1 ========================
2 Django's cache framework
3 ========================
5 A fundamental tradeoff in dynamic Web sites is, well, they're dynamic. Each
6 time a user requests a page, the Web server makes all sorts of calculations --
7 from database queries to template rendering to business logic -- to create the
8 page that your site's visitor sees. This is a lot more expensive, from a
9 processing-overhead perspective, than your standard read-a-file-off-the-filesystem
12 For most Web applications, this overhead isn't a big deal. Most Web
13 applications aren't washingtonpost.com or slashdot.org; they're simply small-
14 to medium-sized sites with so-so traffic. But for medium- to high-traffic
15 sites, it's essential to cut as much overhead as possible.
17 That's where caching comes in.
19 To cache something is to save the result of an expensive calculation so that
20 you don't have to perform the calculation next time. Here's some pseudocode
21 explaining how this would work for a dynamically generated Web page::
23 given a URL, try finding that page in the cache
24 if the page is in the cache:
25 return the cached page
28 save the generated page in the cache (for next time)
29 return the generated page
31 Django comes with a robust cache system that lets you save dynamic pages so
32 they don't have to be calculated for each request. For convenience, Django
33 offers different levels of cache granularity: You can cache the output of
34 specific views, you can cache only the pieces that are difficult to produce, or
35 you can cache your entire site.
37 Django also works well with "upstream" caches, such as Squid
38 (http://www.squid-cache.org/) and browser-based caches. These are the types of
39 caches that you don't directly control but to which you can provide hints (via
40 HTTP headers) about which parts of your site should be cached, and how.
45 The cache system requires a small amount of setup. Namely, you have to tell it
46 where your cached data should live -- whether in a database, on the filesystem
47 or directly in memory. This is an important decision that affects your cache's
48 performance; yes, some cache types are faster than others.
50 Your cache preference goes in the ``CACHE_BACKEND`` setting in your settings
51 file. Here's an explanation of all available values for CACHE_BACKEND.
56 By far the fastest, most efficient type of cache available to Django, Memcached
57 is an entirely memory-based cache framework originally developed to handle high
58 loads at LiveJournal.com and subsequently open-sourced by Danga Interactive.
59 It's used by sites such as Slashdot and Wikipedia to reduce database access and
60 dramatically increase site performance.
62 Memcached is available for free at http://danga.com/memcached/ . It runs as a
63 daemon and is allotted a specified amount of RAM. All it does is provide an
64 interface -- a *super-lightning-fast* interface -- for adding, retrieving and
65 deleting arbitrary data in the cache. All data is stored directly in memory,
66 so there's no overhead of database or filesystem usage.
68 After installing Memcached itself, you'll need to install the Memcached Python
69 bindings. They're in a single Python module, memcache.py, available at
70 ftp://ftp.tummy.com/pub/python-memcached/ . If that URL is no longer valid,
71 just go to the Memcached Web site (http://www.danga.com/memcached/) and get the
72 Python bindings from the "Client APIs" section.
74 To use Memcached with Django, set ``CACHE_BACKEND`` to
75 ``memcached://ip:port/``, where ``ip`` is the IP address of the Memcached
76 daemon and ``port`` is the port on which Memcached is running.
78 In this example, Memcached is running on localhost (127.0.0.1) port 11211::
80 CACHE_BACKEND = 'memcached://127.0.0.1:11211/'
82 One excellent feature of Memcached is its ability to share cache over multiple
83 servers. To take advantage of this feature, include all server addresses in
84 ``CACHE_BACKEND``, separated by semicolons. In this example, the cache is
85 shared over Memcached instances running on IP address 172.19.26.240 and
86 172.19.26.242, both on port 11211::
88 CACHE_BACKEND = 'memcached://172.19.26.240:11211;172.19.26.242:11211/'
90 Memory-based caching has one disadvantage: Because the cached data is stored in
91 memory, the data will be lost if your server crashes. Clearly, memory isn't
92 intended for permanent data storage, so don't rely on memory-based caching as
93 your only data storage. Actually, none of the Django caching backends should be
94 used for permanent storage -- they're all intended to be solutions for caching,
95 not storage -- but we point this out here because memory-based caching is
96 particularly temporary.
