缓存控制标头中多个值的含义 [英] meaning of multiple values in cache-control header

问题描述

我已经阅读了有关单个缓存控制标头值的信息.为了测试我学到的东西，我打开了facebook并进行了检查.这是我得到的Cache-Control响应标头:

I've read about single cache-control header value. To test what I learned, I opened facebook and inspect. This is the Cache-Control response header I get:

cache-control:private, no-cache, no-store, must-revalidate

我很困惑此标头实际告诉我们的内容，因为它一次包含4个值.那么，通过网络发送的资源(如果包含此类标头)会发生什么情况?

I am confused what this header actually tells, because it contains 4 values at once. So what happens with the resource send through the network, if it contains such header?

no-store表示根本不存储，不存储在私有非公共缓存中"，no-cache表示可以缓存，但是请确保在请求资源时重新验证新鲜度".私人说:您可以存储在私人缓存中".它不能同时完成所有3个操作.但是，在这里，我们让它们同时发送响应.似乎还有一些我不知道的其他规则.

no-store says, "do not store at all, not in private not public caches", and no-cache says "yeees you can cache, but make sure you revalidate for freshness when resource is requested". Private says "you can store in the private caches". It cant do all 3 at the same time. But yet, here we are having them send in response at the same time. Looks like there are some additional rules I am not aware of.

推荐答案

RFC 7234 是一个标题的准确含义很好的参考.

RFC 7234 is a good reference for the precise meaning of the headers.

无缓存和无存储意味着不同的东西，例如不能同时服从.

no-cache and no-store mean different things and cannot be obeyed at the same time for example.

他们绝对可以.这些指令是多余的，但并不矛盾.无缓存:

They absolutely can. The directives are redundant, but not contradictory. no-cache:

表示不得使用缓存存储的响应，无法成功满足请求在原始服务器上进行验证.

indicates that a cache MUST NOT use a stored response to satisfy the request without successful validation on the origin server.

和不存储:

表示不得缓存存储此请求或其任何响应的任何部分.

indicates that a cache MUST NOT store any part of either this request or any response to it.

由于 no-store 本质上比 no-cache 更严格，因此结果实际上是 no-store .其他标头类似；我相信:

As no-store is essentially stricter than no-cache, the result is effectively no-store. Similarly for the other headers; I believe:

Cache-control: no-store

将是获得相同结果的一种更简单的方法.但是，您看到的标头可能是建议的组合，而不是故意的一致策略.

would be a simpler way to get the same result. However, it's possible that the header you're seeing is a combination of advice, rather than an intentionally consistent policy.

请注意，如规范所说一样，重复了指令可能无效:

Note that, as the spec says, duplicated directives may be invalid:

当给定指令存在多个值时(例如，两个Expires标头字段，多个Cache-Control:max-age指令)，则该指令的值被视为无效.缓存是鼓励考虑新鲜度无效的回复信息陈旧.

When there is more than one value present for a given directive (e.g., two Expires header fields, multiple Cache-Control: max-age directives), the directive's value is considered invalid. Caches are encouraged to consider responses that have invalid freshness information to be stale.

但是我不认为这是事实.

but I don't believe that's the case here.

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