了解缓存策略

Question

在研究各种搜索引擎和网站本身＃2采用的缓存策略，我不禁注意到在响应报头的细微差别：

谷歌搜索

Cache-Control: private, max-age=0 
Expires: -1 

雅虎搜索

Cache-Control: private 
Connection: Keep-Alive 
Keep-Alive: timeout=60, max=100 

搜索＃1

Cache-Control: private 

必须有通过设置背后的一些合乎逻辑的解释。有人可以照顾解释差异，以便我们每个人都可以学习和获益吗？

Answer 1

从RFC2616 HTTP/1.1 Header Field Definitions, 14.9.1 What is Cacheable：

private 
    Indicates that all or part of the response message is intended for a single 
    user and MUST NOT be cached by a shared cache. This allows an origin server 
    to state that the specified parts of the response are intended for only one 
    user and are not a valid response for requests by other users. A private 
    (non-shared) cache MAY cache the response. 

max-age=0意味着它可以被高速缓存到0秒。值为零意味着不应执行缓存。

Expires=-1当存在max-age时应该忽略，-1是无效日期，应该在过去被解析为一个值（意味着已经过期）。

从RFC2616 HTTP/1.1 Header Field Definitions, 14.21 Expires：

Note: if a response includes a Cache-Control field with the max-age directive 
     (see section 14.9.3), that directive overrides the Expires field 

HTTP/1.1 clients and caches MUST treat other invalid date formats, especially 
including the value "0", as in the past (i.e., "already expired"). 

为persistent connections的Connection: Keep-Alive和Keep-Alive: timeout=60, max=100配置设置。除非另有说明，否则使用HTTP/1.1的所有连接都是持久性的，但这些标头会更改实际的超时值，而不是使用浏览器默认值（其变化很大）。