101 To use a database table as your cache backend, first create a cache table in
102 your database by running this command::
104 python manage.py createcachetable [cache_table_name]
106 ...where ``[cache_table_name]`` is the name of the database table to create.
107 (This name can be whatever you want, as long as it's a valid table name that's
108 not already being used in your database.) This command creates a single table
109 in your database that is in the proper format that Django's database-cache
112 Once you've created that database table, set your ``CACHE_BACKEND`` setting to
113 ``"db://tablename/"``, where ``tablename`` is the name of the database table.
114 In this example, the cache table's name is ``my_cache_table``:
116 CACHE_BACKEND = 'db://my_cache_table'
118 Database caching works best if you've got a fast, well-indexed database server.
123 To store cached items on a filesystem, use the ``"file://"`` cache type for
124 ``CACHE_BACKEND``. For example, to store cached data in ``/var/tmp/django_cache``,
127 CACHE_BACKEND = 'file:///var/tmp/django_cache'
129 Note that there are three forward slashes toward the beginning of that example.
130 The first two are for ``file://``, and the third is the first character of the
131 directory path, ``/var/tmp/django_cache``.
133 The directory path should be absolute -- that is, it should start at the root
134 of your filesystem. It doesn't matter whether you put a slash at the end of the
137 Make sure the directory pointed-to by this setting exists and is readable and
138 writable by the system user under which your Web server runs. Continuing the
139 above example, if your server runs as the user ``apache``, make sure the
140 directory ``/var/tmp/django_cache`` exists and is readable and writable by the
146 If you want the speed advantages of in-memory caching but don't have the
147 capability of running Memcached, consider the local-memory cache backend. This
148 cache is multi-process and thread-safe. To use it, set ``CACHE_BACKEND`` to
149 ``"locmem:///"``. For example::
151 CACHE_BACKEND = 'locmem:///'
153 Simple caching (for development)
154 --------------------------------
156 A simple, single-process memory cache is available as ``"simple:///"``. This
157 merely saves cached data in-process, which means it should only be used in
158 development or testing environments. For example::
160 CACHE_BACKEND = 'simple:///'
162 Dummy caching (for development)
163 -------------------------------
165 Finally, Django comes with a "dummy" cache that doesn't actually cache -- it
166 just implements the cache interface without doing anything.
168 This is useful if you have a production site that uses heavy-duty caching in
169 various places but a development/test environment on which you don't want to
170 cache. In that case, set ``CACHE_BACKEND`` to ``"dummy:///"`` in the settings
171 file for your development environment. As a result, your development
172 environment won't use caching and your production environment still will.
174 CACHE_BACKEND arguments
175 -----------------------
177 All caches may take arguments. They're given in query-string style on the
178 ``CACHE_BACKEND`` setting. Valid arguments are:
181 Default timeout, in seconds, to use for the cache. Defaults to 5
182 minutes (300 seconds).
185 For the simple and database backends, the maximum number of entries
186 allowed in the cache before it is cleaned. Defaults to 300.
189 The percentage of entries that are culled when max_entries is reached.
190 The actual percentage is 1/cull_percentage, so set cull_percentage=3 to
191 cull 1/3 of the entries when max_entries is reached.
193 A value of 0 for cull_percentage means that the entire cache will be
194 dumped when max_entries is reached. This makes culling *much* faster
195 at the expense of more cache misses.
