Django中的CACHE_BACKEND参数和站点级Cache设置
CACHE_BACKEND参数
每个缓存后端都可能使用参数。 它们在CACHE_BACKEND设置中以查询字符串形式给出。 有效参数如下：
- timeout:用于缓存的过期时间，以秒为单位。 这个参数默认被设置为300秒（五分钟）。
- max_entries：对于内存，文件系统和数据库后端，高速缓存允许的最大条目数，超出这个数则旧值将被删除。 这个参数默认是300。
- cull_percentage :当达到 max_entries 的时候,被删除的条目比率。 实际的比率是 1/cull_percentage ,所以设置cull_frequency=2就是在达到 max_entries 的时候去除一半数量的缓存。
把 cull_frequency 的值设置为 0 意味着当达到 max_entries 时,缓存将被清空。 这将以很多缓存丢失为代价,大大提高接受访问的速度。
在这个例子中， timeout 被设成 60
CACHE_BACKEND = "memcached://127.0.0.1:11211/?timeout=60"
而在这个例子中， timeout 设为 30 而 max_entries 为 400 :
CACHE_BACKEND = "locmem:///?timeout=30&max_entries=400"
其中，非法的参数与非法的参数值都将被忽略。
站点级 Cache
一旦高速缓存设置，最简单的方法是使用缓存缓存整个网站。 您 需要添加'django.middleware.cache.UpdateCacheMiddleware'和 ‘django.middleware.cache.FetchFromCacheMiddleware'到您的MIDDLEWARE_CLASSES设置中，在这个例子中是：
MIDDLEWARE_CLASSES = ( 'django.middleware.cache.UpdateCacheMiddleware', 'django.middleware.common.CommonMiddleware', 'django.middleware.cache.FetchFromCacheMiddleware', )
注意：
不，这里并没有排版错误： 修改的中间件，必须放在列表的开始位置，而fectch中间件，必须放在最后。 细节有点费解，如果您想了解完整内幕请参看下面的MIDDLEWARE_CLASSES顺序。
然后，在你的Django settings文件里加入下面所需的设置：
- CACHE_MIDDLEWARE_SECONDS :每个页面应该被缓存的秒数。
- CACHE_MIDDLEWARE_KEY_PREFIX ：如果缓存被多个使用相同Django安装的网站所共享，那么把这个值设成当前网站名，或其他能代表这个Django实例的唯一字符串，以避免key发生冲突。 如果你不在意的话可以设成空字符串。
缓存中间件缓存每个没有GET或者POST参数的页面。 或者，如果CACHE_MIDDLEWARE_ANONYMOUS_ONLY设置为True，只有匿名请求（即不是由登录的用户）将被缓存。 如果想取消用户相关页面（user-specific pages）的缓存，例如Djangos 的管理界面，这是一种既简单又有效的方法。 CACHE_MIDDLEWARE_ANONYMOUS_ONLY，你应该确保你已经启动AuthenticationMiddleware。
此外，缓存中间件为每个HttpResponse自动设置了几个头部信息：
- 当一个新(没缓存的)版本的页面被请求时设置Last-Modified头部为当前日期/时间。
- 设置Expires头部为当前日期/时间加上定义的CACHE_MIDDLEWARE_SECONDS。
- 设置Cache-Control头部来给页面一个最长的有效期，值来自于CACHE_MIDDLEWARE_SECONDS设置。
如果视图设置自己的缓存到期时间（即 它有一个最大年龄在头部信息的Cache-Control中），那么页面将缓存直到过期，而不是CACHE_MIDDLEWARE_SECONDS。使用django.views.decorators.cache装饰器，您可以轻松地设置视图的到期时间（使用cache_control装饰器）或禁用缓存视图（使用never_cache装饰器）。
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