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.
184Please respect copyright.ＰＥＮＡＮＡsM75f2vIea
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:184Please respect copyright.ＰＥＮＡＮＡJgxfnOl1OY
---------------http response with etag----------------184Please respect copyright.ＰＥＮＡＮＡZc0YUAR9ez
HTTP/1.1 200 OK184Please respect copyright.ＰＥＮＡＮＡG5Yf1Vm9r1
Date: Mon, 25 Nov 2024 10:56:12 GMT184Please respect copyright.ＰＥＮＡＮＡCb0zNAmwwq
accept-ranges: bytes184Please respect copyright.ＰＥＮＡＮＡtuhLQmvEUl
ETag: "534231200_1732277116"184Please respect copyright.ＰＥＮＡＮＡq7jfkWNdYi
Content-Length: 534231200184Please respect copyright.ＰＥＮＡＮＡGXoKyPVm7s
Content-Type: application/octet-stream184Please respect copyright.ＰＥＮＡＮＡxpvGKHEw2e
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)184Please respect copyright.ＰＥＮＡＮＡkwKMVWb5t3
-----------------------------------------------------
184Please respect copyright.ＰＥＮＡＮＡif5ULSTfGk
Section B. if-none-match
The browser send back the etag to ask server if it has been changed by header like this:184Please respect copyright.ＰＥＮＡＮＡYlPaOt57y7
-----http request for a cached resource------184Please respect copyright.ＰＥＮＡＮＡWiyHt9aFBF
GET /zen.iso HTTP/1.1184Please respect copyright.ＰＥＮＡＮＡrpIu4p6yWR
if-none-match: "534231200_1732277116"184Please respect copyright.ＰＥＮＡＮＡnNZZ8USqxS
---------------------------------------------
If the etag is unchanged, the server response like this:
1. for get and head requests:184Please respect copyright.ＰＥＮＡＮＡU6bwykP73S
-----------http response for unchanged resource------184Please respect copyright.ＰＥＮＡＮＡs3xXinqf1C
HTTP/1.1 304 Not Modified
(http body canbe empty, the client will used its cache)184Please respect copyright.ＰＥＮＡＮＡ1iC46n8zZk
----------------------------------------------------
2. for other methods requests:184Please respect copyright.ＰＥＮＡＮＡSl7TTD1H01
-----------http response for unchanged resource------184Please respect copyright.ＰＥＮＡＮＡSAtXYS5Ewt
HTTP/1.1 412 Precondition Failed
(http body canbe empty, the client will used its cache)184Please respect copyright.ＰＥＮＡＮＡODDNdpkWsV
----------------------------------------------------
184Please respect copyright.ＰＥＮＡＮＡNJaTP0PlA7
If the etag changes, the server response 200 OK with the new file content like this:184Please respect copyright.ＰＥＮＡＮＡUHTOQNh20R
---------------http response with etag----------------184Please respect copyright.ＰＥＮＡＮＡH1llNQBVic
HTTP/1.1 200 OK184Please respect copyright.ＰＥＮＡＮＡIZEN5kgqhn
Date: Mon, 25 Nov 2024 10:56:12 GMT184Please respect copyright.ＰＥＮＡＮＡ1YOVOsPVHA
accept-ranges: bytes184Please respect copyright.ＰＥＮＡＮＡLeGUlLZ6eH
ETag: "534231200_1732277116"184Please respect copyright.ＰＥＮＡＮＡeKY7tbdGrz
Content-Length: 534231200184Please respect copyright.ＰＥＮＡＮＡ1RQTugQuWm
Content-Type: application/octet-stream184Please respect copyright.ＰＥＮＡＮＡpilRBpP9yO
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)184Please respect copyright.ＰＥＮＡＮＡqd7ldygxDg
-----------------------------------------------------
184Please respect copyright.ＰＥＮＡＮＡOkBBIZm5q6
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:184Please respect copyright.ＰＥＮＡＮＡLkoDeQLvlU
------resume download request-------184Please respect copyright.ＰＥＮＡＮＡp4fXXpM9Tv
GET /zen.iso HTTP/1.1184Please respect copyright.ＰＥＮＡＮＡeA9jKQrDpn
Range: bytes=287718144-184Please respect copyright.ＰＥＮＡＮＡEPgaGNJ4rG
If-Match: "534231200_1732277116"184Please respect copyright.ＰＥＮＡＮＡUrfK3WDfIq
-----------------------------------184Please respect copyright.ＰＥＮＡＮＡifNTve5vkL
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:184Please respect copyright.ＰＥＮＡＮＡItfi2O9NTE
