浅析Python的Django框架中的Memcached
动态网站的问题就在于它是动态的。 也就是说每次用户访问一个页面，服务器要执行数据库查询，启动模板，执行业务逻辑以及最终生成一个你所看到的网页，这一切都是动态即时生成的。 从处理器资源的角度来看，这是比较昂贵的。
对于大多数网络应用来说，过载并不是大问题。 因为大多数网络应用并不是washingtonpost.com或Slashdot；它们通常是很小很简单，或者是中等规模的站点，只有很少的流量。 但是对于中等至大规模流量的站点来说，尽可能地解决过载问题是非常必要的。
这就需要用到缓存了。
缓存的目的是为了避免重复计算，特别是对一些比较耗时间、资源的计算。 下面的伪代码演示了如何对动态页面的结果进行缓存。
given a URL, try finding that page in the cache if the page is in the cache: return the cached page else: generate the page save the generated page in the cache (for next time) return the generated page
为此，Django提供了一个稳定的缓存系统让你缓存动态页面的结果，这样在接下来有相同的请求就可以直接使用缓存中的数据，避免不必要的重复计算。 另外Django还提供了不同粒度数据的缓存，例如： 你可以缓存整个页面，也可以缓存某个部分，甚至缓存整个网站。
Django也和”上游”缓存工作的很好，例如Squid(http://www.squid-cache.org)和基于浏览器的缓存。 这些类型的缓存你不直接控制，但是你可以提供关于你的站点哪部分应该被缓存和怎样缓存的线索(通过HTTP头部)给它们
设定缓存
缓存系统需要一些少量的设定工作。 也就是说，你必须告诉它缓存的数据应该放在哪里，在数据库中，在文件系统，或直接在内存中。 这是一个重要的决定，影响您的高速缓存的性能，是的，有些类型的缓存比其它类型快。
缓存设置在settings文件的 CACHE_BACKEND中。 这里是一个CACHE_BACKEND所有可用值的解释。
内存缓冲
Memcached是迄今为止可用于Django的最快，最有效的缓存类型，Memcached是完全基于内存的缓存框架，最初开发它是用以处理高负荷的LiveJournal.com随后由Danga Interactive公司开源。 它被用于一些站点，例如Facebook和维基百科网站，以减少数据库访问，并大幅提高站点的性能。
Memcached是免费的（http://danga.com/memcached）。它作为一个守护进程运行，并分配了特定数量的内存。 它只是提供了添加，检索和删除缓存中的任意数据的高速接口。 所有数据都直接存储在内存中，所以没有对使用的数据库或文件系统的开销。
在安装了Memcached本身之后，你将需要安装Memcached Python绑定，它没有直接和Django绑定。 这两个可用版本。 选择和安装以下模块之一：
最快的可用选项是一个模块，称为cmemcache，在http://gijsbert.org/cmemcache。
如果您无法安装cmemcache，您可以安装python - Memcached，在ftp://ftp.tummy.com/pub/python-memcached/。如果该网址已不再有效，只要到Memcached的网站http://www.danga.com/memcached/），并从客户端API完成Python绑定。
若要使用Memcached的Django，设置CACHE_BACKEND到memcached：/ / IP：port/，其中IP是Memcached的守护进程的IP地址，port是Memcached运行的端口。
在这个例子中，Memcached运行在本地主机 (127.0.0.1)上,端口为11211：
CACHE_BACKEND = 'memcached://127.0.0.1:11211/'
Memcached的一个极好的特性是它在多个服务器间分享缓存的能力。 这意味着您可以在多台机器上运行Memcached的守护进程，该程序会把这些机器当成一个单一缓存，而无需重复每台机器上的缓存值。 要充分利用此功能，请在CACHE_BACKEND里引入所有服务器的地址，用分号分隔。
这个例子中，缓存在运行在IP地址为172.19.26.240和172.19.26.242，端口号为11211的Memcached实例间分享:
CACHE_BACKEND = 'memcached://172.19.26.240:11211;172.19.26.242:11211/'
这个例子中，缓存在运行在172.19.26.240(端口11211)，172.19.26.242(端口11212)，172.19.26.244(端口11213)的Memcached实例间分享:
CACHE_BACKEND = 'memcached://172.19.26.240:11211;172.19.26.242:11212;172.19.26.244:11213/'
最后有关Memcached的一点是，基于内存的缓存有一个重大的缺点。 由于缓存的数据存储在内存中，所以如果您的服务器崩溃，数据将会消失。 显然，内存不是用来持久化数据的，因此不要把基于内存的缓存作为您唯一的存储数据缓存。 毫无疑问，在Django的缓存后端不应该用于持久化，它们本来就被设计成缓存的解决方案。但我们仍然指出此点，这里是因为基于内存的缓存是暂时的。
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