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.
84Please respect copyright.ＰＥＮＡＮＡ0to7p7zZlZ
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:84Please respect copyright.ＰＥＮＡＮＡLbaIJQKtZQ
---------------http response with etag----------------84Please respect copyright.ＰＥＮＡＮＡpehdesRzXT
HTTP/1.1 200 OK84Please respect copyright.ＰＥＮＡＮＡgCBAcyNukq
Date: Mon, 25 Nov 2024 10:56:12 GMT84Please respect copyright.ＰＥＮＡＮＡEtrm9FIEts
accept-ranges: bytes84Please respect copyright.ＰＥＮＡＮＡz6CC4q2iSx
ETag: "534231200_1732277116"84Please respect copyright.ＰＥＮＡＮＡ64RXGnU1x3
Content-Length: 53423120084Please respect copyright.ＰＥＮＡＮＡFwcBYSa3Ok
Content-Type: application/octet-stream84Please respect copyright.ＰＥＮＡＮＡk6nTJjAsY8
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)84Please respect copyright.ＰＥＮＡＮＡGzvA05ZVuD
-----------------------------------------------------
84Please respect copyright.ＰＥＮＡＮＡLvFlucSvwz
Section B. if-none-match
The browser send back the etag to ask server if it has been changed by header like this:84Please respect copyright.ＰＥＮＡＮＡcvH9hPPGQA
-----http request for a cached resource------84Please respect copyright.ＰＥＮＡＮＡeHdZzj9zVP
GET /zen.iso HTTP/1.184Please respect copyright.ＰＥＮＡＮＡa2BT2VeSDl
if-none-match: "534231200_1732277116"84Please respect copyright.ＰＥＮＡＮＡ6uaKubCUo5
---------------------------------------------
If the etag is unchanged, the server response like this:
1. for get and head requests:84Please respect copyright.ＰＥＮＡＮＡCx88FFphEm
-----------http response for unchanged resource------84Please respect copyright.ＰＥＮＡＮＡSCKAABIfL4
HTTP/1.1 304 Not Modified
(http body canbe empty, the client will used its cache)84Please respect copyright.ＰＥＮＡＮＡGIFVQHi9SD
----------------------------------------------------
2. for other methods requests:84Please respect copyright.ＰＥＮＡＮＡphJk6RQKtW
-----------http response for unchanged resource------84Please respect copyright.ＰＥＮＡＮＡfrhPkhpIKk
HTTP/1.1 412 Precondition Failed
(http body canbe empty, the client will used its cache)84Please respect copyright.ＰＥＮＡＮＡDB85v9jBR8
----------------------------------------------------
84Please respect copyright.ＰＥＮＡＮＡzNNQU4TBDI
If the etag changes, the server response 200 OK with the new file content like this:84Please respect copyright.ＰＥＮＡＮＡvj7OLiCl6z
---------------http response with etag----------------84Please respect copyright.ＰＥＮＡＮＡOf8xsSY7XT
HTTP/1.1 200 OK84Please respect copyright.ＰＥＮＡＮＡpGza76gKM1
Date: Mon, 25 Nov 2024 10:56:12 GMT84Please respect copyright.ＰＥＮＡＮＡ42Kiy5wMka
accept-ranges: bytes84Please respect copyright.ＰＥＮＡＮＡyGAdRS8qQP
ETag: "534231200_1732277116"84Please respect copyright.ＰＥＮＡＮＡSqLRgvQF6J
Content-Length: 53423120084Please respect copyright.ＰＥＮＡＮＡQsuM89Mpxq
Content-Type: application/octet-stream84Please respect copyright.ＰＥＮＡＮＡolVZAIpwTD
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)84Please respect copyright.ＰＥＮＡＮＡ7hfIkk0ppF
-----------------------------------------------------
84Please respect copyright.ＰＥＮＡＮＡ7MNq3GbLIT
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:84Please respect copyright.ＰＥＮＡＮＡdbci5KEFXM
------resume download request-------84Please respect copyright.ＰＥＮＡＮＡi6pOJcrZ8W
GET /zen.iso HTTP/1.184Please respect copyright.ＰＥＮＡＮＡKdhTheh8Gk
Range: bytes=287718144-84Please respect copyright.ＰＥＮＡＮＡ1YksrLS73M
If-Match: "534231200_1732277116"84Please respect copyright.ＰＥＮＡＮＡUgUIasyZwU
-----------------------------------84Please respect copyright.ＰＥＮＡＮＡps2ostBO8l
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:84Please respect copyright.ＰＥＮＡＮＡgv6NQDkO3u
