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.
161Please respect copyright.ＰＥＮＡＮＡERGrPHsx0q
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:161Please respect copyright.ＰＥＮＡＮＡmbJzD0WzXw
---------------http response with etag----------------161Please respect copyright.ＰＥＮＡＮＡK3mmiEuwQs
HTTP/1.1 200 OK161Please respect copyright.ＰＥＮＡＮＡcMn1Lg9Nkh
Date: Mon, 25 Nov 2024 10:56:12 GMT161Please respect copyright.ＰＥＮＡＮＡecxYFHmgl1
accept-ranges: bytes161Please respect copyright.ＰＥＮＡＮＡE9LPOC8Zjj
ETag: "534231200_1732277116"161Please respect copyright.ＰＥＮＡＮＡHsTGMb6pnk
Content-Length: 534231200161Please respect copyright.ＰＥＮＡＮＡbQAxId1Orm
Content-Type: application/octet-stream161Please respect copyright.ＰＥＮＡＮＡZJaUYhxAes
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)161Please respect copyright.ＰＥＮＡＮＡaPtkr5sheC
-----------------------------------------------------
161Please respect copyright.ＰＥＮＡＮＡ4WemJYhTL1
Section B. if-none-match
The browser send back the etag to ask server if it has been changed by header like this:161Please respect copyright.ＰＥＮＡＮＡ6A5nJB69UM
-----http request for a cached resource------161Please respect copyright.ＰＥＮＡＮＡVU97u1ArjN
GET /zen.iso HTTP/1.1161Please respect copyright.ＰＥＮＡＮＡbsSeVmo2oG
if-none-match: "534231200_1732277116"161Please respect copyright.ＰＥＮＡＮＡvGJiLIl8kJ
---------------------------------------------
If the etag is unchanged, the server response like this:
1. for get and head requests:161Please respect copyright.ＰＥＮＡＮＡTTCsLJAlXg
-----------http response for unchanged resource------161Please respect copyright.ＰＥＮＡＮＡR1sd85u1s8
HTTP/1.1 304 Not Modified
(http body canbe empty, the client will used its cache)161Please respect copyright.ＰＥＮＡＮＡt6rptTV7oy
----------------------------------------------------
2. for other methods requests:161Please respect copyright.ＰＥＮＡＮＡPhtMSPPeg7
-----------http response for unchanged resource------161Please respect copyright.ＰＥＮＡＮＡzZkzo9ArZ5
HTTP/1.1 412 Precondition Failed
(http body canbe empty, the client will used its cache)161Please respect copyright.ＰＥＮＡＮＡfhG1Kz5EZC
----------------------------------------------------
161Please respect copyright.ＰＥＮＡＮＡzYQagJNkgx
If the etag changes, the server response 200 OK with the new file content like this:161Please respect copyright.ＰＥＮＡＮＡYX0LYRdtPR
---------------http response with etag----------------161Please respect copyright.ＰＥＮＡＮＡjdSADTfvto
HTTP/1.1 200 OK161Please respect copyright.ＰＥＮＡＮＡ6coCh8L6s5
Date: Mon, 25 Nov 2024 10:56:12 GMT161Please respect copyright.ＰＥＮＡＮＡAnQuK8aWQG
accept-ranges: bytes161Please respect copyright.ＰＥＮＡＮＡtZyeL7F7m5
ETag: "534231200_1732277116"161Please respect copyright.ＰＥＮＡＮＡenkZKX705J
Content-Length: 534231200161Please respect copyright.ＰＥＮＡＮＡHfGJ2crpr6
Content-Type: application/octet-stream161Please respect copyright.ＰＥＮＡＮＡ6fuDDIKclE
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)161Please respect copyright.ＰＥＮＡＮＡde0WgvRY1v
-----------------------------------------------------
161Please respect copyright.ＰＥＮＡＮＡkT3SoBCOK7
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:161Please respect copyright.ＰＥＮＡＮＡhtB27zVHVo
------resume download request-------161Please respect copyright.ＰＥＮＡＮＡrpMMQQGJRv
GET /zen.iso HTTP/1.1161Please respect copyright.ＰＥＮＡＮＡK3HjotVsLb
Range: bytes=287718144-161Please respect copyright.ＰＥＮＡＮＡgepM09kul6
If-Match: "534231200_1732277116"161Please respect copyright.ＰＥＮＡＮＡUkcu7c41la
-----------------------------------161Please respect copyright.ＰＥＮＡＮＡV3zYshEk7U
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:161Please respect copyright.ＰＥＮＡＮＡuAwuBfQnI3
