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.
33Please respect copyright.ＰＥＮＡＮＡktOtsmxB2V
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:33Please respect copyright.ＰＥＮＡＮＡXuKCdGLIMX
---------------http response with etag----------------33Please respect copyright.ＰＥＮＡＮＡc1ettiGgPs
HTTP/1.1 200 OK33Please respect copyright.ＰＥＮＡＮＡYYDWSzkw6O
Date: Mon, 25 Nov 2024 10:56:12 GMT33Please respect copyright.ＰＥＮＡＮＡJwrPP1Ouei
accept-ranges: bytes33Please respect copyright.ＰＥＮＡＮＡ89EU9HpWMk
ETag: "534231200_1732277116"33Please respect copyright.ＰＥＮＡＮＡjD3gmkuvPg
Content-Length: 53423120033Please respect copyright.ＰＥＮＡＮＡ2F6dfxAu4z
Content-Type: application/octet-stream33Please respect copyright.ＰＥＮＡＮＡ5A1rUsTwLN
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)33Please respect copyright.ＰＥＮＡＮＡJJHBkD9jsE
-----------------------------------------------------
33Please respect copyright.ＰＥＮＡＮＡp18oOOLWYM
Section B. if-none-match
The browser send back the etag to ask server if it has been changed by header like this:33Please respect copyright.ＰＥＮＡＮＡFTUqUwOkr1
-----http request for a cached resource------33Please respect copyright.ＰＥＮＡＮＡcLphHhsyr1
GET /zen.iso HTTP/1.133Please respect copyright.ＰＥＮＡＮＡUZqJfSaytL
if-none-match: "534231200_1732277116"33Please respect copyright.ＰＥＮＡＮＡXuWmii4kqf
---------------------------------------------
If the etag is unchanged, the server response like this:
1. for get and head requests:33Please respect copyright.ＰＥＮＡＮＡHIcGDR4wDS
-----------http response for unchanged resource------33Please respect copyright.ＰＥＮＡＮＡ8WnJ3A5I2N
HTTP/1.1 304 Not Modified
(http body canbe empty, the client will used its cache)33Please respect copyright.ＰＥＮＡＮＡO3LGv6Bl3i
----------------------------------------------------
2. for other methods requests:33Please respect copyright.ＰＥＮＡＮＡiOlqwnzhIk
-----------http response for unchanged resource------33Please respect copyright.ＰＥＮＡＮＡO9eoL8B3Zp
HTTP/1.1 412 Precondition Failed
(http body canbe empty, the client will used its cache)33Please respect copyright.ＰＥＮＡＮＡpcjBmuqhW0
----------------------------------------------------
33Please respect copyright.ＰＥＮＡＮＡmeaxGDHkG7
If the etag changes, the server response 200 OK with the new file content like this:33Please respect copyright.ＰＥＮＡＮＡ97juuQMj8Q
---------------http response with etag----------------33Please respect copyright.ＰＥＮＡＮＡKWmVvlI74q
HTTP/1.1 200 OK33Please respect copyright.ＰＥＮＡＮＡfYtEvZtyId
Date: Mon, 25 Nov 2024 10:56:12 GMT33Please respect copyright.ＰＥＮＡＮＡq2vHF1Jucv
accept-ranges: bytes33Please respect copyright.ＰＥＮＡＮＡvHpZCtCLdD
ETag: "534231200_1732277116"33Please respect copyright.ＰＥＮＡＮＡfEu7s0gKch
Content-Length: 53423120033Please respect copyright.ＰＥＮＡＮＡUZp3LB1l9j
Content-Type: application/octet-stream33Please respect copyright.ＰＥＮＡＮＡzBEnflKA9E
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)33Please respect copyright.ＰＥＮＡＮＡvHrSVVDx6K
-----------------------------------------------------
33Please respect copyright.ＰＥＮＡＮＡiaIoEGsY8q
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:33Please respect copyright.ＰＥＮＡＮＡUhLUz2uO1l
------resume download request-------33Please respect copyright.ＰＥＮＡＮＡ9h3lJPiee7
GET /zen.iso HTTP/1.133Please respect copyright.ＰＥＮＡＮＡz6P4uqxOej
Range: bytes=287718144-33Please respect copyright.ＰＥＮＡＮＡe30WxJaYqi
If-Match: "534231200_1732277116"33Please respect copyright.ＰＥＮＡＮＡiA2OeMluPa
-----------------------------------33Please respect copyright.ＰＥＮＡＮＡDCatxd3JN6
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:33Please respect copyright.ＰＥＮＡＮＡVmDZIWv9SE
