Server:www121 Server:www120 Server:NWS_SP 内容被散列,并在响应中放入Etag When to Use Entity-Tags and Last-Modified Dates
1 Request URL:http://www.biyao.com/minisite/bzzx 2 Request Method:GET 3 Status Code:200 OK 4 Remote Address:127.0.0.1:51759 5 Response Headers 6 view source 7 Content-Encoding:gzip 8 Content-Language:en-US 9 Content-Type:text/html;charset=UTF-8 10 Date:Wed, 17 Aug 2016 04:05:59 GMT 11 Server:www120 12 Set-Cookie:uuid=b111a7df-08ca-4704-9780-25581ec7142b; Domain=.biyao.com; Expires=Fri, 10-Aug-2046 04:05:59 GMT; Path=/ 13 Set-Cookie:uuid=b111a7df-08ca-4704-9780-25581ec7142b; Domain=.biyao.com; Expires=Fri, 10-Aug-2046 04:05:59 GMT; Path=/ 14 Transfer-Encoding:chunked 15 Vary:Accept-Encoding 16 Request Headers 17 view source 18 Accept:text/html,application/xhtml+xml,application/xml;q=0.9,image/webp,*/*;q=0.8 19 Accept-Encoding:gzip, deflate, sdch 20 Accept-Language:en-US,en;q=0.8,zh-CN;q=0.6,zh;q=0.4 21 Cache-Control:max-age=0 22 Cookie:DZVISIT=02A2CC8EC82E7002EA08F07B1D3E7EC6D22F652214213EE05F103F7FAA99E7AD5C46A8900EB8A34E43A149CF9CC3F9E9C888976F3F8AA696885C752F70D3485EB1752D2DA1C96E8EE2E523A0AE962099009DEEC9C8A937C8; JSESSIONID=F2F07FC5DE09FCA936984F1D50C5B0DB; uuid=b111a7df-08ca-4704-9780-25581ec7142b; Hm_lvt_8263bc34c44278c176458d5aca724aed=1471405869; Hm_lpvt_8263bc34c44278c176458d5aca724aed=1471406755 23 Host:www.biyao.com 24 Proxy-Connection:keep-alive 25 Referer:http://www.biyao.com/help/tel.html 26 Upgrade-Insecure-Requests:1 27 User-Agent:Mozilla/5.0 (Windows NT 6.1; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/52.0.2743.116 Safari/537.36
1 Request URL:http://www.biyao.com/home/index.html 2 Request Method:GET 3 Status Code:200 OK 4 Remote Address:127.0.0.1:51759 5 Response Headers 6 view source 7 Content-Encoding:gzip 8 Content-Type:text/html 9 Date:Wed, 17 Aug 2016 04:06:59 GMT 10 Etag:W/"57b3da02-7eee" 11 Last-Modified:Wed, 17 Aug 2016 03:29:06 GMT 12 Server:nginx 13 Thanks:Welcome to our wwww!! 14 Transfer-Encoding:chunked 15 Vary:Accept-Encoding 16 Request Headers 17 view source 18 Accept:text/html,application/xhtml+xml,application/xml;q=0.9,image/webp,*/*;q=0.8 19 Accept-Encoding:gzip, deflate, sdch 20 Accept-Language:en-US,en;q=0.8,zh-CN;q=0.6,zh;q=0.4 21 Cookie:DZVISIT=02A2CC8EC82E7002EA08F07B1D3E7EC6D22F652214213EE05F103F7FAA99E7AD5C46A8900EB8A34E43A149CF9CC3F9E9C888976F3F8AA696885C752F70D3485EB1752D2DA1C96E8EE2E523A0AE962099009DEEC9C8A937C8; JSESSIONID=F2F07FC5DE09FCA936984F1D50C5B0DB; uuid=b111a7df-08ca-4704-9780-25581ec7142b; Hm_lvt_8263bc34c44278c176458d5aca724aed=1471405869; Hm_lpvt_8263bc34c44278c176458d5aca724aed=1471406759 22 Host:www.biyao.com 23 Proxy-Connection:keep-alive 24 Upgrade-Insecure-Requests:1 25 User-Agent:Mozilla/5.0 (Windows NT 6.1; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/52.0.2743.116 Safari/537.36