-------------range response-------------------161Please respect copyright.ＰＥＮＡＮＡVS8YfYb8IA
HTTP/1.1 206 Partial Content161Please respect copyright.ＰＥＮＡＮＡngj0mnjTIt
accept-ranges: bytes161Please respect copyright.ＰＥＮＡＮＡ9cBh2GvwiX
content-range: bytes 287718144-534231199/534231200161Please respect copyright.ＰＥＮＡＮＡkeI5eA9Zzj
ETag: "534231200_1732277116"161Please respect copyright.ＰＥＮＡＮＡxdXCBOMnbQ
Content-Length: 246513056161Please respect copyright.ＰＥＮＡＮＡqA4LOvlVRi
Content-Type: application/octet-stream161Please respect copyright.ＰＥＮＡＮＡQOSsOKnwrW
Content-Disposition: attachment; filename="zen.iso"
(http body is the remain file content)161Please respect copyright.ＰＥＮＡＮＡ6C15BzaIq4
-------------------------------------------
If the etag changes, response 412 like this:161Please respect copyright.ＰＥＮＡＮＡwIhumNUT4J
-----------http response for changed resource------161Please respect copyright.ＰＥＮＡＮＡVm32ePC9uZ
HTTP/1.1 412 Precondition Failed
(http body canbe empty, the client will stop downloading the resumed file)161Please respect copyright.ＰＥＮＡＮＡb2Zh0iENPf
---------------------------------------------------
161Please respect copyright.ＰＥＮＡＮＡStIbRPJrJb
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---161Please respect copyright.ＰＥＮＡＮＡ9GcVgHVAJZ
GET /zen.iso HTTP/1.1161Please respect copyright.ＰＥＮＡＮＡCK6nKRc377
Range: bytes=287718144-161Please respect copyright.ＰＥＮＡＮＡiizTifdkSr
If-Range: "534231200_1732277116"161Please respect copyright.ＰＥＮＡＮＡaXudfYUVuj
------------------------------------161Please respect copyright.ＰＥＮＡＮＡor6Yul1qbc
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:161Please respect copyright.ＰＥＮＡＮＡ0UeblLtUk9
-------------range response-------------------161Please respect copyright.ＰＥＮＡＮＡnvNWclgyhE
HTTP/1.1 206 Partial Content161Please respect copyright.ＰＥＮＡＮＡ4ZIQ1lfOyy
accept-ranges: bytes161Please respect copyright.ＰＥＮＡＮＡFK18GSxdH9
content-range: bytes 287718144-534231199/534231200161Please respect copyright.ＰＥＮＡＮＡfVWu34PiVk
ETag: "534231200_1732277116"161Please respect copyright.ＰＥＮＡＮＡJbFRQ5YpX2
Content-Length: 246513056161Please respect copyright.ＰＥＮＡＮＡXSdMJxLozX
Content-Type: application/octet-stream161Please respect copyright.ＰＥＮＡＮＡDYJdJQ5LXL
Content-Disposition: attachment; filename="zen.iso"
(http body is the remain file content)161Please respect copyright.ＰＥＮＡＮＡgbEMn7lLVK
-------------------------------------------
If the etag changes, response 200 from beginning like this:161Please respect copyright.ＰＥＮＡＮＡMtjiCl9YGz
-----------http response from the beginning------161Please respect copyright.ＰＥＮＡＮＡRzRx3XEY4M
HTTP/1.1 200 OK161Please respect copyright.ＰＥＮＡＮＡ6bcPgvIvke
Date: Mon, 25 Nov 2024 10:56:12 GMT161Please respect copyright.ＰＥＮＡＮＡbhGcTRowbC
accept-ranges: bytes161Please respect copyright.ＰＥＮＡＮＡKZ042i3fPo
ETag: "534231200_1732277116"161Please respect copyright.ＰＥＮＡＮＡnEVn96HoX4
Content-Length: 534231200161Please respect copyright.ＰＥＮＡＮＡGmQI7xUhPA
Content-Type: application/octet-stream161Please respect copyright.ＰＥＮＡＮＡQoy09cc45N
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)161Please respect copyright.ＰＥＮＡＮＡGp4SbSsXaQ
---------------------------------------------------
161Please respect copyright.ＰＥＮＡＮＡ8shGgdu9dD
Section X. Thanks
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/If-None-Match161Please respect copyright.ＰＥＮＡＮＡMlQuPAF5wa
https://blog.csdn.net/catoop/article/details/134174653161Please respect copyright.ＰＥＮＡＮＡnsiujO6ydW
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]161Please respect copyright.ＰＥＮＡＮＡdihEbOl5th
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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ns216.73.216.100da2
and then Add to home screen.