php - 缓存系统如何为网站工作

Question

我搜索了缓存主题，结果如下

<meta http-equiv="Cache-control" content="description">

描述

Public - may be cached in public shared caches
Private - may only be cached in private cache
no-Cache - may not be cached
no-Store - may be cached but not archived

它们到底是什么意思

1.公共(public)共享缓存是否意味着文件将从代理缓存和网关缓存下载

2.私有(private)缓存是指在浏览器缓存中引用的文件还是存储在客户端系统中的缓存

3. 当描述设置为 no-Cache 时，它​​会从网络服务器加载新版本。每次加载页面时都会下载新文件。

4. 当他们说未存档时，他们的真正意思是什么。

请简要解释缓存的工作原理，而不是解释每个人都可以访问的公共(public)方式和有限访问的私有(private)方式。

Answer 1

引自 http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.9.1

14.9.1 What is Cacheable

By default, a response is cacheable if the requirements of the request method, request header fields, and the response status indicate that it is cacheable. Section 13.4 summarizes these defaults for cacheability. The following Cache-Control response directives allow an origin server to override the default cacheability of a response:

public

Indicates that the response MAY be cached by any cache, even if it would normally be non-cacheable or cacheable only within a non- shared cache. (See also Authorization, section 14.8, for additional details.)

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.

Note: This usage of the word private only controls where the response may be cached, and cannot ensure the privacy of the message content. no-cache

If the no-cache directive does not specify a field-name, then a cache MUST NOT use the response to satisfy a subsequent request without successful revalidation with the origin server. This allows an origin server to prevent caching even by caches that have been configured to return stale responses to client requests.

If the no-cache directive does specify one or more field-names, then a cache MAY use the response to satisfy a subsequent request, subject to any other restrictions on caching. However, the specified field-name(s) MUST NOT be sent in the response to a subsequent request without successful revalidation with the origin server. This allows an origin server to prevent the re-use of certain header fields in a response, while still allowing caching of the rest of the response.

Note: Most HTTP/1.0 caches will not recognize or obey this directive.

14.9.2 What May be Stored by Caches

no-store

The purpose of the no-store directive is to prevent the inadvertent release or retention of sensitive information (for example, on backup tapes). The no-store directive applies to the entire message, and MAY be sent either in a response or in a request.

If sent in a request, a cache MUST NOT store any part of either this request or any response to it. If sent in a response, a cache MUST NOT store any part of either this response or the request that elicited it. This directive applies to both non- shared and shared caches. "MUST NOT store" in this context means that the cache MUST NOT intentionally store the information in non-volatile storage, and MUST make a best-effort attempt to remove the information from volatile storage as promptly as possible after forwarding it.

Even when this directive is associated with a response, users might explicitly store such a response outside of the caching system (e.g., with a "Save As" dialog). History buffers MAY store such responses as part of their normal operation.

The purpose of this directive is to meet the stated requirements of certain users and service authors who are concerned about accidental releases of information via unanticipated accesses to cache data structures. While the use of this directive might improve privacy in some cases, we caution that it is NOT in any way a reliable or sufficient mechanism for ensuring privacy. In particular, malicious or compromised caches might not recognize or obey this directive, and communications networks might be vulnerable to eavesdropping.