197 In this example, ``timeout`` is set to ``60``::
199 CACHE_BACKEND = "memcached://127.0.0.1:11211/?timeout=60"
201 In this example, ``timeout`` is ``30`` and ``max_entries`` is ``400``::
203 CACHE_BACKEND = "memcached://127.0.0.1:11211/?timeout=30&max_entries=400"
205 Invalid arguments are silently ignored, as are invalid values of known
211 Once the cache is set up, the simplest way to use caching is to cache your
212 entire site. Just add ``'django.middleware.cache.CacheMiddleware'`` to your
213 ``MIDDLEWARE_CLASSES`` setting, as in this example::
215 MIDDLEWARE_CLASSES = (
216 'django.middleware.cache.CacheMiddleware',
217 'django.middleware.common.CommonMiddleware',
220 (The order of ``MIDDLEWARE_CLASSES`` matters. See "Order of MIDDLEWARE_CLASSES"
223 Then, add the following required settings to your Django settings file:
225 * ``CACHE_MIDDLEWARE_SECONDS`` -- The number of seconds each page should be
227 * ``CACHE_MIDDLEWARE_KEY_PREFIX`` -- If the cache is shared across multiple
228 sites using the same Django installation, set this to the name of the site,
229 or some other string that is unique to this Django instance, to prevent key
230 collisions. Use an empty string if you don't care.
232 The cache middleware caches every page that doesn't have GET or POST
233 parameters. Optionally, if the ``CACHE_MIDDLEWARE_ANONYMOUS_ONLY`` setting is
234 ``True``, only anonymous requests (i.e., not those made by a logged-in user)
235 will be cached. This is a simple and effective way of disabling caching for any
236 user-specific pages (include Django's admin interface). Note that if you use
237 ``CACHE_MIDDLEWARE_ANONYMOUS_ONLY``, you should make sure you've activated
238 ``AuthenticationMiddleware`` and that ``AuthenticationMiddleware`` appears
239 before ``CacheMiddleware`` in your ``MIDDLEWARE_CLASSES``.
241 Additionally, ``CacheMiddleware`` automatically sets a few headers in each
244 * Sets the ``Last-Modified`` header to the current date/time when a fresh
245 (uncached) version of the page is requested.
246 * Sets the ``Expires`` header to the current date/time plus the defined
247 ``CACHE_MIDDLEWARE_SECONDS``.
248 * Sets the ``Cache-Control`` header to give a max age for the page -- again,
249 from the ``CACHE_MIDDLEWARE_SECONDS`` setting.
251 See the `middleware documentation`_ for more on middleware.
253 .. _`middleware documentation`: ../middleware/
258 A more granular way to use the caching framework is by caching the output of
259 individual views. ``django.views.decorators.cache`` defines a ``cache_page``
260 decorator that will automatically cache the view's response for you. It's easy
263 from django.views.decorators.cache import cache_page
265 def slashdot_this(request):
268 slashdot_this = cache_page(slashdot_this, 60 * 15)
270 Or, using Python 2.4's decorator syntax::
273 def slashdot_this(request):
276 ``cache_page`` takes a single argument: the cache timeout, in seconds. In the
277 above example, the result of the ``slashdot_this()`` view will be cached for 15
280 The low-level cache API
281 =======================
283 Sometimes, however, caching an entire rendered page doesn't gain you very much.
284 For example, you may find it's only necessary to cache the result of an
285 intensive database query. In cases like this, you can use the low-level cache
286 API to store objects in the cache with any level of granularity you like.
288 The cache API is simple. The cache module, ``django.core.cache``, exports a
289 ``cache`` object that's automatically created from the ``CACHE_BACKEND``
292 >>> from django.core.cache import cache
294 The basic interface is ``set(key, value, timeout_seconds)`` and ``get(key)``::
296 >>> cache.set('my_key', 'hello, world!', 30)
297 >>> cache.get('my_key')
300 The ``timeout_seconds`` argument is optional and defaults to the ``timeout``
301 argument in the ``CACHE_BACKEND`` setting (explained above).
303 If the object doesn't exist in the cache, ``cache.get()`` returns ``None``::
305 >>> cache.get('some_other_key')
308 # Wait 30 seconds for 'my_key' to expire...
