author | Lennard de Rijk <ljvderijk@gmail.com> |
Mon, 13 Jul 2009 00:53:42 +0200 | |
changeset 2616 | 9321e100a7fb |
parent 109 | 620f9b141567 |
permissions | -rw-r--r-- |
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Django's cache framework |
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A fundamental tradeoff in dynamic Web sites is, well, they're dynamic. Each |
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time a user requests a page, the Web server makes all sorts of calculations -- |
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from database queries to template rendering to business logic -- to create the |
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page that your site's visitor sees. This is a lot more expensive, from a |
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processing-overhead perspective, than your standard read-a-file-off-the-filesystem |
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server arrangement. |
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For most Web applications, this overhead isn't a big deal. Most Web |
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applications aren't washingtonpost.com or slashdot.org; they're simply small- |
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to medium-sized sites with so-so traffic. But for medium- to high-traffic |
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sites, it's essential to cut as much overhead as possible. |
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That's where caching comes in. |
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To cache something is to save the result of an expensive calculation so that |
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you don't have to perform the calculation next time. Here's some pseudocode |
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explaining how this would work for a dynamically generated Web page:: |
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given a URL, try finding that page in the cache |
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if the page is in the cache: |
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return the cached page |
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else: |
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generate the page |
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save the generated page in the cache (for next time) |
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return the generated page |
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Django comes with a robust cache system that lets you save dynamic pages so |
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they don't have to be calculated for each request. For convenience, Django |
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offers different levels of cache granularity: You can cache the output of |
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specific views, you can cache only the pieces that are difficult to produce, or |
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you can cache your entire site. |
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Django also works well with "upstream" caches, such as Squid |
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(http://www.squid-cache.org/) and browser-based caches. These are the types of |
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caches that you don't directly control but to which you can provide hints (via |
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HTTP headers) about which parts of your site should be cached, and how. |
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Setting up the cache |
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==================== |
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The cache system requires a small amount of setup. Namely, you have to tell it |
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where your cached data should live -- whether in a database, on the filesystem |
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or directly in memory. This is an important decision that affects your cache's |
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performance; yes, some cache types are faster than others. |
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Your cache preference goes in the ``CACHE_BACKEND`` setting in your settings |
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file. Here's an explanation of all available values for CACHE_BACKEND. |
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Memcached |
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--------- |
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By far the fastest, most efficient type of cache available to Django, Memcached |
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is an entirely memory-based cache framework originally developed to handle high |
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loads at LiveJournal.com and subsequently open-sourced by Danga Interactive. |
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It's used by sites such as Slashdot and Wikipedia to reduce database access and |
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dramatically increase site performance. |
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Memcached is available for free at http://danga.com/memcached/ . It runs as a |
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daemon and is allotted a specified amount of RAM. All it does is provide an |
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interface -- a *super-lightning-fast* interface -- for adding, retrieving and |
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deleting arbitrary data in the cache. All data is stored directly in memory, |
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so there's no overhead of database or filesystem usage. |
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After installing Memcached itself, you'll need to install the Memcached Python |
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bindings. They're in a single Python module, memcache.py, available at |
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ftp://ftp.tummy.com/pub/python-memcached/ . If that URL is no longer valid, |
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just go to the Memcached Web site (http://www.danga.com/memcached/) and get the |
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Python bindings from the "Client APIs" section. |
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To use Memcached with Django, set ``CACHE_BACKEND`` to |
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``memcached://ip:port/``, where ``ip`` is the IP address of the Memcached |
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daemon and ``port`` is the port on which Memcached is running. |
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In this example, Memcached is running on localhost (127.0.0.1) port 11211:: |
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CACHE_BACKEND = 'memcached://127.0.0.1:11211/' |
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One excellent feature of Memcached is its ability to share cache over multiple |
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servers. To take advantage of this feature, include all server addresses in |
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``CACHE_BACKEND``, separated by semicolons. In this example, the cache is |
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shared over Memcached instances running on IP address 172.19.26.240 and |
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172.19.26.242, both on port 11211:: |
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CACHE_BACKEND = 'memcached://172.19.26.240:11211;172.19.26.242:11211/' |
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Memory-based caching has one disadvantage: Because the cached data is stored in |
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memory, the data will be lost if your server crashes. Clearly, memory isn't |
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intended for permanent data storage, so don't rely on memory-based caching as |
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your only data storage. Actually, none of the Django caching backends should be |
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used for permanent storage -- they're all intended to be solutions for caching, |
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not storage -- but we point this out here because memory-based caching is |
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particularly temporary. |
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Database caching |
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---------------- |
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To use a database table as your cache backend, first create a cache table in |
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your database by running this command:: |
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python manage.py createcachetable [cache_table_name] |
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...where ``[cache_table_name]`` is the name of the database table to create. |
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(This name can be whatever you want, as long as it's a valid table name that's |
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not already being used in your database.) This command creates a single table |
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109 |
in your database that is in the proper format that Django's database-cache |
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110 |
system expects. |
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111 |
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112 |
Once you've created that database table, set your ``CACHE_BACKEND`` setting to |
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``"db://tablename/"``, where ``tablename`` is the name of the database table. |
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114 |
In this example, the cache table's name is ``my_cache_table``: |
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115 |
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116 |
CACHE_BACKEND = 'db://my_cache_table' |
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117 |
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118 |
Database caching works best if you've got a fast, well-indexed database server. |
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119 |
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120 |
Filesystem caching |
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121 |
------------------ |
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122 |
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123 |
To store cached items on a filesystem, use the ``"file://"`` cache type for |
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124 |
``CACHE_BACKEND``. For example, to store cached data in ``/var/tmp/django_cache``, |
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125 |
use this setting:: |
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126 |
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CACHE_BACKEND = 'file:///var/tmp/django_cache' |
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128 |
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129 |
Note that there are three forward slashes toward the beginning of that example. |
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The first two are for ``file://``, and the third is the first character of the |
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directory path, ``/var/tmp/django_cache``. |
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132 |
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133 |
The directory path should be absolute -- that is, it should start at the root |
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134 |
of your filesystem. It doesn't matter whether you put a slash at the end of the |
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135 |
setting. |
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136 |
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137 |
