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.
88Please respect copyright.ＰＥＮＡＮＡ1HFo97i41G
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:88Please respect copyright.ＰＥＮＡＮＡsKbNCMyyrx
---------------http response with etag----------------88Please respect copyright.ＰＥＮＡＮＡlHrRsBDXPc
HTTP/1.1 200 OK88Please respect copyright.ＰＥＮＡＮＡvzsDOzARie
Date: Mon, 25 Nov 2024 10:56:12 GMT88Please respect copyright.ＰＥＮＡＮＡM1yu1xzPgn
accept-ranges: bytes88Please respect copyright.ＰＥＮＡＮＡJH7Vi4dwif
ETag: "534231200_1732277116"88Please respect copyright.ＰＥＮＡＮＡBIgNf96cYk
Content-Length: 53423120088Please respect copyright.ＰＥＮＡＮＡwbymV1R0Ye
Content-Type: application/octet-stream88Please respect copyright.ＰＥＮＡＮＡsBusiDHWXi
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)88Please respect copyright.ＰＥＮＡＮＡfugg9byO3F
-----------------------------------------------------
88Please respect copyright.ＰＥＮＡＮＡnauCKrU8h4
Section B. if-none-match
The browser send back the etag to ask server if it has been changed by header like this:88Please respect copyright.ＰＥＮＡＮＡGQVD5Lw2MG
-----http request for a cached resource------88Please respect copyright.ＰＥＮＡＮＡHCgKWIlmfC
GET /zen.iso HTTP/1.188Please respect copyright.ＰＥＮＡＮＡlaz3jiDb18
if-none-match: "534231200_1732277116"88Please respect copyright.ＰＥＮＡＮＡ7FlW3Y58mS
---------------------------------------------
If the etag is unchanged, the server response like this:
1. for get and head requests:88Please respect copyright.ＰＥＮＡＮＡTRrjwWqQb4
-----------http response for unchanged resource------88Please respect copyright.ＰＥＮＡＮＡO9i3bxS1KL
HTTP/1.1 304 Not Modified
(http body canbe empty, the client will used its cache)88Please respect copyright.ＰＥＮＡＮＡGanBi2Bsgp
----------------------------------------------------
2. for other methods requests:88Please respect copyright.ＰＥＮＡＮＡ07771D8SCl
-----------http response for unchanged resource------88Please respect copyright.ＰＥＮＡＮＡwJuapFsNn8
HTTP/1.1 412 Precondition Failed
(http body canbe empty, the client will used its cache)88Please respect copyright.ＰＥＮＡＮＡC929YnqQdT
----------------------------------------------------
88Please respect copyright.ＰＥＮＡＮＡHMhMOaNz3o
If the etag changes, the server response 200 OK with the new file content like this:88Please respect copyright.ＰＥＮＡＮＡF8M4OjJis8
---------------http response with etag----------------88Please respect copyright.ＰＥＮＡＮＡ91ek1CyhHp
HTTP/1.1 200 OK88Please respect copyright.ＰＥＮＡＮＡrWbs9AlnMD
Date: Mon, 25 Nov 2024 10:56:12 GMT88Please respect copyright.ＰＥＮＡＮＡr3ZSs7JQZE
accept-ranges: bytes88Please respect copyright.ＰＥＮＡＮＡxyRAImsvP2
ETag: "534231200_1732277116"88Please respect copyright.ＰＥＮＡＮＡrYZZiZ7Jac
Content-Length: 53423120088Please respect copyright.ＰＥＮＡＮＡHmPqeub8kr
Content-Type: application/octet-stream88Please respect copyright.ＰＥＮＡＮＡB02kgEKUjZ
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)88Please respect copyright.ＰＥＮＡＮＡfOMMrBSNY4
-----------------------------------------------------
88Please respect copyright.ＰＥＮＡＮＡXk6wf66wLv
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:88Please respect copyright.ＰＥＮＡＮＡS1bEL5tsLv
------resume download request-------88Please respect copyright.ＰＥＮＡＮＡF1epjy0dPz
GET /zen.iso HTTP/1.188Please respect copyright.ＰＥＮＡＮＡ2QDO88y7cz
Range: bytes=287718144-88Please respect copyright.ＰＥＮＡＮＡE4euygbY9d
If-Match: "534231200_1732277116"88Please respect copyright.ＰＥＮＡＮＡdJ0PHIq45A
-----------------------------------88Please respect copyright.ＰＥＮＡＮＡCrd68nnHGe
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:88Please respect copyright.ＰＥＮＡＮＡiqekUYw9ot
