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.
86Please respect copyright.ＰＥＮＡＮＡmvJwCzSRlN
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:86Please respect copyright.ＰＥＮＡＮＡvcPTAKSCnw
---------------http response with etag----------------86Please respect copyright.ＰＥＮＡＮＡwBFZTnIK0I
HTTP/1.1 200 OK86Please respect copyright.ＰＥＮＡＮＡDuA0OmvyFR
Date: Mon, 25 Nov 2024 10:56:12 GMT86Please respect copyright.ＰＥＮＡＮＡHpH161wgEQ
accept-ranges: bytes86Please respect copyright.ＰＥＮＡＮＡRX9JOBCJYj
ETag: "534231200_1732277116"86Please respect copyright.ＰＥＮＡＮＡNxCXed3pr8
Content-Length: 53423120086Please respect copyright.ＰＥＮＡＮＡRc9TuIASd5
Content-Type: application/octet-stream86Please respect copyright.ＰＥＮＡＮＡkgmPrx62iS
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)86Please respect copyright.ＰＥＮＡＮＡHln1pPxOfa
-----------------------------------------------------
86Please respect copyright.ＰＥＮＡＮＡIvD1d5CFP1
Section B. if-none-match
The browser send back the etag to ask server if it has been changed by header like this:86Please respect copyright.ＰＥＮＡＮＡNRjJh8tCIZ
-----http request for a cached resource------86Please respect copyright.ＰＥＮＡＮＡ6vC60LVJVI
GET /zen.iso HTTP/1.186Please respect copyright.ＰＥＮＡＮＡBhaoJTwdHN
if-none-match: "534231200_1732277116"86Please respect copyright.ＰＥＮＡＮＡUtqlSK8xwN
---------------------------------------------
If the etag is unchanged, the server response like this:
1. for get and head requests:86Please respect copyright.ＰＥＮＡＮＡM0bUP9hsTU
-----------http response for unchanged resource------86Please respect copyright.ＰＥＮＡＮＡ93jUftF5rt
HTTP/1.1 304 Not Modified
(http body canbe empty, the client will used its cache)86Please respect copyright.ＰＥＮＡＮＡ43x3hevOsG
----------------------------------------------------
2. for other methods requests:86Please respect copyright.ＰＥＮＡＮＡRx12qB2MYd
-----------http response for unchanged resource------86Please respect copyright.ＰＥＮＡＮＡ74TgK6VBF9
HTTP/1.1 412 Precondition Failed
(http body canbe empty, the client will used its cache)86Please respect copyright.ＰＥＮＡＮＡEB7D0Uw8PW
----------------------------------------------------
86Please respect copyright.ＰＥＮＡＮＡqvXrfUV7on
If the etag changes, the server response 200 OK with the new file content like this:86Please respect copyright.ＰＥＮＡＮＡ7veVhRusG2
---------------http response with etag----------------86Please respect copyright.ＰＥＮＡＮＡCqa4tBVHAT
HTTP/1.1 200 OK86Please respect copyright.ＰＥＮＡＮＡPpGAAPMoFg
Date: Mon, 25 Nov 2024 10:56:12 GMT86Please respect copyright.ＰＥＮＡＮＡqywA2xfo2X
accept-ranges: bytes86Please respect copyright.ＰＥＮＡＮＡTfjUhsiyw4
ETag: "534231200_1732277116"86Please respect copyright.ＰＥＮＡＮＡVpPK2SMAi9
Content-Length: 53423120086Please respect copyright.ＰＥＮＡＮＡcn7GDB1BEx
Content-Type: application/octet-stream86Please respect copyright.ＰＥＮＡＮＡkZw6GMhhQa
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)86Please respect copyright.ＰＥＮＡＮＡhwMXNB6DpA
-----------------------------------------------------
86Please respect copyright.ＰＥＮＡＮＡLxh0kKfFYD
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:86Please respect copyright.ＰＥＮＡＮＡHxCSmiXYTf
------resume download request-------86Please respect copyright.ＰＥＮＡＮＡfgnlT16Y1O
GET /zen.iso HTTP/1.186Please respect copyright.ＰＥＮＡＮＡNoQkMU7W78
Range: bytes=287718144-86Please respect copyright.ＰＥＮＡＮＡk5SE8yaE36
If-Match: "534231200_1732277116"86Please respect copyright.ＰＥＮＡＮＡPyyHjoO8bZ
-----------------------------------86Please respect copyright.ＰＥＮＡＮＡ1lSNkVUFMc
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:86Please respect copyright.ＰＥＮＡＮＡFsz0NBIRSy