Make sure the directory pointed-to by this setting exists and is readable and |
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138 |
writable by the system user under which your Web server runs. Continuing the |
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139 |
above example, if your server runs as the user ``apache``, make sure the |
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directory ``/var/tmp/django_cache`` exists and is readable and writable by the |
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141 |
user ``apache``. |
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142 |
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143 |
Local-memory caching |
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144 |
-------------------- |
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145 |
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146 |
If you want the speed advantages of in-memory caching but don't have the |
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147 |
capability of running Memcached, consider the local-memory cache backend. This |
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148 |
cache is multi-process and thread-safe. To use it, set ``CACHE_BACKEND`` to |
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149 |
``"locmem:///"``. For example:: |
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150 |
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151 |
CACHE_BACKEND = 'locmem:///' |
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152 |
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153 |
Simple caching (for development) |
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154 |
-------------------------------- |
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155 |
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156 |
A simple, single-process memory cache is available as ``"simple:///"``. This |
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157 |
merely saves cached data in-process, which means it should only be used in |
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158 |
development or testing environments. For example:: |
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159 |
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160 |
CACHE_BACKEND = 'simple:///' |
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161 |
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162 |
Dummy caching (for development) |
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163 |
------------------------------- |
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164 |
|
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165 |
Finally, Django comes with a "dummy" cache that doesn't actually cache -- it |
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166 |
just implements the cache interface without doing anything. |
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167 |
|
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168 |
This is useful if you have a production site that uses heavy-duty caching in |
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169 |
various places but a development/test environment on which you don't want to |
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170 |
cache. In that case, set ``CACHE_BACKEND`` to ``"dummy:///"`` in the settings |
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171 |
file for your development environment. As a result, your development |
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172 |
environment won't use caching and your production environment still will. |
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173 |
|
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174 |
CACHE_BACKEND arguments |
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175 |
----------------------- |
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176 |
|
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177 |
All caches may take arguments. They're given in query-string style on the |
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178 |
``CACHE_BACKEND`` setting. Valid arguments are: |
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179 |
|
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180 |
timeout |
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181 |
Default timeout, in seconds, to use for the cache. Defaults to 5 |
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182 |
minutes (300 seconds). |
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183 |
|
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184 |
max_entries |
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185 |
For the simple and database backends, the maximum number of entries |
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186 |
allowed in the cache before it is cleaned. Defaults to 300. |
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187 |
|
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188 |
cull_percentage |
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189 |
The percentage of entries that are culled when max_entries is reached. |
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190 |
The actual percentage is 1/cull_percentage, so set cull_percentage=3 to |
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191 |
cull 1/3 of the entries when max_entries is reached. |
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192 |
|
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193 |
A value of 0 for cull_percentage means that the entire cache will be |
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194 |
dumped when max_entries is reached. This makes culling *much* faster |
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195 |
at the expense of more cache misses. |
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196 |
|
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197 |
In this example, ``timeout`` is set to ``60``:: |
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198 |
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199 |
CACHE_BACKEND = "memcached://127.0.0.1:11211/?timeout=60" |
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200 |
|
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201 |
In this example, ``timeout`` is ``30`` and ``max_entries`` is ``400``:: |
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202 |
|
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203 |
CACHE_BACKEND = "memcached://127.0.0.1:11211/?timeout=30&max_entries=400" |
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204 |
|
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205 |
Invalid arguments are silently ignored, as are invalid values of known |
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206 |
arguments. |
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207 |
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208 |
The per-site cache |
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209 |
================== |
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210 |
|
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211 |
Once the cache is set up, the simplest way to use caching is to cache your |
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212 |
entire site. Just add ``'django.middleware.cache.CacheMiddleware'`` to your |
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213 |
``MIDDLEWARE_CLASSES`` setting, as in this example:: |
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214 |
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215 |
MIDDLEWARE_CLASSES = ( |
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216 |
'django.middleware.cache.CacheMiddleware', |
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217 |
'django.middleware.common.CommonMiddleware', |
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218 |
) |
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219 |
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220 |
(The order of ``MIDDLEWARE_CLASSES`` matters. See "Order of MIDDLEWARE_CLASSES" |
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below.) |
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Then, add the following required settings to your Django settings file: |
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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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The cache middleware caches every page that doesn't have GET or POST |
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parameters. Optionally, if the ``CACHE_MIDDLEWARE_ANONYMOUS_ONLY`` setting is |
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``True``, only anonymous requests (i.e., not those made by a logged-in user) |
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will be cached. This is a simple and effective way of disabling caching for any |
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user-specific pages (include Django's admin interface). Note that if you use |
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``CACHE_MIDDLEWARE_ANONYMOUS_ONLY``, you should make sure you've activated |
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``AuthenticationMiddleware`` and that ``AuthenticationMiddleware`` appears |
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before ``CacheMiddleware`` in your ``MIDDLEWARE_CLASSES``. |
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Additionally, ``CacheMiddleware`` automatically sets a few headers in each |
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``HttpResponse``: |
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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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* Sets the ``Expires`` header to the current date/time plus the defined |
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``CACHE_MIDDLEWARE_SECONDS``. |
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* Sets the ``Cache-Control`` header to give a max age for the page -- again, |
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from the ``CACHE_MIDDLEWARE_SECONDS`` setting. |
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See the `middleware documentation`_ for more on middleware. |
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|
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.. _`middleware documentation`: ../middleware/ |
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The per-view 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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|
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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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``cache_page`` takes a single argument: the cache timeout, in seconds. In the |
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above example, the result of the ``slashdot_this()`` view will be cached for 15 |
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minutes. |
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|
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The low-level cache API |
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======================= |
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Sometimes, however, caching an entire rendered page doesn't gain you very much. |
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For example, you may find it's only necessary to cache the result of an |
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intensive database query. In cases like this, you can use the low-level cache |
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API to store objects in the cache with any level of granularity you like. |
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|
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The cache API is simple. The cache module, ``django.core.cache``, exports a |
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``cache`` object that's automatically created from the ``CACHE_BACKEND`` |
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setting:: |
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|
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>>> from django.core.cache import cache |
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|
