HTTP 304&缓存控制：无缓存
我看到对网络服务器的一些调用的以下响应：
初始调用：
HTTP/1.1 200 OK
Date: Mon, 16 Jan 2012 05:46:49 GMT
X-Powered-By: Servlet/2.5 JSP/2.1
Content-Type: text/plain
Content-Length: 78
Content-Encoding: gzip
Etag: "pv2052dae8634d971149a927231e3ceddf"
Cache-Control: no-cache
X-PvInfo: [S10202.C6191.A6057.RA6008.G182D.U3FAE8760].[OT/plaintext.OG/documents]
Vary: Accept-Encoding
Set-Cookie: JSESSIONID=l9pLPT5J1tpgK19Fq2qlT0F15ryByWDLgVLz16ffWPm4qQp6nzzx!-518520380; path=/; HttpOnly
DST=rd319o00000000000000000000ffffac16018bo8200; path=/
Connection: close
后续调用：
HTTP/1.1 304 Not Modified
Date: Mon, 16 Jan 2012 05:48:43 GMT
Connection: close
Etag: "pv2052dae8634d971149a927231e3ceddf"
Cache-Control: no-cache
Vary: Accept-Encoding
我不清楚的是，这两个调用都返回 Cache -Control: 给浏览器的 no-cache 指令。
但是，第二次调用还会返回 304 Not Modified。
如果服务器被指示不缓存早期响应，那么服务器期望页面从哪里提供数据？
有趣的是，我确实看到浏览器中提供的响应，因此浏览器似乎已经缓存了响应，尽管有 no-cache 指令。为什么？
I'm seeing the below response from some calls to a webserver:
Initial call:
HTTP/1.1 200 OK
Date: Mon, 16 Jan 2012 05:46:49 GMT
X-Powered-By: Servlet/2.5 JSP/2.1
Content-Type: text/plain
Content-Length: 78
Content-Encoding: gzip
Etag: "pv2052dae8634d971149a927231e3ceddf"
Cache-Control: no-cache
X-PvInfo: [S10202.C6191.A6057.RA6008.G182D.U3FAE8760].[OT/plaintext.OG/documents]
Vary: Accept-Encoding
Set-Cookie: JSESSIONID=l9pLPT5J1tpgK19Fq2qlT0F15ryByWDLgVLz16ffWPm4qQp6nzzx!-518520380; path=/; HttpOnly
DST=rd319o00000000000000000000ffffac16018bo8200; path=/
Connection: close
Subsequent calls:
HTTP/1.1 304 Not Modified
Date: Mon, 16 Jan 2012 05:48:43 GMT
Connection: close
Etag: "pv2052dae8634d971149a927231e3ceddf"
Cache-Control: no-cache
Vary: Accept-Encoding
What I'm unclear about is that both calls return a Cache-Control: no-cache directive to the browser.
However, the second call also returns a 304 Not Modified.
Where does the server expect the page to serve the data from, given that it's been instructed not to cache the earlier response?
Interestingly, I do see the response served in the browser, so the browser appears to have cached the response, despite the no-cache directive. Why?
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带有
Cache-Control: no-cache的响应 并不意味着响应根本不能存储在客户端，而是意味着：因此，客户端可以将响应存储在本地缓存中，但需要由源服务器重新验证响应。如果服务器表明存储在客户端缓存中的响应仍然有效（即 304 响应），则允许客户端使用存储的响应来满足请求。
A response with
Cache-Control: no-cachedoes not mean that the response must not be stored at the client at all, instead it means:So the client is allowed to store the response in the local cache but it needs to revalidate the response by the origin server. If the server says that the response stored in the client’s cache is still valid (i. e. 304 response), the client is allowed to use the stored response to satisfy the request.