1 Request URL:http://www.biyao.com/minisite/sjzx 2 Request Method:GET 3 Status Code:200 OK 4 Remote Address:127.0.0.1:51759 5 Response Headers 6 view source 7 Content-Encoding:gzip 8 Content-Language:en-US 9 Content-Type:text/html;charset=UTF-8 10 Date:Wed, 17 Aug 2016 04:08:09 GMT 11 Server:www121 12 Set-Cookie:uuid=b111a7df-08ca-4704-9780-25581ec7142b; Domain=.biyao.com; Expires=Fri, 10-Aug-2046 04:08:10 GMT; Path=/ 13 Set-Cookie:uuid=b111a7df-08ca-4704-9780-25581ec7142b; Domain=.biyao.com; Expires=Fri, 10-Aug-2046 04:08:10 GMT; Path=/ 14 Set-Cookie:JSESSIONID=34D6928413A453F6AA2C0FF23FDC5E78; Path=/; HttpOnly 15 Transfer-Encoding:chunked 16 Vary:Accept-Encoding 17 Request Headers 18 view source 19 Accept:text/html,application/xhtml+xml,application/xml;q=0.9,image/webp,*/*;q=0.8 20 Accept-Encoding:gzip, deflate, sdch 21 Accept-Language:en-US,en;q=0.8,zh-CN;q=0.6,zh;q=0.4 22 Cookie:DZVISIT=02A2CC8EC82E7002EA08F07B1D3E7EC6D22F652214213EE05F103F7FAA99E7AD5C46A8900EB8A34E43A149CF9CC3F9E9C888976F3F8AA696885C752F70D3485EB1752D2DA1C96E8EE2E523A0AE962099009DEEC9C8A937C8; JSESSIONID=F2F07FC5DE09FCA936984F1D50C5B0DB; uuid=b111a7df-08ca-4704-9780-25581ec7142b; Hm_lvt_8263bc34c44278c176458d5aca724aed=1471405869; Hm_lpvt_8263bc34c44278c176458d5aca724aed=1471406820 23 Host:www.biyao.com 24 Proxy-Connection:keep-alive 25 Referer:http://www.biyao.com/home/index.html 26 Upgrade-Insecure-Requests:1 27 User-Agent:Mozilla/5.0 (Windows NT 6.1; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/52.0.2743.116 Safari/537.36 28 Name 29 sjzx 30 1 / 27 requests ❘ 7.4 KB / 596 KB transferred ❘ Finish: 11.86 s ❘ DOMContentLoaded: 1.46 s ❘ Load: 10.91 s 31 32 Console
1 Request URL:http://badge.biyao.com/man/index.html?f_upd=fc-27 2 Request Method:GET 3 Status Code:200 OK 4 Remote Address:127.0.0.1:51759 5 Response Headers 6 view source 7 Cache-Control:max-age=600 8 Content-Encoding:gzip 9 Content-Length:7172 10 Content-Type:text/html;charset=utf-8 11 Date:Wed, 17 Aug 2016 04:09:44 GMT 12 Expires:Wed, 17 Aug 2016 04:19:44 GMT 13 Last-Modified:Wed, 17 Aug 2016 02:05:47 GMT 14 Server:NWS_SP 15 X-Cache-Lookup:Hit From Disktank Gz 16 Request Headers 17 view source 18 Accept:text/html,application/xhtml+xml,application/xml;q=0.9,image/webp,*/*;q=0.8 19 Accept-Encoding:gzip, deflate, sdch 20 Accept-Language:en-US,en;q=0.8,zh-CN;q=0.6,zh;q=0.4 21 Cookie:DZVISIT=02A2CC8EC82E7002EA08F07B1D3E7EC6D22F652214213EE05F103F7FAA99E7AD5C46A8900EB8A34E43A149CF9CC3F9E9C888976F3F8AA696885C752F70D3485EB1752D2DA1C96E8EE2E523A0AE962099009DEEC9C8A937C8; uuid=b111a7df-08ca-4704-9780-25581ec7142b; Hm_lvt_8263bc34c44278c176458d5aca724aed=1471405869; Hm_lpvt_8263bc34c44278c176458d5aca724aed=1471406891 22 Host:badge.biyao.com 23 Proxy-Connection:keep-alive 24 Referer:http://www.biyao.com/minisite/sjzx 25 Upgrade-Insecure-Requests:1 26 User-Agent:Mozilla/5.0 (Windows NT 6.1; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/52.0.2743.116 Safari/537.36
https://developer.mozilla.org/zh-CN/docs/Web/HTTP/Headers/ETag
ETag
HTTP响应头是资源的特定版本的标识符。这可以让缓存更高效,并节省带宽,因为如果内容没有改变,Web服务器不需要发送完整的响应。而如果内容发生了变化,使用ETag有助于防止资源的同时更新相互覆盖(“空中碰撞”)。
如果给定URL中的资源更改,则一定要生成新的Etag值。 因此Etags类似于指纹,也可能被某些服务器用于跟踪。 比较etags能快速确定此资源是否变化,但也可能被跟踪服务器永久存留。
Header type | Response header |
---|---|
Forbidden header name | no |
语法
ETag: W/"<etag_value>" ETag: "<etag_value>"
指令
W/
可选'W/'
(大小写敏感) 表示使用弱验证器。 弱验证器很容易生成,但不利于比较。 强验证器是比较的理想选择,但很难有效地生成。 相同资源的两个弱Etag
值可能语义等同,但不是每个字节都相同。- "<etag_value>"
