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.
74Please respect copyright.ＰＥＮＡＮＡ22qdPBecKL
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:74Please respect copyright.ＰＥＮＡＮＡWFtmTi86F9
---------------http response with etag----------------74Please respect copyright.ＰＥＮＡＮＡonFAIGDiYv
HTTP/1.1 200 OK74Please respect copyright.ＰＥＮＡＮＡtyw5fN8PKH
Date: Mon, 25 Nov 2024 10:56:12 GMT74Please respect copyright.ＰＥＮＡＮＡ0fy0xgqUpc
accept-ranges: bytes74Please respect copyright.ＰＥＮＡＮＡi1klCRlsr9
ETag: "534231200_1732277116"74Please respect copyright.ＰＥＮＡＮＡATAnwwSbsA
Content-Length: 53423120074Please respect copyright.ＰＥＮＡＮＡb5ABH1NEFY
Content-Type: application/octet-stream74Please respect copyright.ＰＥＮＡＮＡRKCRgpmi8y
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)74Please respect copyright.ＰＥＮＡＮＡ0ctr4ODwNa
-----------------------------------------------------
74Please respect copyright.ＰＥＮＡＮＡS8y0qbZ46h
Section B. if-none-match
The browser send back the etag to ask server if it has been changed by header like this:74Please respect copyright.ＰＥＮＡＮＡeczgzn5UWX
-----http request for a cached resource------74Please respect copyright.ＰＥＮＡＮＡqiMM4eHhxF
GET /zen.iso HTTP/1.174Please respect copyright.ＰＥＮＡＮＡYkvSqhjjaS
if-none-match: "534231200_1732277116"74Please respect copyright.ＰＥＮＡＮＡ8UvJLxQBfu
---------------------------------------------
If the etag is unchanged, the server response like this:
1. for get and head requests:74Please respect copyright.ＰＥＮＡＮＡ3G7JNfdZwa
-----------http response for unchanged resource------74Please respect copyright.ＰＥＮＡＮＡ8DlhTE4QNl
HTTP/1.1 304 Not Modified
(http body canbe empty, the client will used its cache)74Please respect copyright.ＰＥＮＡＮＡe6etXjGMll
----------------------------------------------------
2. for other methods requests:74Please respect copyright.ＰＥＮＡＮＡ6jxHgJZNDu
-----------http response for unchanged resource------74Please respect copyright.ＰＥＮＡＮＡ3bEO55nBJc
HTTP/1.1 412 Precondition Failed
(http body canbe empty, the client will used its cache)74Please respect copyright.ＰＥＮＡＮＡRgJUCXFARo
----------------------------------------------------
74Please respect copyright.ＰＥＮＡＮＡUEUlSEPYKK
If the etag changes, the server response 200 OK with the new file content like this:74Please respect copyright.ＰＥＮＡＮＡ0IvaUH9VeK
---------------http response with etag----------------74Please respect copyright.ＰＥＮＡＮＡ789EnfLvN3
HTTP/1.1 200 OK74Please respect copyright.ＰＥＮＡＮＡWXfOgNwCHG
Date: Mon, 25 Nov 2024 10:56:12 GMT74Please respect copyright.ＰＥＮＡＮＡjlhc4th3id
accept-ranges: bytes74Please respect copyright.ＰＥＮＡＮＡ6q2h3A2j5C
ETag: "534231200_1732277116"74Please respect copyright.ＰＥＮＡＮＡ1crnFovTSG
Content-Length: 53423120074Please respect copyright.ＰＥＮＡＮＡak2RaLJeY5
Content-Type: application/octet-stream74Please respect copyright.ＰＥＮＡＮＡL48FEwcwWL
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)74Please respect copyright.ＰＥＮＡＮＡjYo05tEBHx
-----------------------------------------------------
74Please respect copyright.ＰＥＮＡＮＡXy5TzkXxRd
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:74Please respect copyright.ＰＥＮＡＮＡ3w2gMRMFJ0
------resume download request-------74Please respect copyright.ＰＥＮＡＮＡYiLEYRpoS3
GET /zen.iso HTTP/1.174Please respect copyright.ＰＥＮＡＮＡb6SqhbmL8M
Range: bytes=287718144-74Please respect copyright.ＰＥＮＡＮＡF7WiZsann8
If-Match: "534231200_1732277116"74Please respect copyright.ＰＥＮＡＮＡJ5tPeVYEGU
-----------------------------------74Please respect copyright.ＰＥＮＡＮＡI8iBwdyOQk
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:74Please respect copyright.ＰＥＮＡＮＡN1of2SVmkp
