瞭解緩存策略

Question

在研究各種搜索引擎和網站本身＃2採用的緩存策略，我不禁注意到在響應報頭的細微差別：

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 

必須有通過設置背後的一些合乎邏輯的解釋。有人可以照顧解釋差異，以便我們每個人都可以學習和獲益嗎？

Answer 1

從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緩存到0秒。值爲零意味着不應執行緩存。

Expires=-1當存在max-age時應該忽略，-1是無效日期，應該在過去被解析爲一個值（意味着已經過期）。

從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"). 

爲persistent connections的Connection: Keep-Alive和Keep-Alive: timeout=60, max=100配置設置。除非另有說明，否則使用HTTP/1.1的所有連接都是持久性的，但這些標頭會更改實際的超時值，而不是使用瀏覽器默認值（其變化很大）。