-------------range response-------------------184Please respect copyright.ＰＥＮＡＮＡz8nqWQgrrF
HTTP/1.1 206 Partial Content184Please respect copyright.ＰＥＮＡＮＡiYtzaidRtY
accept-ranges: bytes184Please respect copyright.ＰＥＮＡＮＡiXheSfmP43
content-range: bytes 287718144-534231199/534231200184Please respect copyright.ＰＥＮＡＮＡlMsi2ptdLV
ETag: "534231200_1732277116"184Please respect copyright.ＰＥＮＡＮＡi5f0Qo4l5B
Content-Length: 246513056184Please respect copyright.ＰＥＮＡＮＡkPRf8QF690
Content-Type: application/octet-stream184Please respect copyright.ＰＥＮＡＮＡPNOYnUZdiB
Content-Disposition: attachment; filename="zen.iso"
(http body is the remain file content)184Please respect copyright.ＰＥＮＡＮＡT6xQs70U5A
-------------------------------------------
If the etag changes, response 412 like this:184Please respect copyright.ＰＥＮＡＮＡQX2EjOkYiK
-----------http response for changed resource------184Please respect copyright.ＰＥＮＡＮＡPvS8m2KIsu
HTTP/1.1 412 Precondition Failed
(http body canbe empty, the client will stop downloading the resumed file)184Please respect copyright.ＰＥＮＡＮＡDpT0ePavF2
---------------------------------------------------
184Please respect copyright.ＰＥＮＡＮＡUIIpGfuqi3
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---184Please respect copyright.ＰＥＮＡＮＡ7C5bTQtoWO
GET /zen.iso HTTP/1.1184Please respect copyright.ＰＥＮＡＮＡ2rMFMBpdJ0
Range: bytes=287718144-184Please respect copyright.ＰＥＮＡＮＡZRBFFlzOp4
If-Range: "534231200_1732277116"184Please respect copyright.ＰＥＮＡＮＡMhUACRW2Ha
------------------------------------184Please respect copyright.ＰＥＮＡＮＡSVULwnfGzx
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:184Please respect copyright.ＰＥＮＡＮＡ0k2vnZ9lHE
-------------range response-------------------184Please respect copyright.ＰＥＮＡＮＡ2qkU1sSNMY
HTTP/1.1 206 Partial Content184Please respect copyright.ＰＥＮＡＮＡ0a3WTu1Gl2
accept-ranges: bytes184Please respect copyright.ＰＥＮＡＮＡvYcruz8HpM
content-range: bytes 287718144-534231199/534231200184Please respect copyright.ＰＥＮＡＮＡkVVwnYPFer
ETag: "534231200_1732277116"184Please respect copyright.ＰＥＮＡＮＡW86w3Q6P8R
Content-Length: 246513056184Please respect copyright.ＰＥＮＡＮＡpEbQkeW9Hi
Content-Type: application/octet-stream184Please respect copyright.ＰＥＮＡＮＡlTh2cGfC1K
Content-Disposition: attachment; filename="zen.iso"
(http body is the remain file content)184Please respect copyright.ＰＥＮＡＮＡ1qIJ6Q2gGp
-------------------------------------------
If the etag changes, response 200 from beginning like this:184Please respect copyright.ＰＥＮＡＮＡOWyGs4LyeJ
-----------http response from the beginning------184Please respect copyright.ＰＥＮＡＮＡUI3vXBcpUa
HTTP/1.1 200 OK184Please respect copyright.ＰＥＮＡＮＡ4FSmKdXLn5
Date: Mon, 25 Nov 2024 10:56:12 GMT184Please respect copyright.ＰＥＮＡＮＡqHrWJIE0lE
accept-ranges: bytes184Please respect copyright.ＰＥＮＡＮＡZQTlpXZwQo
ETag: "534231200_1732277116"184Please respect copyright.ＰＥＮＡＮＡbZhwh4XJNF
Content-Length: 534231200184Please respect copyright.ＰＥＮＡＮＡdRhbxO9NZG
Content-Type: application/octet-stream184Please respect copyright.ＰＥＮＡＮＡ9XdTOImr0h
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)184Please respect copyright.ＰＥＮＡＮＡWxj16nsKf9
---------------------------------------------------
184Please respect copyright.ＰＥＮＡＮＡXiRxhIKlXX
Section X. Thanks
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/If-None-Match184Please respect copyright.ＰＥＮＡＮＡcH4dWaS1Mu
https://blog.csdn.net/catoop/article/details/134174653184Please respect copyright.ＰＥＮＡＮＡLfg5FHY8d2
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]184Please respect copyright.ＰＥＮＡＮＡfhbhpB0sju
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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