310 >>> cache.get('my_key')
313 get() can take a ``default`` argument::
315 >>> cache.get('my_key', 'has expired')
318 There's also a get_many() interface that only hits the cache once. get_many()
319 returns a dictionary with all the keys you asked for that actually exist in the
320 cache (and haven't expired)::
322 >>> cache.set('a', 1)
323 >>> cache.set('b', 2)
324 >>> cache.set('c', 3)
325 >>> cache.get_many(['a', 'b', 'c'])
326 {'a': 1, 'b': 2, 'c': 3}
328 Finally, you can delete keys explicitly with ``delete()``. This is an easy way
329 of clearing the cache for a particular object::
331 >>> cache.delete('a')
333 That's it. The cache has very few restrictions: You can cache any object that
334 can be pickled safely, although keys must be strings.
339 So far, this document has focused on caching your *own* data. But another type
340 of caching is relevant to Web development, too: caching performed by "upstream"
341 caches. These are systems that cache pages for users even before the request
342 reaches your Web site.
344 Here are a few examples of upstream caches:
346 * Your ISP may cache certain pages, so if you requested a page from
347 somedomain.com, your ISP would send you the page without having to access
348 somedomain.com directly.
350 * Your Django Web site may sit behind a Squid Web proxy
351 (http://www.squid-cache.org/) that caches pages for performance. In this
352 case, each request first would be handled by Squid, and it'd only be
353 passed to your application if needed.
355 * Your Web browser caches pages, too. If a Web page sends out the right
356 headers, your browser will use the local (cached) copy for subsequent
357 requests to that page.
359 Upstream caching is a nice efficiency boost, but there's a danger to it:
360 Many Web pages' contents differ based on authentication and a host of other
361 variables, and cache systems that blindly save pages based purely on URLs could
362 expose incorrect or sensitive data to subsequent visitors to those pages.
364 For example, say you operate a Web e-mail system, and the contents of the
365 "inbox" page obviously depend on which user is logged in. If an ISP blindly
366 cached your site, then the first user who logged in through that ISP would have
367 his user-specific inbox page cached for subsequent visitors to the site. That's
370 Fortunately, HTTP provides a solution to this problem: A set of HTTP headers
371 exist to instruct caching mechanisms to differ their cache contents depending
372 on designated variables, and to tell caching mechanisms not to cache particular
378 One of these headers is ``Vary``. It defines which request headers a cache
379 mechanism should take into account when building its cache key. For example, if
380 the contents of a Web page depend on a user's language preference, the page is
381 said to "vary on language."
383 By default, Django's cache system creates its cache keys using the requested
384 path -- e.g., ``"/stories/2005/jun/23/bank_robbed/"``. This means every request
385 to that URL will use the same cached version, regardless of user-agent
386 differences such as cookies or language preferences.
388 That's where ``Vary`` comes in.
390 If your Django-powered page outputs different content based on some difference
391 in request headers -- such as a cookie, or language, or user-agent -- you'll
392 need to use the ``Vary`` header to tell caching mechanisms that the page output
393 depends on those things.
395 To do this in Django, use the convenient ``vary_on_headers`` view decorator,
398 from django.views.decorators.vary import vary_on_headers
401 def my_view(request):
403 my_view = vary_on_headers(my_view, 'User-Agent')
405 # Python 2.4 decorator syntax.
406 @vary_on_headers('User-Agent')
407 def my_view(request):
410 In this case, a caching mechanism (such as Django's own cache middleware) will
411 cache a separate version of the page for each unique user-agent.
413 The advantage to using the ``vary_on_headers`` decorator rather than manually
414 setting the ``Vary`` header (using something like
415 ``response['Vary'] = 'user-agent'``) is that the decorator adds to the ``Vary``
416 header (which may already exist) rather than setting it from scratch.