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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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The ``timeout_seconds`` argument is optional and defaults to the ``timeout`` |
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argument in the ``CACHE_BACKEND`` setting (explained above). |
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If the object doesn't exist in the cache, ``cache.get()`` returns ``None``:: |
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>>> cache.get('some_other_key') |
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None |
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# Wait 30 seconds for 'my_key' to expire... |
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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. get_many() |
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returns a dictionary with all the keys you asked for that actually exist in the |
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cache (and haven't expired):: |
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|
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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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Finally, you can delete keys explicitly with ``delete()``. This is an easy way |
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of clearing the cache for a particular object:: |
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|
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>>> cache.delete('a') |
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|
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333 |
That's it. The cache has very few restrictions: You can cache any object that |
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|
334 |
can be pickled safely, although keys must be strings. |
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|
335 |
|
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|
336 |
Upstream caches |
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|
337 |
=============== |
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|
338 |
|
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|
339 |
So far, this document has focused on caching your *own* data. But another type |
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parents:
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|
340 |
of caching is relevant to Web development, too: caching performed by "upstream" |
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|
341 |
caches. These are systems that cache pages for users even before the request |
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|
342 |
reaches your Web site. |
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|
343 |
|
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|
344 |
Here are a few examples of upstream caches: |
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345 |
|
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|
346 |
* Your ISP may cache certain pages, so if you requested a page from |
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parents:
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|
347 |
somedomain.com, your ISP would send you the page without having to access |
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parents:
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|
348 |
somedomain.com directly. |
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|
349 |
|
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|
350 |
* Your Django Web site may sit behind a Squid Web proxy |
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|
351 |
(http://www.squid-cache.org/) that caches pages for performance. In this |
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parents:
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|
352 |
case, each request first would be handled by Squid, and it'd only be |
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|
353 |
passed to your application if needed. |
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|
354 |
|
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parents:
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|
355 |
* Your Web browser caches pages, too. If a Web page sends out the right |
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parents:
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|
356 |
headers, your browser will use the local (cached) copy for subsequent |
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|
357 |
requests to that page. |
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|
358 |
|
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parents:
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|
359 |
Upstream caching is a nice efficiency boost, but there's a danger to it: |
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parents:
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|
360 |
Many Web pages' contents differ based on authentication and a host of other |
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parents:
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|
361 |
variables, and cache systems that blindly save pages based purely on URLs could |
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|
362 |
expose incorrect or sensitive data to subsequent visitors to those pages. |
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|
363 |
|
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|
364 |
For example, say you operate a Web e-mail system, and the contents of the |
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parents:
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|
365 |
"inbox" page obviously depend on which user is logged in. If an ISP blindly |
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parents:
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|
366 |
cached your site, then the first user who logged in through that ISP would have |
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parents:
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|
367 |
his user-specific inbox page cached for subsequent visitors to the site. That's |
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parents:
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|
368 |
not cool. |
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|
369 |
|
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|
370 |
Fortunately, HTTP provides a solution to this problem: A set of HTTP headers |
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parents:
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|
371 |
exist to instruct caching mechanisms to differ their cache contents depending |
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parents:
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|
372 |
on designated variables, and to tell caching mechanisms not to cache particular |
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|
373 |
pages. |
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|
374 |
|
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|
375 |
Using Vary headers |
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|
376 |
================== |
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|
377 |
|
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parents:
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|
378 |
One of these headers is ``Vary``. It defines which request headers a cache |
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parents:
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|
379 |
mechanism should take into account when building its cache key. For example, if |
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parents:
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|
380 |
the contents of a Web page depend on a user's language preference, the page is |
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parents:
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|
381 |
said to "vary on language." |
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parents:
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|
382 |
|
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parents:
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|
383 |
By default, Django's cache system creates its cache keys using the requested |
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parents:
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changeset
|
384 |
path -- e.g., ``"/stories/2005/jun/23/bank_robbed/"``. This means every request |
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parents:
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changeset
|
385 |
to that URL will use the same cached version, regardless of user-agent |
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parents:
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changeset
|
386 |
differences such as cookies or language preferences. |
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parents:
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changeset
|
387 |
|
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parents:
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changeset
|
388 |
That's where ``Vary`` comes in. |
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parents:
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|
389 |
|
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parents:
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changeset
|
390 |
If your Django-powered page outputs different content based on some difference |
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parents:
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changeset
|
391 |
in request headers -- such as a cookie, or language, or user-agent -- you'll |
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parents:
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changeset
|
392 |
need to use the ``Vary`` header to tell caching mechanisms that the page output |
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parents:
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|
393 |
depends on those things. |
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parents:
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|
394 |
|
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parents:
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changeset
|
395 |
To do this in Django, use the convenient ``vary_on_headers`` view decorator, |
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parents:
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changeset
|
396 |
like so:: |
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parents:
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changeset
|
397 |
|
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parents:
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changeset
|
398 |
from django.views.decorators.vary import vary_on_headers |
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parents:
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changeset
|
399 |
|
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Todd Larsen <tlarsen@google.com>
parents:
diff
changeset
|
400 |
# Python 2.3 syntax. |
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Todd Larsen <tlarsen@google.com>
parents:
diff
changeset
|
401 |
def my_view(request): |
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Todd Larsen <tlarsen@google.com>
parents:
diff
changeset
|
402 |
... |
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parents:
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changeset
|
403 |
my_view = vary_on_headers(my_view, 'User-Agent') |
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Todd Larsen <tlarsen@google.com>
parents:
diff
changeset
|
404 |
|
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Todd Larsen <tlarsen@google.com>
parents:
diff
changeset
|
405 |
# Python 2.4 decorator syntax. |
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Todd Larsen <tlarsen@google.com>
parents:
diff
changeset
|
406 |
@vary_on_headers('User-Agent') |
620f9b141567
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Todd Larsen <tlarsen@google.com>
parents:
diff
changeset
|
407 |
def my_view(request): |
620f9b141567
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Todd Larsen <tlarsen@google.com>
parents:
diff
changeset
|
408 |
... |
620f9b141567
Load ../../google_appengine into trunk/thirdparty/google_appengine.
