3. Storing Responses in Caches
rfc 7234 3. Storing Responses in Caches
判断response是否能被存入cache
优先使用1.进行判断，如果没有结果则使用2.进行判断
1.Cache Control Extension
如果cache认识(recognized)这个Cache Control Extension，则reponse是否可以被存进cache只以这个extension定义为准。因为Extension可以override其他条件：
Note that any of the requirements listed above can be overridden by a
cache-control extension;
当没Extension或者cache不认识(unrecognized)这个Extension，则使用2.进行判断.
2.其余条件全部满足
注意到每个条件后面使用的是"and",这意味着所有的都得满足，此时的判断逻辑如下
// 1.没有判断出结果时
2. = (条件A) AND (条件B) AND (条件C) AND (条件D) AND (条件E) AND (条件F)
条件"o the response either"的理解
注意到使用的是"either or",所以只需要其中至少一个情况满足此条件就满足。
contains an Expires header field (see Section 5.3), or
只要包含Expires就行，不管值如何
Expires : 0
上述这种值也是满足条件的，如果一个被存入cache的response的Expires为0，则被判定为stale，cache会去服务器验证，服务器如果返回304，则本地缓存的response也 能发挥作用。
contains a max-age response directive (see Section 5.2.2.8), or
max-age也是类似，只要包含条件就满足，不管值是多少。
这个不懂是什么意思（RFC_TODO）:
contains a Cache Control Extension (see Section 5.2.3) that
allows it to be cached
也许能参考后面code 302的分析？
最终判断逻辑表达
可以存储 = (1.) or (2.)
条件中内容解释
cacheable method
cacheable指method具备了参与"是否被reuse"判断的资格。目前具有cacheable的method如下:
GET cacheable
HEAD cacheable
POST cacheable,当response包含explicit freshness information时
PATCH cacheable,当response包含explicit freshness information时
GET
复用场景
1.GET -> GET
2.GET -> HEAD
HEAD
复用场景
HEAD -> HEAD
POST
复用场景
POST -> GET
cacheable条件：
Responses to POST requests are only cacheable when they include
explicit freshness information (see Section 4.2.1 of [RFC7234]).
However, POST caching is not widely implemented. For cases where an
origin server wishes the client to be able to cache the result of a
POST in a way that can be reused by a later GET, the origin server
MAY send a 200 (OK) response containing the result and a
Content-Location header field that has the same value as the POST's
effective request URI (Section 3.1.4.2).
PATCH
复用场景
1.PATCH -> GET
2.PATCH -> HEAD
cacheable条件：：
A response to this method is only cacheable if it
contains explicit freshness information (such as an Expires header or
"Cache-Control: max-age" directive) as well as the Content-Location
header matching the Request-URI, indicating that the PATCH response
body is a resource representation. A cached PATCH response can only
be used to respond to subsequent GET and HEAD requests; it MUST NOT
be used to respond to other methods (in particular, PATCH).
cacheable status code
默认情况下为cacheable的code
200, 203, 204, 206, 300, 301, 404, 405, 410, 414, and 501:
- cacheable code列表 rfc 7231 Overview of Status Codes
这章中每个code的定义中会标注是否是cacheable rfc 7231 Response Status Codes
308:
308 cacheable的定义在这里rfc 7238 3. 308 Permanent Redirect
A 308 response is cacheable by default; i.e., unless otherwise
indicated by the method definition or explicit cache controls (see
[RFC7234], Section 4.2.2)
默认不是cacheable，但是满足特定条件以后是
302:
rfc 1945:302 Moved Temporarily
rfc 2616:302 Found
rfc 9110:302 Found
目前只在rfc 2616中发现了对302的cachealbe定义:
This response is only cacheable if indicated by a Cache-Control or Expires header
field.
OkHttp中对应的cacheable判定:
// These codes can only be cached with the right response headers.
// http://tools.ietf.org/html/rfc7234#section-3
// s-maxage is not checked because OkHttp is a private cache that should ignore s-maxage.
if (
// Expires
response.header("Expires") == null &&
// Cache-Control
response.cacheControl.maxAgeSeconds == -1 &&
!response.cacheControl.isPublic &&
!response.cacheControl.isPrivate
) {
return false
}
所以暂时(RFC_GUSS)将302可以cacheable的条件理解为stored response满足如下任意一个条件:
- 包含Expires header
- 包含Cache-Control:s-maxage
- 包含Cache-Control:max-age
- 包含Cache-Control:public
- 包含Cache-Control:private
307:
rfc 2616:307 Temporary Redirect
rfc 9110:307 Temporary Redirect
目前只在rfc 2616中发现了对307的cachealbe定义:
This response is only cacheable if indicated by a Cache-Control or Expires header
field.
这句话的意思参考code 302中的分析。
Authorization header
o the Authorization header field (see Section 4.2 of [RFC7235]) does
not appear in the request, if the cache is shared, unless the
response explicitly allows it (see Section 3.2), and
shared cache 不能(MUST NOT)缓存request(Authorization header)-response,除非response至少携带:
- public
- s-maxage
- must-revalidate
