本文描述了HTTP协议使用Etag对内容进行缓存以便减少不必要的网络传输以提高运行效率的机制。后面用英文写，但我的英文不太好，如有错误之处，希望大家帮我纠正，我的联系方式在后面。
This article describes the HTTP procotol mechanism of using Etag to cache resources and resume file download in order to reduce unnecessory network transfer so the performance can increase.
91Please respect copyright.ＰＥＮＡＮＡiLZze0uV8y
Section A. ETag
Etag is an identity of a resource, for example, we can use "10000_20241125150500" as the identity of a file where 10000 is the file size and 20241125150500 is the last modifed time of the file. We tell client browsers the etag of a file, then the browser cache the file and its etag, when next time the file is requested again, the browser can first just send the etag of the file to server, and if the server find the etag remain unchanged, then the browser can use the cache, no need to get the file from server once again.
The server send the etag of a resource by header like this:91Please respect copyright.ＰＥＮＡＮＡfmwoXa9nZG
---------------http response with etag----------------91Please respect copyright.ＰＥＮＡＮＡshAOmhyEsa
HTTP/1.1 200 OK91Please respect copyright.ＰＥＮＡＮＡpwoVXx0aA9
Date: Mon, 25 Nov 2024 10:56:12 GMT91Please respect copyright.ＰＥＮＡＮＡHAwlzN1Gnc
accept-ranges: bytes91Please respect copyright.ＰＥＮＡＮＡkG1CiteNYw
ETag: "534231200_1732277116"91Please respect copyright.ＰＥＮＡＮＡGvjyrFbdt3
Content-Length: 53423120091Please respect copyright.ＰＥＮＡＮＡLS0GVXeuxl
Content-Type: application/octet-stream91Please respect copyright.ＰＥＮＡＮＡljtR4TQPbq
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)91Please respect copyright.ＰＥＮＡＮＡuDRndRJZ7q
-----------------------------------------------------
91Please respect copyright.ＰＥＮＡＮＡjwPdFT50qM
Section B. if-none-match
The browser send back the etag to ask server if it has been changed by header like this:91Please respect copyright.ＰＥＮＡＮＡws3qQtsBth
-----http request for a cached resource------91Please respect copyright.ＰＥＮＡＮＡIp0QKCdyzi
GET /zen.iso HTTP/1.191Please respect copyright.ＰＥＮＡＮＡ0aB9nqbV2V
if-none-match: "534231200_1732277116"91Please respect copyright.ＰＥＮＡＮＡJJaIq59OqL
---------------------------------------------
If the etag is unchanged, the server response like this:
1. for get and head requests:91Please respect copyright.ＰＥＮＡＮＡKWNAShtUhz
-----------http response for unchanged resource------91Please respect copyright.ＰＥＮＡＮＡr0N3nGQaw8
HTTP/1.1 304 Not Modified
(http body canbe empty, the client will used its cache)91Please respect copyright.ＰＥＮＡＮＡGqrAcKr3Ge
----------------------------------------------------
2. for other methods requests:91Please respect copyright.ＰＥＮＡＮＡnmWPYY7piz
-----------http response for unchanged resource------91Please respect copyright.ＰＥＮＡＮＡy6x47T2yxh
HTTP/1.1 412 Precondition Failed
(http body canbe empty, the client will used its cache)91Please respect copyright.ＰＥＮＡＮＡzzFwh5t5Oq
----------------------------------------------------
91Please respect copyright.ＰＥＮＡＮＡom899VULnA
If the etag changes, the server response 200 OK with the new file content like this:91Please respect copyright.ＰＥＮＡＮＡT2GStnvJPr
---------------http response with etag----------------91Please respect copyright.ＰＥＮＡＮＡX33BDKsJlj
HTTP/1.1 200 OK91Please respect copyright.ＰＥＮＡＮＡZGNA7jhxdJ
Date: Mon, 25 Nov 2024 10:56:12 GMT91Please respect copyright.ＰＥＮＡＮＡnED9t7b6eU
accept-ranges: bytes91Please respect copyright.ＰＥＮＡＮＡyuotsJ37DY
ETag: "534231200_1732277116"91Please respect copyright.ＰＥＮＡＮＡtvH7Wcjtsp
Content-Length: 53423120091Please respect copyright.ＰＥＮＡＮＡ8guokGD3AO
Content-Type: application/octet-stream91Please respect copyright.ＰＥＮＡＮＡdHLnGNe1fW
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)91Please respect copyright.ＰＥＮＡＮＡvV9zvzOlPO
-----------------------------------------------------
91Please respect copyright.ＰＥＮＡＮＡHV4exZVaKf
Section C. if-match
When resuming a download in the browser download manager, the browser sends a range request with the if-match header like this:91Please respect copyright.ＰＥＮＡＮＡQO3fUYA36C
------resume download request-------91Please respect copyright.ＰＥＮＡＮＡQHdX47iKom
GET /zen.iso HTTP/1.191Please respect copyright.ＰＥＮＡＮＡvjY4yPshym
Range: bytes=287718144-91Please respect copyright.ＰＥＮＡＮＡCqXajhN0ec
If-Match: "534231200_1732277116"91Please respect copyright.ＰＥＮＡＮＡPC4t6Ulo5S
-----------------------------------91Please respect copyright.ＰＥＮＡＮＡXh96X39dbW
It tells the server to check the etag in If-Match, if unchanged, resume to get the remain content; otherwise, return 412 Precondition Failed.