Todd Larsen <tlarsen@google.com>
parents:
diff
changeset
|
409 |
|
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Todd Larsen <tlarsen@google.com>
parents:
diff
changeset
|
410 |
In this case, a caching mechanism (such as Django's own cache middleware) will |
620f9b141567
Load ../../google_appengine into trunk/thirdparty/google_appengine.
Todd Larsen <tlarsen@google.com>
parents:
diff
changeset
|
411 |
cache a separate version of the page for each unique user-agent. |
620f9b141567
Load ../../google_appengine into trunk/thirdparty/google_appengine.
Todd Larsen <tlarsen@google.com>
parents:
diff
changeset
|
412 |
|
620f9b141567
Load ../../google_appengine into trunk/thirdparty/google_appengine.
Todd Larsen <tlarsen@google.com>
parents:
diff
changeset
|
413 |
The advantage to using the ``vary_on_headers`` decorator rather than manually |
620f9b141567
Load ../../google_appengine into trunk/thirdparty/google_appengine.
Todd Larsen <tlarsen@google.com>
parents:
diff
changeset
|
414 |
setting the ``Vary`` header (using something like |
620f9b141567
Load ../../google_appengine into trunk/thirdparty/google_appengine.
Todd Larsen <tlarsen@google.com>
parents:
diff
changeset
|
415 |
``response['Vary'] = 'user-agent'``) is that the decorator adds to the ``Vary`` |
620f9b141567
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Todd Larsen <tlarsen@google.com>
parents:
diff
changeset
|
416 |
header (which may already exist) rather than setting it from scratch. |
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Load ../../google_appengine into trunk/thirdparty/google_appengine.
Todd Larsen <tlarsen@google.com>
parents:
diff
changeset
|
417 |
|
620f9b141567
Load ../../google_appengine into trunk/thirdparty/google_appengine.
Todd Larsen <tlarsen@google.com>
parents:
diff
changeset
|
418 |
You can pass multiple headers to ``vary_on_headers()``:: |
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Load ../../google_appengine into trunk/thirdparty/google_appengine.
Todd Larsen <tlarsen@google.com>
parents:
diff
changeset
|
419 |
|
620f9b141567
Load ../../google_appengine into trunk/thirdparty/google_appengine.
Todd Larsen <tlarsen@google.com>
parents:
diff
changeset
|
420 |
@vary_on_headers('User-Agent', 'Cookie') |
620f9b141567
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Todd Larsen <tlarsen@google.com>
parents:
diff
changeset
|
421 |
def my_view(request): |
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Todd Larsen <tlarsen@google.com>
parents:
diff
changeset
|
422 |
... |
620f9b141567
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Todd Larsen <tlarsen@google.com>
parents:
diff
changeset
|
423 |
|
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Load ../../google_appengine into trunk/thirdparty/google_appengine.
Todd Larsen <tlarsen@google.com>
parents:
diff
changeset
|
424 |
Because varying on cookie is such a common case, there's a ``vary_on_cookie`` |
620f9b141567
Load ../../google_appengine into trunk/thirdparty/google_appengine.
Todd Larsen <tlarsen@google.com>
parents:
diff
changeset
|
425 |
decorator. These two views are equivalent:: |
620f9b141567
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Todd Larsen <tlarsen@google.com>
parents:
diff
changeset
|
426 |
|
620f9b141567
Load ../../google_appengine into trunk/thirdparty/google_appengine.
Todd Larsen <tlarsen@google.com>
parents:
diff
changeset
|
427 |
@vary_on_cookie |
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Load ../../google_appengine into trunk/thirdparty/google_appengine.
Todd Larsen <tlarsen@google.com>
parents:
diff
changeset
|
428 |
def my_view(request): |
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Todd Larsen <tlarsen@google.com>
parents:
diff
changeset
|
429 |
... |
620f9b141567
Load ../../google_appengine into trunk/thirdparty/google_appengine.
Todd Larsen <tlarsen@google.com>
parents:
diff
changeset
|
430 |
|
620f9b141567
Load ../../google_appengine into trunk/thirdparty/google_appengine.
Todd Larsen <tlarsen@google.com>
parents:
diff
changeset
|
431 |
@vary_on_headers('Cookie') |
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Load ../../google_appengine into trunk/thirdparty/google_appengine.
Todd Larsen <tlarsen@google.com>
parents:
diff
changeset
|
432 |
def my_view(request): |
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Todd Larsen <tlarsen@google.com>
parents:
diff
changeset
|
433 |
... |
620f9b141567
Load ../../google_appengine into trunk/thirdparty/google_appengine.
Todd Larsen <tlarsen@google.com>
parents:
diff
changeset
|
434 |
|
620f9b141567
Load ../../google_appengine into trunk/thirdparty/google_appengine.