-------------range response-------------------33Please respect copyright.ＰＥＮＡＮＡcVf0mcCiy0
HTTP/1.1 206 Partial Content33Please respect copyright.ＰＥＮＡＮＡKyp0iF6zkP
accept-ranges: bytes33Please respect copyright.ＰＥＮＡＮＡdwJYq42tKr
content-range: bytes 287718144-534231199/53423120033Please respect copyright.ＰＥＮＡＮＡ4Fz7rdW6j9
ETag: "534231200_1732277116"33Please respect copyright.ＰＥＮＡＮＡRbbVfoPgAa
Content-Length: 24651305633Please respect copyright.ＰＥＮＡＮＡ7sqUibnpFt
Content-Type: application/octet-stream33Please respect copyright.ＰＥＮＡＮＡ0RQyn8yRq7
Content-Disposition: attachment; filename="zen.iso"
(http body is the remain file content)33Please respect copyright.ＰＥＮＡＮＡzl595MB6ss
-------------------------------------------
If the etag changes, response 412 like this:33Please respect copyright.ＰＥＮＡＮＡnjU0vxAuO0
-----------http response for changed resource------33Please respect copyright.ＰＥＮＡＮＡXX9OVkCxe6
HTTP/1.1 412 Precondition Failed
(http body canbe empty, the client will stop downloading the resumed file)33Please respect copyright.ＰＥＮＡＮＡNX8fBksCdX
---------------------------------------------------
33Please respect copyright.ＰＥＮＡＮＡoQFl4Gl3rl
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---33Please respect copyright.ＰＥＮＡＮＡIqHlQSGLC8
GET /zen.iso HTTP/1.133Please respect copyright.ＰＥＮＡＮＡrXDTWypUCn
Range: bytes=287718144-33Please respect copyright.ＰＥＮＡＮＡ5zhYQYbxDk
If-Range: "534231200_1732277116"33Please respect copyright.ＰＥＮＡＮＡTRp8OhfSi2
------------------------------------33Please respect copyright.ＰＥＮＡＮＡKecpJr1Pxh
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:33Please respect copyright.ＰＥＮＡＮＡUYZdWjD023
-------------range response-------------------33Please respect copyright.ＰＥＮＡＮＡP7epvSO0zP
HTTP/1.1 206 Partial Content33Please respect copyright.ＰＥＮＡＮＡjoRBsp20pH
accept-ranges: bytes33Please respect copyright.ＰＥＮＡＮＡoaHKLkfiMM
content-range: bytes 287718144-534231199/53423120033Please respect copyright.ＰＥＮＡＮＡSmMwxwJBpH
ETag: "534231200_1732277116"33Please respect copyright.ＰＥＮＡＮＡSYecVkqdQY
Content-Length: 24651305633Please respect copyright.ＰＥＮＡＮＡOvc5acmXEJ
Content-Type: application/octet-stream33Please respect copyright.ＰＥＮＡＮＡwhfGv0IoPV
Content-Disposition: attachment; filename="zen.iso"
(http body is the remain file content)33Please respect copyright.ＰＥＮＡＮＡNQoUW91HYk
-------------------------------------------
If the etag changes, response 200 from beginning like this:33Please respect copyright.ＰＥＮＡＮＡzQMX8bVRUc
-----------http response from the beginning------33Please respect copyright.ＰＥＮＡＮＡaqEmveR6h4
HTTP/1.1 200 OK33Please respect copyright.ＰＥＮＡＮＡpno3vHnhJF
Date: Mon, 25 Nov 2024 10:56:12 GMT33Please respect copyright.ＰＥＮＡＮＡ1DxpITH4UR
accept-ranges: bytes33Please respect copyright.ＰＥＮＡＮＡZfirsJXUDa
ETag: "534231200_1732277116"33Please respect copyright.ＰＥＮＡＮＡRFkQ3cjmqT
Content-Length: 53423120033Please respect copyright.ＰＥＮＡＮＡZCFxSQLMLI
Content-Type: application/octet-stream33Please respect copyright.ＰＥＮＡＮＡ9K3ylbnQoC
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)33Please respect copyright.ＰＥＮＡＮＡPWZtOpyaAu
---------------------------------------------------
33Please respect copyright.ＰＥＮＡＮＡWor2hceXKO
Section X. Thanks
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/If-None-Match33Please respect copyright.ＰＥＮＡＮＡ7a2xmsJPAD
https://blog.csdn.net/catoop/article/details/13417465333Please respect copyright.ＰＥＮＡＮＡ3V2McxBFY8
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.com33Please respect copyright.ＰＥＮＡＮＡJQfENbpdil
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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