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140 lines
5.3 KiB
Plaintext
140 lines
5.3 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 two methods of caching dynamic pages in
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memory or in a database: you can automatically cache the entire page, or you can manually
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cache only the pieces that are difficult to produce.
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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
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(mostly useful as a fallback), a database-backed cache, and a memcached_
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backend (by 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 quasi-URI; examples are:
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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; these are 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
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to 5 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-page 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 entire view functions. ``django.views.decorators.cache`` defines
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a ``cache_page`` decorator that will automatically cache the view's response
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for you. Using it couldn't be easier::
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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. We often find that we only need to cache a list of object IDs
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from an intensive database query, for example. In cases like these, you
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can use the cache API to store objects in the cache with any level of
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granularity you like.
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The cache API is almost shockingly 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
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# to what 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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>>> cache.get_many(['a', 'b', 'c'])
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{'a': 1, 'b': 2, 'c': 3}
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# get_many returns a dict 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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# there's also a way to explicitly delete keys
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>>> cache.delete('a')
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Really, that's the entire API! There's 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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