Todd Larsen <tlarsen@google.com>
parents:
diff
changeset
|
435 |
Also note that the headers you pass to ``vary_on_headers`` are not case |
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Todd Larsen <tlarsen@google.com>
parents:
diff
changeset
|
436 |
sensitive. ``"User-Agent"`` is the same thing as ``"user-agent"``. |
620f9b141567
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Todd Larsen <tlarsen@google.com>
parents:
diff
changeset
|
437 |
|
620f9b141567
Load ../../google_appengine into trunk/thirdparty/google_appengine.
Todd Larsen <tlarsen@google.com>
parents:
diff
changeset
|
438 |
You can also use a helper function, ``django.utils.cache.patch_vary_headers``, |
620f9b141567
Load ../../google_appengine into trunk/thirdparty/google_appengine.
Todd Larsen <tlarsen@google.com>
parents:
diff
changeset
|
439 |
directly:: |
620f9b141567
Load ../../google_appengine into trunk/thirdparty/google_appengine.
Todd Larsen <tlarsen@google.com>
parents:
diff
changeset
|
440 |
|
620f9b141567
Load ../../google_appengine into trunk/thirdparty/google_appengine.
Todd Larsen <tlarsen@google.com>
parents:
diff
changeset
|
441 |
from django.utils.cache import patch_vary_headers |
620f9b141567
Load ../../google_appengine into trunk/thirdparty/google_appengine.
Todd Larsen <tlarsen@google.com>
parents:
diff
changeset
|
442 |
def my_view(request): |
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Load ../../google_appengine into trunk/thirdparty/google_appengine.
Todd Larsen <tlarsen@google.com>
parents:
diff
changeset
|
443 |
... |
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Load ../../google_appengine into trunk/thirdparty/google_appengine.
Todd Larsen <tlarsen@google.com>
parents:
diff
changeset
|
444 |
response = render_to_response('template_name', context) |
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Load ../../google_appengine into trunk/thirdparty/google_appengine.
Todd Larsen <tlarsen@google.com>
parents:
diff
changeset
|
445 |
patch_vary_headers(response, ['Cookie']) |
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Load ../../google_appengine into trunk/thirdparty/google_appengine.
Todd Larsen <tlarsen@google.com>
parents:
diff
changeset
|
446 |
return response |
620f9b141567
Load ../../google_appengine into trunk/thirdparty/google_appengine.
Todd Larsen <tlarsen@google.com>
parents:
diff
changeset
|
447 |
|
620f9b141567
Load ../../google_appengine into trunk/thirdparty/google_appengine.
Todd Larsen <tlarsen@google.com>
parents:
diff
changeset
|
448 |
``patch_vary_headers`` takes an ``HttpResponse`` instance as its first argument |
620f9b141567
Load ../../google_appengine into trunk/thirdparty/google_appengine.
Todd Larsen <tlarsen@google.com>
parents:
diff
changeset
|
449 |
and a list/tuple of header names as its second argument. |
620f9b141567
Load ../../google_appengine into trunk/thirdparty/google_appengine.
Todd Larsen <tlarsen@google.com>
parents:
diff
changeset
|
450 |
|
620f9b141567
Load ../../google_appengine into trunk/thirdparty/google_appengine.
Todd Larsen <tlarsen@google.com>
parents:
diff
changeset
|
451 |
For more on Vary headers, see the `official Vary spec`_. |
620f9b141567
Load ../../google_appengine into trunk/thirdparty/google_appengine.
Todd Larsen <tlarsen@google.com>
parents:
diff
changeset
|
452 |
|
620f9b141567
Load ../../google_appengine into trunk/thirdparty/google_appengine.
Todd Larsen <tlarsen@google.com>
parents:
diff
changeset
|
453 |
.. _`official Vary spec`: http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.44 |
620f9b141567
Load ../../google_appengine into trunk/thirdparty/google_appengine.
Todd Larsen <tlarsen@google.com>
parents:
diff
changeset
|
454 |
|
620f9b141567
Load ../../google_appengine into trunk/thirdparty/google_appengine.
Todd Larsen <tlarsen@google.com>
parents:
diff
changeset
|
455 |
Controlling cache: Using other headers |
620f9b141567
Load ../../google_appengine into trunk/thirdparty/google_appengine.
Todd Larsen <tlarsen@google.com>
parents:
diff
changeset
|
456 |
====================================== |
620f9b141567
Load ../../google_appengine into trunk/thirdparty/google_appengine.
Todd Larsen <tlarsen@google.com>
parents:
diff
changeset
|
457 |
|
620f9b141567
Load ../../google_appengine into trunk/thirdparty/google_appengine.
Todd Larsen <tlarsen@google.com>
parents:
diff
changeset
|
458 |
Another problem with caching is the privacy of data and the question of where |
620f9b141567
Load ../../google_appengine into trunk/thirdparty/google_appengine.
Todd Larsen <tlarsen@google.com>
parents:
diff
changeset
|
459 |
data should be stored in a cascade of caches. |
620f9b141567
Load ../../google_appengine into trunk/thirdparty/google_appengine.
Todd Larsen <tlarsen@google.com>
parents:
diff
changeset
|
460 |
|
620f9b141567
Load ../../google_appengine into trunk/thirdparty/google_appengine.
Todd Larsen <tlarsen@google.com>
parents:
diff
changeset
|
461 |
A user usually faces two kinds of caches: his own browser cache (a private |
620f9b141567
Load ../../google_appengine into trunk/thirdparty/google_appengine.
Todd Larsen <tlarsen@google.com>
parents:
diff
changeset
|
462 |
cache) and his provider's cache (a public cache). A public cache is used by |
620f9b141567
Load ../../google_appengine into trunk/thirdparty/google_appengine.
