWARE_KEY_PREFIX`` -- If the cache is shared across multiple
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sites using the same Django installation, set this to the name of the site,
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or some other string that is unique to this Django instance, to prevent key
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collisions. Use an empty string if you don't care.
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* ``CACHE_MIDDLEWARE_GZIP`` -- Either ``True`` or ``False``. If this is
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enabled, Django will gzip all content for users whose browsers support gzip
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encoding. Using gzip adds a level of overhead to page requests, but the
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overhead generally is cancelled out by the fact that gzipped pages are stored
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in the cache. That means subsequent requests won't have the overhead of
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zipping, and the cache will hold more pages because each one is smaller.
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Pages with GET or POST parameters won't be cached.
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The cache middleware also makes a few more optimizations:
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* Sets and deals with ``ETag`` headers.
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* Sets the ``Content-Length`` header.
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* Sets the ``Last-Modified`` header to the current date/time when a fresh
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(uncached) version of the page is requested.
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It doesn't matter where in the middleware stack you put the cache middleware.
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The per-page cache
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==================
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A more granular way to use the caching framework is by caching the output of
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individual views. ``django.views.decorators.cache`` defines a ``cache_page``
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decorator that will automatically cache the view's response for you. It's easy
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to use::
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from django.views.decorators.cache import cache_page
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def slashdot_this(request):
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...
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slashdot_this = cache_page(slashdot_this, 60 * 15)
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Or, using Python 2.4's decorator syntax::
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@cache_page(60 * 15)
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def slashdot_this(request):
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...
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This will cache the result of that view for 15 minutes. (The cache timeout is
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in seconds.)
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The low-level cache API
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=======================
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There are times, however, that caching an entire rendered page doesn't gain
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you very much. The Django developers have found it's only necessary to cache a
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list of object IDs from an intensive database query, for example. In cases like
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these, you can use the cache API to store objects in the cache with any level
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of granularity you like.
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The cache API is simple::
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# the cache module exports a cache object that's automatically
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# created from the CACHE_BACKEND setting
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>>> from django.core.cache import cache
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# The basic interface is set(key, value, timeout_seconds) and get(key)
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>>> cache.set('my_key', 'hello, world!', 30)
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>>> cache.get('my_key')
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'hello, world!'
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# (Wait 30 seconds...)
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>>> cache.get('my_key')
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None
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# get() can take a default argument
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>>> cache.get('my_key', 'has_expired')
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'has_expired'
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# There's also a get_many() interface that only hits the cache once.
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# Also, note that the timeout argument is optional and defaults to what
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# you've given in the settings file.
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>>> cache.set('a', 1)
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>>> cache.set('b', 2)
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>>> cache.set('c', 3)
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# get_many() returns a dictionary with all the keys you asked for that
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# actually exist in the cache (and haven't expired).
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>>> cache.get_many(['a', 'b', 'c'])
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{'a': 1, 'b': 2, 'c': 3}
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# There's also a way to delete keys explicitly.
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>>> cache.delete('a')
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Really, that's the entire API! There are very few restrictions on what you can
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use the cache for; you can store any object in the cache that can be pickled
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safely, although keys must be strings.
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.. _memcached: http://www.danga.com/memcached/
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