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.
206Please respect copyright.ＰＥＮＡＮＡyOp5zFhy39
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:206Please respect copyright.ＰＥＮＡＮＡPNRWHOObFp
---------------http response with etag----------------206Please respect copyright.ＰＥＮＡＮＡ6q2nsDM0Yu
HTTP/1.1 200 OK206Please respect copyright.ＰＥＮＡＮＡ2iBq5VuWtw
Date: Mon, 25 Nov 2024 10:56:12 GMT206Please respect copyright.ＰＥＮＡＮＡt8r82ViuMv
accept-ranges: bytes206Please respect copyright.ＰＥＮＡＮＡ7s8jZAZd1h
ETag: "534231200_1732277116"206Please respect copyright.ＰＥＮＡＮＡA4qPgiV2vZ
Content-Length: 534231200206Please respect copyright.ＰＥＮＡＮＡge70FPNQVT
Content-Type: application/octet-stream206Please respect copyright.ＰＥＮＡＮＡlaVDaduaZl
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)206Please respect copyright.ＰＥＮＡＮＡIptTMbPstR
-----------------------------------------------------
206Please respect copyright.ＰＥＮＡＮＡMire38DgOd
Section B. if-none-match
The browser send back the etag to ask server if it has been changed by header like this:206Please respect copyright.ＰＥＮＡＮＡX77Vgi2h6B
-----http request for a cached resource------206Please respect copyright.ＰＥＮＡＮＡQJM4xwkD0r
GET /zen.iso HTTP/1.1206Please respect copyright.ＰＥＮＡＮＡBn7rwI77YL
if-none-match: "534231200_1732277116"206Please respect copyright.ＰＥＮＡＮＡUxZbW5k6pU
---------------------------------------------
If the etag is unchanged, the server response like this:
1. for get and head requests:206Please respect copyright.ＰＥＮＡＮＡsEP8KmCegR
-----------http response for unchanged resource------206Please respect copyright.ＰＥＮＡＮＡjzUJHe0N7q
HTTP/1.1 304 Not Modified
(http body canbe empty, the client will used its cache)206Please respect copyright.ＰＥＮＡＮＡYdnOscuBuE
----------------------------------------------------
2. for other methods requests:206Please respect copyright.ＰＥＮＡＮＡckC1Z14ygT
-----------http response for unchanged resource------206Please respect copyright.ＰＥＮＡＮＡiyzsfqJo0d
HTTP/1.1 412 Precondition Failed
(http body canbe empty, the client will used its cache)206Please respect copyright.ＰＥＮＡＮＡsRn8YV8lxc
----------------------------------------------------
206Please respect copyright.ＰＥＮＡＮＡv0EoUXTObY
If the etag changes, the server response 200 OK with the new file content like this:206Please respect copyright.ＰＥＮＡＮＡgCXiWPwy5Y
---------------http response with etag----------------206Please respect copyright.ＰＥＮＡＮＡBJGhzBKjlX
HTTP/1.1 200 OK206Please respect copyright.ＰＥＮＡＮＡ4EkMVWImyI
Date: Mon, 25 Nov 2024 10:56:12 GMT206Please respect copyright.ＰＥＮＡＮＡvTuthzrPco
accept-ranges: bytes206Please respect copyright.ＰＥＮＡＮＡ3WGarUJuwn
ETag: "534231200_1732277116"206Please respect copyright.ＰＥＮＡＮＡ2Nh0ZiHZ4a
Content-Length: 534231200206Please respect copyright.ＰＥＮＡＮＡlHCOO0ua65
Content-Type: application/octet-stream206Please respect copyright.ＰＥＮＡＮＡaAJk0Ut6pR
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)206Please respect copyright.ＰＥＮＡＮＡCp24UnUaJu
-----------------------------------------------------
206Please respect copyright.ＰＥＮＡＮＡpvtVlRwYJX
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:206Please respect copyright.ＰＥＮＡＮＡhHY2UeOpOv
------resume download request-------206Please respect copyright.ＰＥＮＡＮＡxQboi3HkIy
GET /zen.iso HTTP/1.1206Please respect copyright.ＰＥＮＡＮＡ85HdvCeXkr
Range: bytes=287718144-206Please respect copyright.ＰＥＮＡＮＡmEte57q4sR
If-Match: "534231200_1732277116"206Please respect copyright.ＰＥＮＡＮＡ0cb27MBnsU
-----------------------------------206Please respect copyright.ＰＥＮＡＮＡOatwC7i5HI
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:206Please respect copyright.ＰＥＮＡＮＡqSqI4fSbzU