Todd Larsen <tlarsen@google.com>
parents:
diff
changeset
|
463 |
multiple users and controlled by someone else. This poses problems with |
620f9b141567
Load ../../google_appengine into trunk/thirdparty/google_appengine.
Todd Larsen <tlarsen@google.com>
parents:
diff
changeset
|
464 |
sensitive data: You don't want, say, your banking-account number stored in a |
620f9b141567
Load ../../google_appengine into trunk/thirdparty/google_appengine.
Todd Larsen <tlarsen@google.com>
parents:
diff
changeset
|
465 |
public cache. So Web applications need a way to tell caches which data is |
620f9b141567
Load ../../google_appengine into trunk/thirdparty/google_appengine.
Todd Larsen <tlarsen@google.com>
parents:
diff
changeset
|
466 |
private and which is public. |
620f9b141567
Load ../../google_appengine into trunk/thirdparty/google_appengine.
Todd Larsen <tlarsen@google.com>
parents:
diff
changeset
|
467 |
|
620f9b141567
Load ../../google_appengine into trunk/thirdparty/google_appengine.
Todd Larsen <tlarsen@google.com>
parents:
diff
changeset
|
468 |
The solution is to indicate a page's cache should be "private." To do this in |
620f9b141567
Load ../../google_appengine into trunk/thirdparty/google_appengine.
Todd Larsen <tlarsen@google.com>
parents:
diff
changeset
|
469 |
Django, use the ``cache_control`` view decorator. Example:: |
620f9b141567
Load ../../google_appengine into trunk/thirdparty/google_appengine.
Todd Larsen <tlarsen@google.com>
parents:
diff
changeset
|
470 |
|
620f9b141567
Load ../../google_appengine into trunk/thirdparty/google_appengine.
Todd Larsen <tlarsen@google.com>
parents:
diff
changeset
|
471 |
from django.views.decorators.cache import cache_control |
620f9b141567
Load ../../google_appengine into trunk/thirdparty/google_appengine.
Todd Larsen <tlarsen@google.com>
parents:
diff
changeset
|
472 |
@cache_control(private=True) |
620f9b141567
Load ../../google_appengine into trunk/thirdparty/google_appengine.
Todd Larsen <tlarsen@google.com>
parents:
diff
changeset
|
473 |
def my_view(request): |
620f9b141567
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Todd Larsen <tlarsen@google.com>
parents:
diff
changeset
|
474 |
... |
620f9b141567
Load ../../google_appengine into trunk/thirdparty/google_appengine.
Todd Larsen <tlarsen@google.com>
parents:
diff
changeset
|
475 |
|
620f9b141567
Load ../../google_appengine into trunk/thirdparty/google_appengine.
Todd Larsen <tlarsen@google.com>
parents:
diff
changeset
|
476 |
This decorator takes care of sending out the appropriate HTTP header behind the |
620f9b141567
Load ../../google_appengine into trunk/thirdparty/google_appengine.
Todd Larsen <tlarsen@google.com>
parents:
diff
changeset
|
477 |
scenes. |
620f9b141567
Load ../../google_appengine into trunk/thirdparty/google_appengine.
Todd Larsen <tlarsen@google.com>
parents:
diff
changeset
|
478 |
|
620f9b141567
Load ../../google_appengine into trunk/thirdparty/google_appengine.
Todd Larsen <tlarsen@google.com>
parents:
diff
changeset
|
479 |
There are a few other ways to control cache parameters. For example, HTTP |
620f9b141567
Load ../../google_appengine into trunk/thirdparty/google_appengine.
Todd Larsen <tlarsen@google.com>
parents:
diff
changeset
|
480 |
allows applications to do the following: |
620f9b141567
Load ../../google_appengine into trunk/thirdparty/google_appengine.
Todd Larsen <tlarsen@google.com>
parents:
diff
changeset
|
481 |
|
620f9b141567
Load ../../google_appengine into trunk/thirdparty/google_appengine.
Todd Larsen <tlarsen@google.com>
parents:
diff
changeset
|
482 |
* Define the maximum time a page should be cached. |
620f9b141567
Load ../../google_appengine into trunk/thirdparty/google_appengine.
Todd Larsen <tlarsen@google.com>
parents:
diff
changeset
|
483 |
* Specify whether a cache should always check for newer versions, only |
620f9b141567
Load ../../google_appengine into trunk/thirdparty/google_appengine.
Todd Larsen <tlarsen@google.com>
parents:
diff
changeset
|
484 |
delivering the cached content when there are no changes. (Some caches |
620f9b141567
Load ../../google_appengine into trunk/thirdparty/google_appengine.
Todd Larsen <tlarsen@google.com>
parents:
diff
changeset
|
485 |
might deliver cached content even if the server page changed -- simply |
620f9b141567
Load ../../google_appengine into trunk/thirdparty/google_appengine.
Todd Larsen <tlarsen@google.com>
parents:
diff
changeset
|
486 |
because the cache copy isn't yet expired.) |
620f9b141567
Load ../../google_appengine into trunk/thirdparty/google_appengine.
Todd Larsen <tlarsen@google.com>
parents:
diff
changeset
|
487 |
|
620f9b141567
Load ../../google_appengine into trunk/thirdparty/google_appengine.
Todd Larsen <tlarsen@google.com>
parents:
diff
changeset
|
488 |
In Django, use the ``cache_control`` view decorator to specify these cache |
620f9b141567
Load ../../google_appengine into trunk/thirdparty/google_appengine.
Todd Larsen <tlarsen@google.com>
parents:
diff
changeset
|
489 |
parameters. In this example, ``cache_control`` tells caches to revalidate the |
620f9b141567
Load ../../google_appengine into trunk/thirdparty/google_appengine.