- 实体标签唯一地表示所请求的资源。 它们是位于双引号之间的ASCII字符串(如“675af34563dc-tr34”)。 没有明确指定生成ETag值的方法。 通常,使用内容的散列,最后修改时间戳的哈希值,或简单地使用版本号。 例如,MDN使用wiki内容的十六进制数字的哈希值。
示例
ETag: "33a64df551425fcc55e4d42a148795d9f25f89d4"
ETag: W/"0815"
避免“空中碰撞”
在ETag
和 If-Match
头部的帮助下,您可以检测到"空中碰撞"的编辑冲突。
例如,当编辑MDN时,当前的wiki内容被散列,并在响应中放入Etag
:
ETag: "33a64df551425fcc55e4d42a148795d9f25f89d4
将更改保存到Wiki页面(发布数据)时,POST
请求将包含有ETag值的If-Match
头来检查是否为最新版本。
If-Match: "33a64df551425fcc55e4d42a148795d9f25f89d4"
如果哈希值不匹配,则意味着文档已经被编辑,抛出412
前提条件失败错误。
缓存未更改的资源
ETag
头的另一个典型用例是缓存未更改的资源。 如果用户再次访问给定的URL(设有ETag
字段),显示资源过期了且不可用,客户端就发送值为ETag
的If-None-Match
header字段:
If-None-Match: "33a64df551425fcc55e4d42a148795d9f25f89d4"
服务器将客户端的ETag(作为If-None-Match字段的值一起发送)与其当前版本的资源的ETag进行比较,如果两个值匹配(即资源未更改),服务器将返回不带任何内容的304
未修改状态,告诉客户端缓存版本可用(新鲜)。
规范
Specification | Title |
---|---|
RFC 7232, section 2.3: ETag | Hypertext Transfer Protocol (HTTP/1.1): Conditional Requests |
https://tools.ietf.org/html/rfc7232#section-2.3
2.3. ETag
The "ETag" header field in a response provides the current entity-tag
for the selected representation, as determined at the conclusion of
handling the request. An entity-tag is an opaque validator for
differentiating between multiple representations of the same
resource, regardless of whether those multiple representations are
due to resource state changes over time, content negotiation
resulting in multiple representations being valid at the same time,
or both. An entity-tag consists of an opaque quoted string, possibly
prefixed by a weakness indicator.
ETag = entity-tag
entity-tag = [ weak ] opaque-tag
weak = %x57.2F ; "W/", case-sensitive
opaque-tag = DQUOTE *etagc DQUOTE
etagc = %x21 / %x23-7E / obs-text
; VCHAR except double quotes, plus obs-text
Note: Previously, opaque-tag was defined to be a quoted-string
([RFC2616], Section 3.11); thus, some recipients might perform
backslash unescaping. Servers therefore ought to avoid backslash
characters in entity tags.
An entity-tag can be more reliable for validation than a modification
date in situations where it is inconvenient to store modification
dates, where the one-second resolution of HTTP date values is not
sufficient, or where modification dates are not consistently
maintained.
Examples:
ETag: "xyzzy"
ETag: W/"xyzzy"
ETag: ""
Fielding & Reschke Standards Track [Page 9]
RFC 7232 HTTP/1.1 Conditional Requests June 2014 An entity-tag can be either a weak or strong validator, with strong being the default. If an origin server provides an entity-tag for a representation and the generation of that entity-tag does not satisfy all of the characteristics of a strong validator (Section 2.1), then the origin server MUST mark the entity-tag as weak by prefixing its opaque value with "W/" (case-sensitive).
2.3.1. Generation
The principle behind entity-tags is that only the service author
knows the implementation of a resource well enough to select the most
accurate and efficient validation mechanism for that resource, and
that any such mechanism can be mapped to a simple sequence of octets
for easy comparison. Since the value is opaque, there is no need for
the client to be aware of how each entity-tag is constructed.
