May 2024
Intermediate
278 pages
5h 24m
Chinese
Content preview from 构建可扩展分布式系统：方法与实践
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108
｜
第
6
章
这与旁路缓存模式（在这种模式下应用逻辑必须感知到缓存未命中）形成了鲜明的
对比。
通读、通写和后写策略需要这样一种缓存技术，该技术可以通过特定应用的处理程序进
行扩充，以便在应用程序访问缓存时执行数据库的读取和写入。例如，
NCache
（
https://
oreil.ly/v6Xio
）支持
提供者接口
（
provider interface
）由应用程序实现。接口会在通读缓
存未命中和通写缓存写入时自动调用。其他此类缓存本质上都是专用的数据库缓存，因
此需要缓存访问与底层数据库模型一致。
Amazon
的
DynamoDB
加速器（
DAX
；
https://
oreil.ly/lfltM
）就是一个例子。
DAX
位于应用代码和
DynamoDB
之间，透明地充当高速
内存缓存，以减少数据库访问时间。
旁路缓存策略的一个显著优势是它对缓存故障更具弹性。在缓存不可用的情况下，所有
请求基本上都当作缓存未命中来处理。性能会受到影响，但服务仍能满足请求。此外，
扩展
Redis
和
memcached
之类的旁路缓存平台非常简单，得益于其简单的分布式哈希表
模型。因此旁路缓存模式是大规模可扩展系统使用的主要方法。
6.2 Web
缓存
网站响应速度如此之快的原因之一是互联网上充满着
Web
缓存。
Web
缓存会在定义的时
间段内存储给定资源（例如，网页或图像）的副本。缓存会拦截客户端请求，如果它们
在本地缓存了请求的资源，就会返回副本而不是将请求转发到目标服务。因此，可以在
不给服务增加负担的情况下满足许多请求。此外，由于缓存在物理上更靠近客户端，因 ...
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I'm always learning. So when I got on to O'Reilly, I was like a kid in a candy store. There are playlists. There are answers. There's on-demand training. It's worth its weight in gold, in terms of what it allows me to do.