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.
39Please respect copyright.ＰＥＮＡＮＡFwJPRS972b
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:39Please respect copyright.ＰＥＮＡＮＡiXFi3iiDTg
---------------http response with etag----------------39Please respect copyright.ＰＥＮＡＮＡLMKFRaszdf
HTTP/1.1 200 OK39Please respect copyright.ＰＥＮＡＮＡBLpPJkOgcB
Date: Mon, 25 Nov 2024 10:56:12 GMT39Please respect copyright.ＰＥＮＡＮＡK7J6aVDHsp
accept-ranges: bytes39Please respect copyright.ＰＥＮＡＮＡOM8NRLTMOG
ETag: "534231200_1732277116"39Please respect copyright.ＰＥＮＡＮＡbfpk1gAory
Content-Length: 53423120039Please respect copyright.ＰＥＮＡＮＡhxWiXrj2S9
Content-Type: application/octet-stream39Please respect copyright.ＰＥＮＡＮＡoD35zSs6zP
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)39Please respect copyright.ＰＥＮＡＮＡyeL65h9FvC
-----------------------------------------------------
39Please respect copyright.ＰＥＮＡＮＡ0IkNLJ7swU
Section B. if-none-match
The browser send back the etag to ask server if it has been changed by header like this:39Please respect copyright.ＰＥＮＡＮＡGR8NoZlqkr
-----http request for a cached resource------39Please respect copyright.ＰＥＮＡＮＡH8A5iO3Xk9
GET /zen.iso HTTP/1.139Please respect copyright.ＰＥＮＡＮＡCSbuak0fHx
if-none-match: "534231200_1732277116"39Please respect copyright.ＰＥＮＡＮＡTqAJbWC5dq
---------------------------------------------
If the etag is unchanged, the server response like this:
1. for get and head requests:39Please respect copyright.ＰＥＮＡＮＡF7wo4oZ74w
-----------http response for unchanged resource------39Please respect copyright.ＰＥＮＡＮＡNh1CcfWSGF
HTTP/1.1 304 Not Modified
(http body canbe empty, the client will used its cache)39Please respect copyright.ＰＥＮＡＮＡrW8GdeZogW
----------------------------------------------------
2. for other methods requests:39Please respect copyright.ＰＥＮＡＮＡLnRGNlTb2b
-----------http response for unchanged resource------39Please respect copyright.ＰＥＮＡＮＡBgXN4HUtk7
HTTP/1.1 412 Precondition Failed
(http body canbe empty, the client will used its cache)39Please respect copyright.ＰＥＮＡＮＡmmFtrz5xW8
----------------------------------------------------
39Please respect copyright.ＰＥＮＡＮＡLRhNNKSoeP
If the etag changes, the server response 200 OK with the new file content like this:39Please respect copyright.ＰＥＮＡＮＡpuf8AqrN2J
---------------http response with etag----------------39Please respect copyright.ＰＥＮＡＮＡzSJesRqTGL
HTTP/1.1 200 OK39Please respect copyright.ＰＥＮＡＮＡPw4eZbNjw4
Date: Mon, 25 Nov 2024 10:56:12 GMT39Please respect copyright.ＰＥＮＡＮＡ6PMuiQ3f4P
accept-ranges: bytes39Please respect copyright.ＰＥＮＡＮＡp3HiPM3gOj
ETag: "534231200_1732277116"39Please respect copyright.ＰＥＮＡＮＡUucfVwNJWX
Content-Length: 53423120039Please respect copyright.ＰＥＮＡＮＡnrWPRzZdda
Content-Type: application/octet-stream39Please respect copyright.ＰＥＮＡＮＡTuMxyamHoV
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)39Please respect copyright.ＰＥＮＡＮＡIBO8iA05wy
-----------------------------------------------------
39Please respect copyright.ＰＥＮＡＮＡqdmY06wQk9
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:39Please respect copyright.ＰＥＮＡＮＡ1QQp6nKmp5
------resume download request-------39Please respect copyright.ＰＥＮＡＮＡdFNgO815Pd
GET /zen.iso HTTP/1.139Please respect copyright.ＰＥＮＡＮＡn6aAc6rxOu
Range: bytes=287718144-39Please respect copyright.ＰＥＮＡＮＡDm51igehdy
If-Match: "534231200_1732277116"39Please respect copyright.ＰＥＮＡＮＡEEpyO4I8Lw
-----------------------------------39Please respect copyright.ＰＥＮＡＮＡspVjMR25dc
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:39Please respect copyright.ＰＥＮＡＮＡJr2mLHntPO
