缓存机制
参考文档
- MDN - HTTP cachingopen in new window
- RFC 7234: Hypertext Transfer Protocol (HTTP/1.1): 4.2.2. Calculating Heuristic Freshnessopen in new window
浏览器默认过期时间（启发式新鲜度检查）
A 页面 HTML 的响应头：
# Response headers
Connection: keep-alive
Content-Encoding: gzip
Content-Type: text/html; charset=utf-8
Date: Wed, 29 Dec 2021 10:41:07 GMT
Transfer-Encoding: chunked
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3
4
5
6
B 页面 HTML 的响应头：
# Response headers
Connection: keep-alive
Date: Wed, 29 Dec 2021 10:39:40 GMT
ETag: "61cadb92-1cf6"
Last-Modified: Tue, 28 Dec 2021 09:40:34 GMT
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3
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问：浏览器会缓存页面 A、B 的 HTML 吗？
答：浏览器会缓存页面 B 的 HTML，但不会缓存页面 A 的 HTML。
以上两个 HTML 的响应头里都没有Cache-Control和Expires，且响应头里也不包含其他缓存相关的限制，因此浏览器默认会采用一个启发式的算法, 通常会取响应头的Date值 - Last-Modified值的10%作为缓存时间。
MDN 原文如下：
Heuristic freshness checking
If an origin server does not explicitly specify freshness (for example, using Cache-Control or Expires header) then a heuristic approach may be used.
If this is the case, look for a Last-Modified header. If the header is present, then the cache's freshness lifetime is equal to the value of the Date header minus the value of the Last-modified header divided by 10. The expiration time is computed as follows:
expirationTime = responseTime + freshnessLifetime - currentAge
where responseTime is the time at which the response was received according to the browser. For more information see RFC 7234: Hypertext Transfer Protocol (HTTP/1.1): 4.2.2. Calculating Heuristic Freshnessopen in new window