-------------range response-------------------84Please respect copyright.ＰＥＮＡＮＡqJqCQKMqfC
HTTP/1.1 206 Partial Content84Please respect copyright.ＰＥＮＡＮＡzG2JewH9w6
accept-ranges: bytes84Please respect copyright.ＰＥＮＡＮＡd971ckoeE0
content-range: bytes 287718144-534231199/53423120084Please respect copyright.ＰＥＮＡＮＡxWynz0p4Q7
ETag: "534231200_1732277116"84Please respect copyright.ＰＥＮＡＮＡfxPIcQES0e
Content-Length: 24651305684Please respect copyright.ＰＥＮＡＮＡxhrwlthPE8
Content-Type: application/octet-stream84Please respect copyright.ＰＥＮＡＮＡn64nFeCoas
Content-Disposition: attachment; filename="zen.iso"
(http body is the remain file content)84Please respect copyright.ＰＥＮＡＮＡUBjeEFDFQg
-------------------------------------------
If the etag changes, response 412 like this:84Please respect copyright.ＰＥＮＡＮＡZ6upoCrRVu
-----------http response for changed resource------84Please respect copyright.ＰＥＮＡＮＡVlzDajAvtk
HTTP/1.1 412 Precondition Failed
(http body canbe empty, the client will stop downloading the resumed file)84Please respect copyright.ＰＥＮＡＮＡMDRldYXuZb
---------------------------------------------------
84Please respect copyright.ＰＥＮＡＮＡP4U1GFwWSG
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---84Please respect copyright.ＰＥＮＡＮＡBJJphES8jm
GET /zen.iso HTTP/1.184Please respect copyright.ＰＥＮＡＮＡyUbhLi0wQu
Range: bytes=287718144-84Please respect copyright.ＰＥＮＡＮＡSXxOkEjfQa
If-Range: "534231200_1732277116"84Please respect copyright.ＰＥＮＡＮＡlrp5AMcpR9
------------------------------------84Please respect copyright.ＰＥＮＡＮＡFbuLCD82Dj
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:84Please respect copyright.ＰＥＮＡＮＡLtRNQ9gFYX
-------------range response-------------------84Please respect copyright.ＰＥＮＡＮＡklXKaDDjx2
HTTP/1.1 206 Partial Content84Please respect copyright.ＰＥＮＡＮＡaZmMw0HBYn
accept-ranges: bytes84Please respect copyright.ＰＥＮＡＮＡXDwGethL59
content-range: bytes 287718144-534231199/53423120084Please respect copyright.ＰＥＮＡＮＡKgBggLAtbp
ETag: "534231200_1732277116"84Please respect copyright.ＰＥＮＡＮＡb8WQZlUOzt
Content-Length: 24651305684Please respect copyright.ＰＥＮＡＮＡc4W7Fe62dA
Content-Type: application/octet-stream84Please respect copyright.ＰＥＮＡＮＡVPZSoEkls4
Content-Disposition: attachment; filename="zen.iso"
(http body is the remain file content)84Please respect copyright.ＰＥＮＡＮＡlgJD5u4Yw2
-------------------------------------------
If the etag changes, response 200 from beginning like this:84Please respect copyright.ＰＥＮＡＮＡGaaZohYi0S
-----------http response from the beginning------84Please respect copyright.ＰＥＮＡＮＡJICcvK4MNE
HTTP/1.1 200 OK84Please respect copyright.ＰＥＮＡＮＡ4PPQJ6gesE
Date: Mon, 25 Nov 2024 10:56:12 GMT84Please respect copyright.ＰＥＮＡＮＡGmDV6VQrOX
accept-ranges: bytes84Please respect copyright.ＰＥＮＡＮＡUrsmnI8Mod
ETag: "534231200_1732277116"84Please respect copyright.ＰＥＮＡＮＡHagUOHsZkO
Content-Length: 53423120084Please respect copyright.ＰＥＮＡＮＡ0YQDwtMoXx
Content-Type: application/octet-stream84Please respect copyright.ＰＥＮＡＮＡGxU1iqlUtt
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)84Please respect copyright.ＰＥＮＡＮＡswIKD0vo4W
---------------------------------------------------
84Please respect copyright.ＰＥＮＡＮＡhOWEN6GGeu
Section X. Thanks
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/If-None-Match84Please respect copyright.ＰＥＮＡＮＡHgqFY0eO0E
https://blog.csdn.net/catoop/article/details/13417465384Please respect copyright.ＰＥＮＡＮＡ0AfuAF4zDN
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]84Please respect copyright.ＰＥＮＡＮＡVyDXBwZgi3
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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