The server response for unchanged etag is like this:91Please respect copyright.ＰＥＮＡＮＡeaK1ubNW0t
-------------range response-------------------91Please respect copyright.ＰＥＮＡＮＡMe2tJ8lpGU
HTTP/1.1 206 Partial Content91Please respect copyright.ＰＥＮＡＮＡnM4nnQz7Es
accept-ranges: bytes91Please respect copyright.ＰＥＮＡＮＡ2sAy2RDB24
content-range: bytes 287718144-534231199/53423120091Please respect copyright.ＰＥＮＡＮＡDHUvmjzn0N
ETag: "534231200_1732277116"91Please respect copyright.ＰＥＮＡＮＡiHL6HXyKHg
Content-Length: 24651305691Please respect copyright.ＰＥＮＡＮＡv5CX5sexuc
Content-Type: application/octet-stream91Please respect copyright.ＰＥＮＡＮＡfV7jdXrYEP
Content-Disposition: attachment; filename="zen.iso"
(http body is the remain file content)91Please respect copyright.ＰＥＮＡＮＡq8g5pbc0l9
-------------------------------------------
If the etag changes, response 412 like this:91Please respect copyright.ＰＥＮＡＮＡRWckZ9gmnT
-----------http response for changed resource------91Please respect copyright.ＰＥＮＡＮＡmgTOrDz9rU
HTTP/1.1 412 Precondition Failed
(http body canbe empty, the client will stop downloading the resumed file)91Please respect copyright.ＰＥＮＡＮＡCB55p1dczq
---------------------------------------------------
91Please respect copyright.ＰＥＮＡＮＡoepjJVrlE6
Section D. if-range
If-Range is similar with If-Match, it's used by chrome when resuming a download in the download manager, whereas firefox use If-Match. They are the same except that when the etag changes, if-range response the resource content from beginning, so the resumed download will start downloading from beginning, while if-match just response the 412 error to stop the resumed download. I think chrome do a better job.
---resume download request by chrome---91Please respect copyright.ＰＥＮＡＮＡbYZiBvsKh1
GET /zen.iso HTTP/1.191Please respect copyright.ＰＥＮＡＮＡrd1oBjzo0i
Range: bytes=287718144-91Please respect copyright.ＰＥＮＡＮＡo2ZZVpmyPv
If-Range: "534231200_1732277116"91Please respect copyright.ＰＥＮＡＮＡKwrgGFRJtT
------------------------------------91Please respect copyright.ＰＥＮＡＮＡtTQfnVMeJG
It tells the server to check the etag in If-Range, if unchanged, resume to get the remain content; otherwise, return 200 OK and the file content from the beginning.
The server response for unchanged etag is like this:91Please respect copyright.ＰＥＮＡＮＡG7uarOedmw
-------------range response-------------------91Please respect copyright.ＰＥＮＡＮＡCn14Bi5Ftg
HTTP/1.1 206 Partial Content91Please respect copyright.ＰＥＮＡＮＡptghuIMZJb
accept-ranges: bytes91Please respect copyright.ＰＥＮＡＮＡCIKAyqUEd9
content-range: bytes 287718144-534231199/53423120091Please respect copyright.ＰＥＮＡＮＡb1T5nSYKMQ
ETag: "534231200_1732277116"91Please respect copyright.ＰＥＮＡＮＡ3veVppTBbg
Content-Length: 24651305691Please respect copyright.ＰＥＮＡＮＡqUYRSwAAjH
Content-Type: application/octet-stream91Please respect copyright.ＰＥＮＡＮＡPqXYaHSThJ
Content-Disposition: attachment; filename="zen.iso"
(http body is the remain file content)91Please respect copyright.ＰＥＮＡＮＡmu15pTlHd4
-------------------------------------------
If the etag changes, response 200 from beginning like this:91Please respect copyright.ＰＥＮＡＮＡrrmpdLogp4
-----------http response from the beginning------91Please respect copyright.ＰＥＮＡＮＡNxTaimIbMC
HTTP/1.1 200 OK91Please respect copyright.ＰＥＮＡＮＡEmYRw17oCH
Date: Mon, 25 Nov 2024 10:56:12 GMT91Please respect copyright.ＰＥＮＡＮＡmKT790eQED
accept-ranges: bytes91Please respect copyright.ＰＥＮＡＮＡ3CdEkZ1KDS
ETag: "534231200_1732277116"91Please respect copyright.ＰＥＮＡＮＡAAijyPn15V
Content-Length: 53423120091Please respect copyright.ＰＥＮＡＮＡIS19cCopID
Content-Type: application/octet-stream91Please respect copyright.ＰＥＮＡＮＡI3zqp8Rkza
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)91Please respect copyright.ＰＥＮＡＮＡISpycXmWYm
---------------------------------------------------
91Please respect copyright.ＰＥＮＡＮＡLoYMDkYv3R
Section X. Thanks
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/If-None-Match91Please respect copyright.ＰＥＮＡＮＡ7aIPUSruVo
https://blog.csdn.net/catoop/article/details/13417465391Please respect copyright.ＰＥＮＡＮＡGYGiqGdxCU
https://blog.csdn.net/phker/article/details/50722619
Section Y. Contacts Me
If you found any errors or have any suggestions for this article, please let me know, my wechat: si_jinmin, my email: [email protected]91Please respect copyright.ＰＥＮＡＮＡ4efED3sDcJ
如果您发现本文有任何错误，或者对本文有好的建议，欢迎与我联系探讨，我的微信: si_jinmin, 我的email: [email protected]
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