-------------range response-------------------88Please respect copyright.ＰＥＮＡＮＡaN8902nwMb
HTTP/1.1 206 Partial Content88Please respect copyright.ＰＥＮＡＮＡmH4vJHt4iZ
accept-ranges: bytes88Please respect copyright.ＰＥＮＡＮＡ3PNF0Dt7i1
content-range: bytes 287718144-534231199/53423120088Please respect copyright.ＰＥＮＡＮＡVCKgA3Giwj
ETag: "534231200_1732277116"88Please respect copyright.ＰＥＮＡＮＡj7ULo8aLEL
Content-Length: 24651305688Please respect copyright.ＰＥＮＡＮＡpAFxHwNqVc
Content-Type: application/octet-stream88Please respect copyright.ＰＥＮＡＮＡE6Jc4R3vs4
Content-Disposition: attachment; filename="zen.iso"
(http body is the remain file content)88Please respect copyright.ＰＥＮＡＮＡ0sZNfY2549
-------------------------------------------
If the etag changes, response 412 like this:88Please respect copyright.ＰＥＮＡＮＡ5ix72kObTh
-----------http response for changed resource------88Please respect copyright.ＰＥＮＡＮＡHRbIPsNAnu
HTTP/1.1 412 Precondition Failed
(http body canbe empty, the client will stop downloading the resumed file)88Please respect copyright.ＰＥＮＡＮＡ6lYjsbMrE4
---------------------------------------------------
88Please respect copyright.ＰＥＮＡＮＡGqAR2pMNLI
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---88Please respect copyright.ＰＥＮＡＮＡkCWzD8FRf1
GET /zen.iso HTTP/1.188Please respect copyright.ＰＥＮＡＮＡRnVLfDAhpb
Range: bytes=287718144-88Please respect copyright.ＰＥＮＡＮＡwymkur0wwK
If-Range: "534231200_1732277116"88Please respect copyright.ＰＥＮＡＮＡtZCpUR7Jp9
------------------------------------88Please respect copyright.ＰＥＮＡＮＡmlS0zzd4bQ
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:88Please respect copyright.ＰＥＮＡＮＡNOkmm6nxvk
-------------range response-------------------88Please respect copyright.ＰＥＮＡＮＡC1qxlzJ3Ct
HTTP/1.1 206 Partial Content88Please respect copyright.ＰＥＮＡＮＡO95uPtKcin
accept-ranges: bytes88Please respect copyright.ＰＥＮＡＮＡRtVazKL7ZL
content-range: bytes 287718144-534231199/53423120088Please respect copyright.ＰＥＮＡＮＡS9ATVLG6z4
ETag: "534231200_1732277116"88Please respect copyright.ＰＥＮＡＮＡgycbrGlNKN
Content-Length: 24651305688Please respect copyright.ＰＥＮＡＮＡsJ172BTMHB
Content-Type: application/octet-stream88Please respect copyright.ＰＥＮＡＮＡHXvuwrasGq
Content-Disposition: attachment; filename="zen.iso"
(http body is the remain file content)88Please respect copyright.ＰＥＮＡＮＡaKHktVDtmw
-------------------------------------------
If the etag changes, response 200 from beginning like this:88Please respect copyright.ＰＥＮＡＮＡQ1Qwi9hqFX
-----------http response from the beginning------88Please respect copyright.ＰＥＮＡＮＡRw7SLMJ1vS
HTTP/1.1 200 OK88Please respect copyright.ＰＥＮＡＮＡrIi5dQVh7A
Date: Mon, 25 Nov 2024 10:56:12 GMT88Please respect copyright.ＰＥＮＡＮＡP3Y9Pwz9jY
accept-ranges: bytes88Please respect copyright.ＰＥＮＡＮＡK58ojtnceB
ETag: "534231200_1732277116"88Please respect copyright.ＰＥＮＡＮＡSiZkzz7B6X
Content-Length: 53423120088Please respect copyright.ＰＥＮＡＮＡgX1ZACS6AE
Content-Type: application/octet-stream88Please respect copyright.ＰＥＮＡＮＡvtQWeMB1oa
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)88Please respect copyright.ＰＥＮＡＮＡ5UZqyrq1GI
---------------------------------------------------
88Please respect copyright.ＰＥＮＡＮＡtfhiRo0lgi
Section X. Thanks
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/If-None-Match88Please respect copyright.ＰＥＮＡＮＡZ1oCnSbq5s
https://blog.csdn.net/catoop/article/details/13417465388Please respect copyright.ＰＥＮＡＮＡNFPY3Baa0t
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]88Please respect copyright.ＰＥＮＡＮＡG5u399vrvL
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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