418 You can pass multiple headers to ``vary_on_headers()``::
420 @vary_on_headers('User-Agent', 'Cookie')
421 def my_view(request):
424 Because varying on cookie is such a common case, there's a ``vary_on_cookie``
425 decorator. These two views are equivalent::
428 def my_view(request):
431 @vary_on_headers('Cookie')
432 def my_view(request):
435 Also note that the headers you pass to ``vary_on_headers`` are not case
436 sensitive. ``"User-Agent"`` is the same thing as ``"user-agent"``.
438 You can also use a helper function, ``django.utils.cache.patch_vary_headers``,
441 from django.utils.cache import patch_vary_headers
442 def my_view(request):
444 response = render_to_response('template_name', context)
445 patch_vary_headers(response, ['Cookie'])
448 ``patch_vary_headers`` takes an ``HttpResponse`` instance as its first argument
449 and a list/tuple of header names as its second argument.
451 For more on Vary headers, see the `official Vary spec`_.
453 .. _`official Vary spec`: http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.44
455 Controlling cache: Using other headers
456 ======================================
458 Another problem with caching is the privacy of data and the question of where
459 data should be stored in a cascade of caches.
461 A user usually faces two kinds of caches: his own browser cache (a private
462 cache) and his provider's cache (a public cache). A public cache is used by
463 multiple users and controlled by someone else. This poses problems with
464 sensitive data: You don't want, say, your banking-account number stored in a
465 public cache. So Web applications need a way to tell caches which data is
466 private and which is public.
468 The solution is to indicate a page's cache should be "private." To do this in
469 Django, use the ``cache_control`` view decorator. Example::
471 from django.views.decorators.cache import cache_control
472 @cache_control(private=True)
473 def my_view(request):
476 This decorator takes care of sending out the appropriate HTTP header behind the
479 There are a few other ways to control cache parameters. For example, HTTP
480 allows applications to do the following:
482 * Define the maximum time a page should be cached.
483 * Specify whether a cache should always check for newer versions, only
484 delivering the cached content when there are no changes. (Some caches
485 might deliver cached content even if the server page changed -- simply
486 because the cache copy isn't yet expired.)
488 In Django, use the ``cache_control`` view decorator to specify these cache
489 parameters. In this example, ``cache_control`` tells caches to revalidate the
490 cache on every access and to store cached versions for, at most, 3600 seconds::
492 from django.views.decorators.cache import cache_control
493 @cache_control(must_revalidate=True, max_age=3600)
494 def my_view(request):
497 Any valid ``Cache-Control`` HTTP directive is valid in ``cache_control()``.
503 * ``no_transform=True``
504 * ``must_revalidate=True``
505 * ``proxy_revalidate=True``
506 * ``max_age=num_seconds``
507 * ``s_maxage=num_seconds``
509 For explanation of Cache-Control HTTP directives, see the `Cache-Control spec`_.
511 (Note that the caching middleware already sets the cache header's max-age with
512 the value of the ``CACHE_MIDDLEWARE_SETTINGS`` setting. If you use a custom
513 ``max_age`` in a ``cache_control`` decorator, the decorator will take
514 precedence, and the header values will be merged correctly.)
516 .. _`Cache-Control spec`: http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.9
521 Django comes with a few other pieces of middleware that can help optimize your
524 * ``django.middleware.http.ConditionalGetMiddleware`` adds support for
525 conditional GET. This makes use of ``ETag`` and ``Last-Modified``
528 * ``django.middleware.gzip.GZipMiddleware`` compresses content for browsers
529 that understand gzip compression (all modern browsers).
531 Order of MIDDLEWARE_CLASSES
532 ===========================
534 If you use ``CacheMiddleware``, it's important to put it in the right place
535 within the ``MIDDLEWARE_CLASSES`` setting, because the cache middleware needs
536 to know which headers by which to vary the cache storage. Middleware always
537 adds something the ``Vary`` response header when it can.
539 Put the ``CacheMiddleware`` after any middlewares that might add something to
540 the ``Vary`` header. The following middlewares do so:
542 * ``SessionMiddleware`` adds ``Cookie``
543 * ``GZipMiddleware`` adds ``Accept-Encoding``