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.ＰＥＮＡＮＡrlslkLIY0l
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.ＰＥＮＡＮＡKNrR4ON6fp
---------------http response with etag----------------206Please respect copyright.ＰＥＮＡＮＡ5PlHFEa7As
HTTP/1.1 200 OK206Please respect copyright.ＰＥＮＡＮＡfuLfEinTiO
Date: Mon, 25 Nov 2024 10:56:12 GMT206Please respect copyright.ＰＥＮＡＮＡAm69YHvEQ5
accept-ranges: bytes206Please respect copyright.ＰＥＮＡＮＡfGDkMA3fZh
ETag: "534231200_1732277116"206Please respect copyright.ＰＥＮＡＮＡms0bskaOG4
Content-Length: 534231200206Please respect copyright.ＰＥＮＡＮＡzhgF2k0iz5
Content-Type: application/octet-stream206Please respect copyright.ＰＥＮＡＮＡWwR28VHMat
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)206Please respect copyright.ＰＥＮＡＮＡKnFf6ZJlUq
-----------------------------------------------------
206Please respect copyright.ＰＥＮＡＮＡdEg8s3uKtN
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.ＰＥＮＡＮＡshoriGn5Jc
-----http request for a cached resource------206Please respect copyright.ＰＥＮＡＮＡi6C8SCF6Dd
GET /zen.iso HTTP/1.1206Please respect copyright.ＰＥＮＡＮＡaH7EORva80
if-none-match: "534231200_1732277116"206Please respect copyright.ＰＥＮＡＮＡHt8X350cDg
---------------------------------------------
If the etag is unchanged, the server response like this:
1. for get and head requests:206Please respect copyright.ＰＥＮＡＮＡCc9vO4B009
-----------http response for unchanged resource------206Please respect copyright.ＰＥＮＡＮＡLKN4ndsUwD
HTTP/1.1 304 Not Modified
(http body canbe empty, the client will used its cache)206Please respect copyright.ＰＥＮＡＮＡ4jtpRnM0S6
----------------------------------------------------
2. for other methods requests:206Please respect copyright.ＰＥＮＡＮＡlrbut3voLi
-----------http response for unchanged resource------206Please respect copyright.ＰＥＮＡＮＡ8IVLeBfa6Q
HTTP/1.1 412 Precondition Failed
(http body canbe empty, the client will used its cache)206Please respect copyright.ＰＥＮＡＮＡSiTo45dI0K
----------------------------------------------------
206Please respect copyright.ＰＥＮＡＮＡdzaVkWryOd
If the etag changes, the server response 200 OK with the new file content like this:206Please respect copyright.ＰＥＮＡＮＡXkKlNvr12O
---------------http response with etag----------------206Please respect copyright.ＰＥＮＡＮＡgYDE9XWgOk
HTTP/1.1 200 OK206Please respect copyright.ＰＥＮＡＮＡMbAJ46Wtk0
Date: Mon, 25 Nov 2024 10:56:12 GMT206Please respect copyright.ＰＥＮＡＮＡAfg9bJnHkD
accept-ranges: bytes206Please respect copyright.ＰＥＮＡＮＡQDniQ5XSIN
ETag: "534231200_1732277116"206Please respect copyright.ＰＥＮＡＮＡxJAPehPPfV
Content-Length: 534231200206Please respect copyright.ＰＥＮＡＮＡYuG9eE9va6
Content-Type: application/octet-stream206Please respect copyright.ＰＥＮＡＮＡDJaFeeMUmI
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)206Please respect copyright.ＰＥＮＡＮＡ9ECb9vd7HB
-----------------------------------------------------
206Please respect copyright.ＰＥＮＡＮＡRxL1GAqJOV
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.ＰＥＮＡＮＡC4SR1fR4B6
------resume download request-------206Please respect copyright.ＰＥＮＡＮＡ2jLnL6l9ON
GET /zen.iso HTTP/1.1206Please respect copyright.ＰＥＮＡＮＡiZ2mB88v5r
Range: bytes=287718144-206Please respect copyright.ＰＥＮＡＮＡ3Ko3Ha7ut2
If-Match: "534231200_1732277116"206Please respect copyright.ＰＥＮＡＮＡk2buE54AJC
-----------------------------------206Please respect copyright.ＰＥＮＡＮＡmYcrhXr2UW
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.ＰＥＮＡＮＡ1OKx1yNGhs
-------------range response-------------------206Please respect copyright.ＰＥＮＡＮＡtch7loBvlY