-------------range response-------------------74Please respect copyright.ＰＥＮＡＮＡvVLX4Xzvez
HTTP/1.1 206 Partial Content74Please respect copyright.ＰＥＮＡＮＡbEnELeT7re
accept-ranges: bytes74Please respect copyright.ＰＥＮＡＮＡVlg1KZn5J6
content-range: bytes 287718144-534231199/53423120074Please respect copyright.ＰＥＮＡＮＡIqBRUkGPbJ
ETag: "534231200_1732277116"74Please respect copyright.ＰＥＮＡＮＡUCSBTOTwl5
Content-Length: 24651305674Please respect copyright.ＰＥＮＡＮＡnmPy4peyUL
Content-Type: application/octet-stream74Please respect copyright.ＰＥＮＡＮＡARBhzSCCxf
Content-Disposition: attachment; filename="zen.iso"
(http body is the remain file content)74Please respect copyright.ＰＥＮＡＮＡtFwprZ8dOs
-------------------------------------------
If the etag changes, response 412 like this:74Please respect copyright.ＰＥＮＡＮＡZsR74dx2VH
-----------http response for changed resource------74Please respect copyright.ＰＥＮＡＮＡ9rwRz6TsUC
HTTP/1.1 412 Precondition Failed
(http body canbe empty, the client will stop downloading the resumed file)74Please respect copyright.ＰＥＮＡＮＡNZmX4oTodX
---------------------------------------------------
74Please respect copyright.ＰＥＮＡＮＡvBmsu1jbCv
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---74Please respect copyright.ＰＥＮＡＮＡB058ueihng
GET /zen.iso HTTP/1.174Please respect copyright.ＰＥＮＡＮＡakqt4z1abT
Range: bytes=287718144-74Please respect copyright.ＰＥＮＡＮＡghcNLV6hF9
If-Range: "534231200_1732277116"74Please respect copyright.ＰＥＮＡＮＡ5CDIvs9HgI
------------------------------------74Please respect copyright.ＰＥＮＡＮＡUYpM8Eb2yM
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:74Please respect copyright.ＰＥＮＡＮＡAQZw5IlH57
-------------range response-------------------74Please respect copyright.ＰＥＮＡＮＡtijfB0F9fN
HTTP/1.1 206 Partial Content74Please respect copyright.ＰＥＮＡＮＡ0qGgeRFuEa
accept-ranges: bytes74Please respect copyright.ＰＥＮＡＮＡMi9ojGs6HU
content-range: bytes 287718144-534231199/53423120074Please respect copyright.ＰＥＮＡＮＡqyLGwCqHeu
ETag: "534231200_1732277116"74Please respect copyright.ＰＥＮＡＮＡoih6uklQXg
Content-Length: 24651305674Please respect copyright.ＰＥＮＡＮＡfnNxWCmfAv
Content-Type: application/octet-stream74Please respect copyright.ＰＥＮＡＮＡeLZG0V2nYV
Content-Disposition: attachment; filename="zen.iso"
(http body is the remain file content)74Please respect copyright.ＰＥＮＡＮＡoTRK64lAkI
-------------------------------------------
If the etag changes, response 200 from beginning like this:74Please respect copyright.ＰＥＮＡＮＡkGDoTcRE5G
-----------http response from the beginning------74Please respect copyright.ＰＥＮＡＮＡSk265UOo2F
HTTP/1.1 200 OK74Please respect copyright.ＰＥＮＡＮＡtdMOtAOloW
Date: Mon, 25 Nov 2024 10:56:12 GMT74Please respect copyright.ＰＥＮＡＮＡmeToljnTRA
accept-ranges: bytes74Please respect copyright.ＰＥＮＡＮＡYPcZGwcgha
ETag: "534231200_1732277116"74Please respect copyright.ＰＥＮＡＮＡAZHqMBBGHZ
Content-Length: 53423120074Please respect copyright.ＰＥＮＡＮＡ9Fo4Yy7EG1
Content-Type: application/octet-stream74Please respect copyright.ＰＥＮＡＮＡwcrTuk8Yim
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)74Please respect copyright.ＰＥＮＡＮＡlJaDdY9Oxm
---------------------------------------------------
74Please respect copyright.ＰＥＮＡＮＡXjhT8NJ9MU
Section X. Thanks
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/If-None-Match74Please respect copyright.ＰＥＮＡＮＡ9bF579HJ1s
https://blog.csdn.net/catoop/article/details/13417465374Please respect copyright.ＰＥＮＡＮＡeuGt4YCM0B
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: si.jinmin@gmail.com74Please respect copyright.ＰＥＮＡＮＡUlTDgbLWPi
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: si.jinmin@gmail.com
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