-------------range response-------------------86Please respect copyright.ＰＥＮＡＮＡzQpKOMdnM0
HTTP/1.1 206 Partial Content86Please respect copyright.ＰＥＮＡＮＡlrILMfp08e
accept-ranges: bytes86Please respect copyright.ＰＥＮＡＮＡFY3u9iMN6Y
content-range: bytes 287718144-534231199/53423120086Please respect copyright.ＰＥＮＡＮＡBJsDcJyePn
ETag: "534231200_1732277116"86Please respect copyright.ＰＥＮＡＮＡv4ifGSvdFb
Content-Length: 24651305686Please respect copyright.ＰＥＮＡＮＡzbIH8srhTe
Content-Type: application/octet-stream86Please respect copyright.ＰＥＮＡＮＡ2tQBhWSvQX
Content-Disposition: attachment; filename="zen.iso"
(http body is the remain file content)86Please respect copyright.ＰＥＮＡＮＡlDHXsK8epM
-------------------------------------------
If the etag changes, response 412 like this:86Please respect copyright.ＰＥＮＡＮＡBNwUVZabWa
-----------http response for changed resource------86Please respect copyright.ＰＥＮＡＮＡf5swUBDHCg
HTTP/1.1 412 Precondition Failed
(http body canbe empty, the client will stop downloading the resumed file)86Please respect copyright.ＰＥＮＡＮＡgmaxgrHuy2
---------------------------------------------------
86Please respect copyright.ＰＥＮＡＮＡlilEX5Q5eJ
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---86Please respect copyright.ＰＥＮＡＮＡ1xFg3nRuws
GET /zen.iso HTTP/1.186Please respect copyright.ＰＥＮＡＮＡolYPFkoUQt
Range: bytes=287718144-86Please respect copyright.ＰＥＮＡＮＡnF7lCqwmx9
If-Range: "534231200_1732277116"86Please respect copyright.ＰＥＮＡＮＡzShx9HCN4m
------------------------------------86Please respect copyright.ＰＥＮＡＮＡf6byXACsEs
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:86Please respect copyright.ＰＥＮＡＮＡrJWI46mVJB
-------------range response-------------------86Please respect copyright.ＰＥＮＡＮＡUlOIaI1icO
HTTP/1.1 206 Partial Content86Please respect copyright.ＰＥＮＡＮＡXKpePWuu8F
accept-ranges: bytes86Please respect copyright.ＰＥＮＡＮＡzpTEeIJMV1
content-range: bytes 287718144-534231199/53423120086Please respect copyright.ＰＥＮＡＮＡ7TOtXweAFi
ETag: "534231200_1732277116"86Please respect copyright.ＰＥＮＡＮＡsg8SWQSPHm
Content-Length: 24651305686Please respect copyright.ＰＥＮＡＮＡznM5Prt0ZA
Content-Type: application/octet-stream86Please respect copyright.ＰＥＮＡＮＡm73ZIbMKJn
Content-Disposition: attachment; filename="zen.iso"
(http body is the remain file content)86Please respect copyright.ＰＥＮＡＮＡHXWXamxFiE
-------------------------------------------
If the etag changes, response 200 from beginning like this:86Please respect copyright.ＰＥＮＡＮＡd0YgZrNm4A
-----------http response from the beginning------86Please respect copyright.ＰＥＮＡＮＡbaDa0kC5a2
HTTP/1.1 200 OK86Please respect copyright.ＰＥＮＡＮＡnnlYtKAQjG
Date: Mon, 25 Nov 2024 10:56:12 GMT86Please respect copyright.ＰＥＮＡＮＡcPJr8xgzT2
accept-ranges: bytes86Please respect copyright.ＰＥＮＡＮＡrO2SioAnBa
ETag: "534231200_1732277116"86Please respect copyright.ＰＥＮＡＮＡn0QPJ75org
Content-Length: 53423120086Please respect copyright.ＰＥＮＡＮＡlVelWeB0W4
Content-Type: application/octet-stream86Please respect copyright.ＰＥＮＡＮＡpqxOLPo7lG
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)86Please respect copyright.ＰＥＮＡＮＡ58LOEjsduZ
---------------------------------------------------
86Please respect copyright.ＰＥＮＡＮＡsL8LALtv4s
Section X. Thanks
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/If-None-Match86Please respect copyright.ＰＥＮＡＮＡHeBElU3wX5
https://blog.csdn.net/catoop/article/details/13417465386Please respect copyright.ＰＥＮＡＮＡuvBs7dR3sr
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]86Please respect copyright.ＰＥＮＡＮＡey8EzhobNr
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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