Todd Larsen <tlarsen@google.com>
parents:
diff
changeset
|
490 |
cache on every access and to store cached versions for, at most, 3600 seconds:: |
620f9b141567
Load ../../google_appengine into trunk/thirdparty/google_appengine.
Todd Larsen <tlarsen@google.com>
parents:
diff
changeset
|
491 |
|
620f9b141567
Load ../../google_appengine into trunk/thirdparty/google_appengine.
Todd Larsen <tlarsen@google.com>
parents:
diff
changeset
|
492 |
from django.views.decorators.cache import cache_control |
620f9b141567
Load ../../google_appengine into trunk/thirdparty/google_appengine.
Todd Larsen <tlarsen@google.com>
parents:
diff
changeset
|
493 |
@cache_control(must_revalidate=True, max_age=3600) |
620f9b141567
Load ../../google_appengine into trunk/thirdparty/google_appengine.
Todd Larsen <tlarsen@google.com>
parents:
diff
changeset
|
494 |
def my_view(request): |
620f9b141567
Load ../../google_appengine into trunk/thirdparty/google_appengine.
Todd Larsen <tlarsen@google.com>
parents:
diff
changeset
|
495 |
... |
620f9b141567
Load ../../google_appengine into trunk/thirdparty/google_appengine.
Todd Larsen <tlarsen@google.com>
parents:
diff
changeset
|
496 |
|
620f9b141567
Load ../../google_appengine into trunk/thirdparty/google_appengine.
Todd Larsen <tlarsen@google.com>
parents:
diff
changeset
|
497 |
Any valid ``Cache-Control`` HTTP directive is valid in ``cache_control()``. |
620f9b141567
Load ../../google_appengine into trunk/thirdparty/google_appengine.
Todd Larsen <tlarsen@google.com>
parents:
diff
changeset
|
498 |
Here's a full list: |
620f9b141567
Load ../../google_appengine into trunk/thirdparty/google_appengine.
Todd Larsen <tlarsen@google.com>
parents:
diff
changeset
|
499 |
|
620f9b141567
Load ../../google_appengine into trunk/thirdparty/google_appengine.
Todd Larsen <tlarsen@google.com>
parents:
diff
changeset
|
500 |
* ``public=True`` |
620f9b141567
Load ../../google_appengine into trunk/thirdparty/google_appengine.
Todd Larsen <tlarsen@google.com>
parents:
diff
changeset
|
501 |
* ``private=True`` |
620f9b141567
Load ../../google_appengine into trunk/thirdparty/google_appengine.
Todd Larsen <tlarsen@google.com>
parents:
diff
changeset
|
502 |
* ``no_cache=True`` |
620f9b141567
Load ../../google_appengine into trunk/thirdparty/google_appengine.
Todd Larsen <tlarsen@google.com>
parents:
diff
changeset
|
503 |
* ``no_transform=True`` |
620f9b141567
Load ../../google_appengine into trunk/thirdparty/google_appengine.
Todd Larsen <tlarsen@google.com>
parents:
diff
changeset
|
504 |
* ``must_revalidate=True`` |
620f9b141567
Load ../../google_appengine into trunk/thirdparty/google_appengine.
Todd Larsen <tlarsen@google.com>
parents:
diff
changeset
|
505 |
* ``proxy_revalidate=True`` |
620f9b141567
Load ../../google_appengine into trunk/thirdparty/google_appengine.
Todd Larsen <tlarsen@google.com>
parents:
diff
changeset
|
506 |
* ``max_age=num_seconds`` |
620f9b141567
Load ../../google_appengine into trunk/thirdparty/google_appengine.
Todd Larsen <tlarsen@google.com>
parents:
diff
changeset
|
507 |
* ``s_maxage=num_seconds`` |
620f9b141567
Load ../../google_appengine into trunk/thirdparty/google_appengine.
Todd Larsen <tlarsen@google.com>
parents:
diff
changeset
|
508 |
|
620f9b141567
Load ../../google_appengine into trunk/thirdparty/google_appengine.
Todd Larsen <tlarsen@google.com>
parents:
diff
changeset
|
509 |
For explanation of Cache-Control HTTP directives, see the `Cache-Control spec`_. |
620f9b141567
Load ../../google_appengine into trunk/thirdparty/google_appengine.
Todd Larsen <tlarsen@google.com>
parents:
diff
changeset
|
510 |
|
620f9b141567
Load ../../google_appengine into trunk/thirdparty/google_appengine.
Todd Larsen <tlarsen@google.com>
parents:
diff
changeset
|
511 |
(Note that the caching middleware already sets the cache header's max-age with |
620f9b141567
Load ../../google_appengine into trunk/thirdparty/google_appengine.
Todd Larsen <tlarsen@google.com>
parents:
diff
changeset
|
512 |
the value of the ``CACHE_MIDDLEWARE_SETTINGS`` setting. If you use a custom |
620f9b141567
Load ../../google_appengine into trunk/thirdparty/google_appengine.
Todd Larsen <tlarsen@google.com>
parents:
diff
changeset
|
513 |
``max_age`` in a ``cache_control`` decorator, the decorator will take |
620f9b141567
Load ../../google_appengine into trunk/thirdparty/google_appengine.
Todd Larsen <tlarsen@google.com>
parents:
diff
changeset
|
514 |
precedence, and the header values will be merged correctly.) |
620f9b141567
Load ../../google_appengine into trunk/thirdparty/google_appengine.
Todd Larsen <tlarsen@google.com>
parents:
diff
changeset
|
515 |
|
620f9b141567
Load ../../google_appengine into trunk/thirdparty/google_appengine.
Todd Larsen <tlarsen@google.com>
parents:
diff
changeset
|
516 |
.. _`Cache-Control spec`: http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.9 |
620f9b141567
Load ../../google_appengine into trunk/thirdparty/google_appengine.