For example, a resource that has implementation-specific versioning
applied to all changes might use an internal revision number, perhaps
combined with a variance identifier for content negotiation, to
accurately differentiate between representations. Other
implementations might use a collision-resistant hash of
representation content, a combination of various file attributes, or
a modification timestamp that has sub-second resolution.
An origin server SHOULD send an ETag for any selected representation
for which detection of changes can be reasonably and consistently
determined, since the entity-tag's use in conditional requests and
evaluating cache freshness ([RFC7234]) can result in a substantial
reduction of HTTP network traffic and can be a significant factor in
improving service scalability and reliability.
2.3.2. Comparison
There are two entity-tag comparison functions, depending on whether
or not the comparison context allows the use of weak validators:
o Strong comparison: two entity-tags are equivalent if both are not
weak and their opaque-tags match character-by-character.
o Weak comparison: two entity-tags are equivalent if their
opaque-tags match character-by-character, regardless of either or
both being tagged as "weak".
Fielding & Reschke Standards Track [Page 10]
RFC 7232 HTTP/1.1 Conditional Requests June 2014 The example below shows the results for a set of entity-tag pairs and both the weak and strong comparison function results: +--------+--------+-------------------+-----------------+ | ETag 1 | ETag 2 | Strong Comparison | Weak Comparison | +--------+--------+-------------------+-----------------+ | W/"1" | W/"1" | no match | match | | W/"1" | W/"2" | no match | no match | | W/"1" | "1" | no match | match | | "1" | "1" | match | match | +--------+--------+-------------------+-----------------+
2.3.3. Example: Entity-Tags Varying on Content-Negotiated Resources
Consider a resource that is subject to content negotiation (Section
3.4 of [RFC7231]), and where the representations sent in response to
a GET request vary based on the Accept-Encoding request header field
(Section 5.3.4 of [RFC7231]):
>> Request:
GET /index HTTP/1.1
Host: www.example.com
Accept-Encoding: gzip
In this case, the response might or might not use the gzip content
coding. If it does not, the response might look like:
>> Response:
HTTP/1.1 200 OK
Date: Fri, 26 Mar 2010 00:05:00 GMT
ETag: "123-a"
Content-Length: 70
Vary: Accept-Encoding
Content-Type: text/plain
Hello World!
Hello World!
Hello World!
Hello World!
Hello World!
Fielding & Reschke Standards Track [Page 11]
RFC 7232 HTTP/1.1 Conditional Requests June 2014 An alternative representation that does use gzip content coding would be: >> Response: HTTP/1.1 200 OK Date: Fri, 26 Mar 2010 00:05:00 GMT ETag: "123-b" Content-Length: 43 Vary: Accept-Encoding Content-Type: text/plain Content-Encoding: gzip ...binary data... Note: Content codings are a property of the representation data, so a strong entity-tag for a content-encoded representation has to be distinct from the entity tag of an unencoded representation to prevent potential conflicts during cache updates and range requests. In contrast, transfer codings (Section 4 of [RFC7230]) apply only during message transfer and do not result in distinct entity-tags.
2.4. When to Use Entity-Tags and Last-Modified Dates
In 200 (OK) responses to GET or HEAD, an origin server:
o SHOULD send an entity-tag validator unless it is not feasible to
generate one.
o MAY send a weak entity-tag instead of a strong entity-tag, if
performance considerations support the use of weak entity-tags, or
if it is unfeasible to send a strong entity-tag.
o SHOULD send a Last-Modified value if it is feasible to send one.
In other words, the preferred behavior for an origin server is to
send both a strong entity-tag and a Last-Modified value in successful
responses to a retrieval request.
A client:
o MUST send that entity-tag in any cache validation request (using
If-Match or If-None-Match) if an entity-tag has been provided by
the origin server.
Fielding & Reschke Standards Track [Page 12]
RFC 7232 HTTP/1.1 Conditional Requests June 2014 o SHOULD send the Last-Modified value in non-subrange cache validation requests (using If-Modified-Since) if only a Last-Modified value has been provided by the origin server. o MAY send the Last-Modified value in subrange cache validation requests (using If-Unmodified-Since) if only a Last-Modified value has been provided by an HTTP/1.0 origin server. The user agent SHOULD provide a way to disable this, in case of difficulty. o SHOULD send both validators in cache validation requests if both an entity-tag and a Last-Modified value have been provided by the origin server. This allows both HTTP/1.0 and HTTP/1.1 caches to respond appropriately.