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HTTP缓存机制 #6

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AILINGANGEL opened this issue May 27, 2019 · 0 comments

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AILINGANGEL commented May 27, 2019

http缓存分为强缓存和协商缓存，强缓存的优先级高于协商缓存

强缓存有两种方式: expires响应头(http1.0)和cache-control(http1.1)

expires
服务器在请求头上带上这个字段告诉浏览器什么时间过期
cache-control
max-age字段设置多少s之后缓存失效

协商缓存也有两种方式: last-modified和if-modified-since 以及 etag和if-none-match

last-modified和if-modified-since
如果强缓存过期就浏览器就会发送请求到服务器并带上if-modified-since字段，服务器收到之后会返回last-modified字段，如果没炒作if-modified-since字段就会告诉浏览器当前的缓存还是有效的，服务器没有修改过

这种方式存在的问题就在于时间精度问题，如果在1s内修改了文件服务器无法检测出来

etag和if-none-match
服务器为每个文件生成一个etag值，只要文件不变这个etag的值就不变。浏览器请求资源的时候会在if-none-match中带上这个etag值，如果服务器检测发现etag没变就会告诉浏览器当前缓存还是有效的

协商缓存和http 304相关联

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