HTTP/1.1 206 Partial Content206Please respect copyright.ＰＥＮＡＮＡfdFKkKKXwu
accept-ranges: bytes206Please respect copyright.ＰＥＮＡＮＡ2tuLf9kFoH
content-range: bytes 287718144-534231199/534231200206Please respect copyright.ＰＥＮＡＮＡWripcXChtF
ETag: "534231200_1732277116"206Please respect copyright.ＰＥＮＡＮＡc5rcntAJjr
Content-Length: 246513056206Please respect copyright.ＰＥＮＡＮＡ1zFhznIj3F
Content-Type: application/octet-stream206Please respect copyright.ＰＥＮＡＮＡ6ftl9EIRYD
Content-Disposition: attachment; filename="zen.iso"
(http body is the remain file content)206Please respect copyright.ＰＥＮＡＮＡA9TXIT0FoL
-------------------------------------------
If the etag changes, response 412 like this:206Please respect copyright.ＰＥＮＡＮＡGeb6wtvv32
-----------http response for changed resource------206Please respect copyright.ＰＥＮＡＮＡDhyT0MqAE5
HTTP/1.1 412 Precondition Failed
(http body canbe empty, the client will stop downloading the resumed file)206Please respect copyright.ＰＥＮＡＮＡYwCEmu1dJy
---------------------------------------------------
206Please respect copyright.ＰＥＮＡＮＡKbcVBNKTmt
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.ＰＥＮＡＮＡBQTDsVjgn9
GET /zen.iso HTTP/1.1206Please respect copyright.ＰＥＮＡＮＡlvOkC12Q52
Range: bytes=287718144-206Please respect copyright.ＰＥＮＡＮＡutVc6vgMQJ
If-Range: "534231200_1732277116"206Please respect copyright.ＰＥＮＡＮＡmCvEeh6hma
------------------------------------206Please respect copyright.ＰＥＮＡＮＡhd036pa5eK
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.ＰＥＮＡＮＡjMS3J8FAzc
-------------range response-------------------206Please respect copyright.ＰＥＮＡＮＡVeOT64LupX
HTTP/1.1 206 Partial Content206Please respect copyright.ＰＥＮＡＮＡsiRadBEls6
accept-ranges: bytes206Please respect copyright.ＰＥＮＡＮＡWAx3Gq6kr6
content-range: bytes 287718144-534231199/534231200206Please respect copyright.ＰＥＮＡＮＡ7nBUxBQFEN
ETag: "534231200_1732277116"206Please respect copyright.ＰＥＮＡＮＡ96a7p403lb
Content-Length: 246513056206Please respect copyright.ＰＥＮＡＮＡ36G2O2kPjc
Content-Type: application/octet-stream206Please respect copyright.ＰＥＮＡＮＡfzriZsQ3hm
Content-Disposition: attachment; filename="zen.iso"
(http body is the remain file content)206Please respect copyright.ＰＥＮＡＮＡORv4zuts8D
-------------------------------------------
If the etag changes, response 200 from beginning like this:206Please respect copyright.ＰＥＮＡＮＡPKCkLCnHrW
-----------http response from the beginning------206Please respect copyright.ＰＥＮＡＮＡub3QLRGwBi
HTTP/1.1 200 OK206Please respect copyright.ＰＥＮＡＮＡzqoLDNbgdo
Date: Mon, 25 Nov 2024 10:56:12 GMT206Please respect copyright.ＰＥＮＡＮＡnwzLbhbzbZ
accept-ranges: bytes206Please respect copyright.ＰＥＮＡＮＡfQSRKWVsES
ETag: "534231200_1732277116"206Please respect copyright.ＰＥＮＡＮＡ4Cxh9zzjQN
Content-Length: 534231200206Please respect copyright.ＰＥＮＡＮＡKrMIkPRPGT
Content-Type: application/octet-stream206Please respect copyright.ＰＥＮＡＮＡWzg7sR52yl
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)206Please respect copyright.ＰＥＮＡＮＡdZ7btXA8Jq
---------------------------------------------------
206Please respect copyright.ＰＥＮＡＮＡztVXltMavK
Section X. Thanks
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/If-None-Match206Please respect copyright.ＰＥＮＡＮＡSoIxNRuTaG
https://blog.csdn.net/catoop/article/details/134174653206Please respect copyright.ＰＥＮＡＮＡhwGxEuJ533
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.ＰＥＮＡＮＡyxSyhKWaZT
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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