-------------range response-------------------39Please respect copyright.ＰＥＮＡＮＡbVYUdtd9ee
HTTP/1.1 206 Partial Content39Please respect copyright.ＰＥＮＡＮＡC7TlTE2R50
accept-ranges: bytes39Please respect copyright.ＰＥＮＡＮＡ33u108tJwP
content-range: bytes 287718144-534231199/53423120039Please respect copyright.ＰＥＮＡＮＡatTQe5Ktoy
ETag: "534231200_1732277116"39Please respect copyright.ＰＥＮＡＮＡXbnyZjvmya
Content-Length: 24651305639Please respect copyright.ＰＥＮＡＮＡ4VIefCRN3b
Content-Type: application/octet-stream39Please respect copyright.ＰＥＮＡＮＡvwzCVECcPW
Content-Disposition: attachment; filename="zen.iso"
(http body is the remain file content)39Please respect copyright.ＰＥＮＡＮＡNMJM9LgTc8
-------------------------------------------
If the etag changes, response 412 like this:39Please respect copyright.ＰＥＮＡＮＡe4zbEy8mzN
-----------http response for changed resource------39Please respect copyright.ＰＥＮＡＮＡj3JIxtRe1s
HTTP/1.1 412 Precondition Failed
(http body canbe empty, the client will stop downloading the resumed file)39Please respect copyright.ＰＥＮＡＮＡcvpt3dynkB
---------------------------------------------------
39Please respect copyright.ＰＥＮＡＮＡ4wgPmmqjGA
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---39Please respect copyright.ＰＥＮＡＮＡJRcRAjw1xk
GET /zen.iso HTTP/1.139Please respect copyright.ＰＥＮＡＮＡ9riVoS0dc5
Range: bytes=287718144-39Please respect copyright.ＰＥＮＡＮＡPTQDAchxN0
If-Range: "534231200_1732277116"39Please respect copyright.ＰＥＮＡＮＡhMptRRT7Qk
------------------------------------39Please respect copyright.ＰＥＮＡＮＡL4Choj3Shv
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:39Please respect copyright.ＰＥＮＡＮＡVDBaVcwy6A
-------------range response-------------------39Please respect copyright.ＰＥＮＡＮＡHhkxUaJSBH
HTTP/1.1 206 Partial Content39Please respect copyright.ＰＥＮＡＮＡksGDQ0OfMX
accept-ranges: bytes39Please respect copyright.ＰＥＮＡＮＡBR5285XxfH
content-range: bytes 287718144-534231199/53423120039Please respect copyright.ＰＥＮＡＮＡJ0gfmVoxlA
ETag: "534231200_1732277116"39Please respect copyright.ＰＥＮＡＮＡf0i1pNjsNK
Content-Length: 24651305639Please respect copyright.ＰＥＮＡＮＡEXeGkZnWnh
Content-Type: application/octet-stream39Please respect copyright.ＰＥＮＡＮＡfDPcva1gHb
Content-Disposition: attachment; filename="zen.iso"
(http body is the remain file content)39Please respect copyright.ＰＥＮＡＮＡ7S17kaVa6W
-------------------------------------------
If the etag changes, response 200 from beginning like this:39Please respect copyright.ＰＥＮＡＮＡ7oSq9L3vio
-----------http response from the beginning------39Please respect copyright.ＰＥＮＡＮＡL1npSc9sp5
HTTP/1.1 200 OK39Please respect copyright.ＰＥＮＡＮＡza1m2TICe4
Date: Mon, 25 Nov 2024 10:56:12 GMT39Please respect copyright.ＰＥＮＡＮＡvYnGfefM9T
accept-ranges: bytes39Please respect copyright.ＰＥＮＡＮＡp2ThW4Q9ru
ETag: "534231200_1732277116"39Please respect copyright.ＰＥＮＡＮＡDfwLX4sHAD
Content-Length: 53423120039Please respect copyright.ＰＥＮＡＮＡuqjeinHpbd
Content-Type: application/octet-stream39Please respect copyright.ＰＥＮＡＮＡPr6D2wco6p
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)39Please respect copyright.ＰＥＮＡＮＡWFYZXNgRKV
---------------------------------------------------
39Please respect copyright.ＰＥＮＡＮＡVPpi0UdZc4
Section X. Thanks
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/If-None-Match39Please respect copyright.ＰＥＮＡＮＡbmpn4vV3Wm
https://blog.csdn.net/catoop/article/details/13417465339Please respect copyright.ＰＥＮＡＮＡ20CcH5RYfM
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.com39Please respect copyright.ＰＥＮＡＮＡU0tJpvt03N
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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