-------------range response-------------------206Please respect copyright.ＰＥＮＡＮＡEnjdjcClr2
HTTP/1.1 206 Partial Content206Please respect copyright.ＰＥＮＡＮＡdm9mOl10pt
accept-ranges: bytes206Please respect copyright.ＰＥＮＡＮＡ2e21m6BEWQ
content-range: bytes 287718144-534231199/534231200206Please respect copyright.ＰＥＮＡＮＡIs9Uw3RkRO
ETag: "534231200_1732277116"206Please respect copyright.ＰＥＮＡＮＡDm4nXPC2OS
Content-Length: 246513056206Please respect copyright.ＰＥＮＡＮＡ93tepMfCsf
Content-Type: application/octet-stream206Please respect copyright.ＰＥＮＡＮＡNz5auJLvaF
Content-Disposition: attachment; filename="zen.iso"
(http body is the remain file content)206Please respect copyright.ＰＥＮＡＮＡ3g2O7eul1i
-------------------------------------------
If the etag changes, response 412 like this:206Please respect copyright.ＰＥＮＡＮＡ0Fu8CY1MEC
-----------http response for changed resource------206Please respect copyright.ＰＥＮＡＮＡTlIsExRGoc
HTTP/1.1 412 Precondition Failed
(http body canbe empty, the client will stop downloading the resumed file)206Please respect copyright.ＰＥＮＡＮＡMjlzOXvRts
---------------------------------------------------
206Please respect copyright.ＰＥＮＡＮＡUShm5bcfqC
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---206Please respect copyright.ＰＥＮＡＮＡe5KIljHbaD
GET /zen.iso HTTP/1.1206Please respect copyright.ＰＥＮＡＮＡoCI1BRU6dR
Range: bytes=287718144-206Please respect copyright.ＰＥＮＡＮＡOD70nJDMiG
If-Range: "534231200_1732277116"206Please respect copyright.ＰＥＮＡＮＡJLHSRVhUkT
------------------------------------206Please respect copyright.ＰＥＮＡＮＡS4LB0CERpO
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:206Please respect copyright.ＰＥＮＡＮＡzViyyqEfVA
-------------range response-------------------206Please respect copyright.ＰＥＮＡＮＡJ1I8F2x4EB
HTTP/1.1 206 Partial Content206Please respect copyright.ＰＥＮＡＮＡbCeGj5criM
accept-ranges: bytes206Please respect copyright.ＰＥＮＡＮＡvbKB178S3g
content-range: bytes 287718144-534231199/534231200206Please respect copyright.ＰＥＮＡＮＡEg9GU8ZgVZ
ETag: "534231200_1732277116"206Please respect copyright.ＰＥＮＡＮＡ92kzelz3RT
Content-Length: 246513056206Please respect copyright.ＰＥＮＡＮＡw5EQUlpX6K
Content-Type: application/octet-stream206Please respect copyright.ＰＥＮＡＮＡXqF1Hrhcwv
Content-Disposition: attachment; filename="zen.iso"
(http body is the remain file content)206Please respect copyright.ＰＥＮＡＮＡsrzrmmjEt8
-------------------------------------------
If the etag changes, response 200 from beginning like this:206Please respect copyright.ＰＥＮＡＮＡ6LOkqOUosU
-----------http response from the beginning------206Please respect copyright.ＰＥＮＡＮＡtBzbYVsL2H
HTTP/1.1 200 OK206Please respect copyright.ＰＥＮＡＮＡCvHC2s9G1u
Date: Mon, 25 Nov 2024 10:56:12 GMT206Please respect copyright.ＰＥＮＡＮＡGXDj8fSpzy
accept-ranges: bytes206Please respect copyright.ＰＥＮＡＮＡnywdrqCmKl
ETag: "534231200_1732277116"206Please respect copyright.ＰＥＮＡＮＡp8X2fRpHVE
Content-Length: 534231200206Please respect copyright.ＰＥＮＡＮＡOXHKhmfd23
Content-Type: application/octet-stream206Please respect copyright.ＰＥＮＡＮＡx8XHflyZ14
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)206Please respect copyright.ＰＥＮＡＮＡpsvUBMSn5l
---------------------------------------------------
206Please respect copyright.ＰＥＮＡＮＡ9vEV9TQpud
Section X. Thanks
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/If-None-Match206Please respect copyright.ＰＥＮＡＮＡI719IeYnOb
https://blog.csdn.net/catoop/article/details/134174653206Please respect copyright.ＰＥＮＡＮＡgtGM5pXBZS
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]206Please respect copyright.ＰＥＮＡＮＡEs9eZ7ZnsO
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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