Todd Larsen <tlarsen@google.com>
parents:
diff
changeset
|
517 |
|
620f9b141567
Load ../../google_appengine into trunk/thirdparty/google_appengine.
Todd Larsen <tlarsen@google.com>
parents:
diff
changeset
|
518 |
Other optimizations |
620f9b141567
Load ../../google_appengine into trunk/thirdparty/google_appengine.
Todd Larsen <tlarsen@google.com>
parents:
diff
changeset
|
519 |
=================== |
620f9b141567
Load ../../google_appengine into trunk/thirdparty/google_appengine.
Todd Larsen <tlarsen@google.com>
parents:
diff
changeset
|
520 |
|
620f9b141567
Load ../../google_appengine into trunk/thirdparty/google_appengine.
Todd Larsen <tlarsen@google.com>
parents:
diff
changeset
|
521 |
Django comes with a few other pieces of middleware that can help optimize your |
620f9b141567
Load ../../google_appengine into trunk/thirdparty/google_appengine.
Todd Larsen <tlarsen@google.com>
parents:
diff
changeset
|
522 |
apps' performance: |
620f9b141567
Load ../../google_appengine into trunk/thirdparty/google_appengine.
Todd Larsen <tlarsen@google.com>
parents:
diff
changeset
|
523 |
|
620f9b141567
Load ../../google_appengine into trunk/thirdparty/google_appengine.
Todd Larsen <tlarsen@google.com>
parents:
diff
changeset
|
524 |
* ``django.middleware.http.ConditionalGetMiddleware`` adds support for |
620f9b141567
Load ../../google_appengine into trunk/thirdparty/google_appengine.
Todd Larsen <tlarsen@google.com>
parents:
diff
changeset
|
525 |
conditional GET. This makes use of ``ETag`` and ``Last-Modified`` |
620f9b141567
Load ../../google_appengine into trunk/thirdparty/google_appengine.
Todd Larsen <tlarsen@google.com>
parents:
diff
changeset
|
526 |
headers. |
620f9b141567
Load ../../google_appengine into trunk/thirdparty/google_appengine.
Todd Larsen <tlarsen@google.com>
parents:
diff
changeset
|
527 |
|
620f9b141567
Load ../../google_appengine into trunk/thirdparty/google_appengine.
Todd Larsen <tlarsen@google.com>
parents:
diff
changeset
|
528 |
* ``django.middleware.gzip.GZipMiddleware`` compresses content for browsers |
620f9b141567
Load ../../google_appengine into trunk/thirdparty/google_appengine.
Todd Larsen <tlarsen@google.com>
parents:
diff
changeset
|
529 |
that understand gzip compression (all modern browsers). |
620f9b141567
Load ../../google_appengine into trunk/thirdparty/google_appengine.
Todd Larsen <tlarsen@google.com>
parents:
diff
changeset
|
530 |
|
620f9b141567
Load ../../google_appengine into trunk/thirdparty/google_appengine.
Todd Larsen <tlarsen@google.com>
parents:
diff
changeset
|
531 |
Order of MIDDLEWARE_CLASSES |
620f9b141567
Load ../../google_appengine into trunk/thirdparty/google_appengine.
Todd Larsen <tlarsen@google.com>
parents:
diff
changeset
|
532 |
=========================== |
620f9b141567
Load ../../google_appengine into trunk/thirdparty/google_appengine.
Todd Larsen <tlarsen@google.com>
parents:
diff
changeset
|
533 |
|
620f9b141567
Load ../../google_appengine into trunk/thirdparty/google_appengine.
Todd Larsen <tlarsen@google.com>
parents:
diff
changeset
|
534 |
If you use ``CacheMiddleware``, it's important to put it in the right place |
620f9b141567
Load ../../google_appengine into trunk/thirdparty/google_appengine.
Todd Larsen <tlarsen@google.com>
parents:
diff
changeset
|
535 |
within the ``MIDDLEWARE_CLASSES`` setting, because the cache middleware needs |
620f9b141567
Load ../../google_appengine into trunk/thirdparty/google_appengine.
Todd Larsen <tlarsen@google.com>
parents:
diff
changeset
|
536 |
to know which headers by which to vary the cache storage. Middleware always |
620f9b141567
Load ../../google_appengine into trunk/thirdparty/google_appengine.
Todd Larsen <tlarsen@google.com>
parents:
diff
changeset
|
537 |
adds something the ``Vary`` response header when it can. |
620f9b141567
Load ../../google_appengine into trunk/thirdparty/google_appengine.
Todd Larsen <tlarsen@google.com>
parents:
diff
changeset
|
538 |
|
620f9b141567
Load ../../google_appengine into trunk/thirdparty/google_appengine.
Todd Larsen <tlarsen@google.com>
parents:
diff
changeset
|
539 |
Put the ``CacheMiddleware`` after any middlewares that might add something to |
620f9b141567
Load ../../google_appengine into trunk/thirdparty/google_appengine.
Todd Larsen <tlarsen@google.com>
parents:
diff
changeset
|
540 |
the ``Vary`` header. The following middlewares do so: |
620f9b141567
Load ../../google_appengine into trunk/thirdparty/google_appengine.
Todd Larsen <tlarsen@google.com>
parents:
diff
changeset
|
541 |
|
620f9b141567
Load ../../google_appengine into trunk/thirdparty/google_appengine.
Todd Larsen <tlarsen@google.com>
parents:
diff
changeset
|
542 |
* ``SessionMiddleware`` adds ``Cookie`` |
620f9b141567
Load ../../google_appengine into trunk/thirdparty/google_appengine.
Todd Larsen <tlarsen@google.com>
parents:
diff
changeset
|
543 |
* ``GZipMiddleware`` adds ``Accept-Encoding`` |