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.
146Please respect copyright.ＰＥＮＡＮＡP67cb83cwU
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:146Please respect copyright.ＰＥＮＡＮＡfBouKJeSOG
---------------http response with etag----------------146Please respect copyright.ＰＥＮＡＮＡqab3tK3bPy
HTTP/1.1 200 OK146Please respect copyright.ＰＥＮＡＮＡbDZUsZlZZT
Date: Mon, 25 Nov 2024 10:56:12 GMT146Please respect copyright.ＰＥＮＡＮＡuCYTeu5q7w
accept-ranges: bytes146Please respect copyright.ＰＥＮＡＮＡ1Q78NFWZ8F
ETag: "534231200_1732277116"146Please respect copyright.ＰＥＮＡＮＡjloGzfCAvO
Content-Length: 534231200146Please respect copyright.ＰＥＮＡＮＡKqLUYuuQkc
Content-Type: application/octet-stream146Please respect copyright.ＰＥＮＡＮＡLcwO6X7znp
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)146Please respect copyright.ＰＥＮＡＮＡRhxEqFJL7G
-----------------------------------------------------
146Please respect copyright.ＰＥＮＡＮＡMPOK0sSB01
Section B. if-none-match
The browser send back the etag to ask server if it has been changed by header like this:146Please respect copyright.ＰＥＮＡＮＡ2lYWBhixSS
-----http request for a cached resource------146Please respect copyright.ＰＥＮＡＮＡM1cPI17Cfy
GET /zen.iso HTTP/1.1146Please respect copyright.ＰＥＮＡＮＡocRE2zEl32
if-none-match: "534231200_1732277116"146Please respect copyright.ＰＥＮＡＮＡwZJuWy54VX
---------------------------------------------
If the etag is unchanged, the server response like this:
1. for get and head requests:146Please respect copyright.ＰＥＮＡＮＡv09xAK7tB5
-----------http response for unchanged resource------146Please respect copyright.ＰＥＮＡＮＡH7KtBhvlIU
HTTP/1.1 304 Not Modified
(http body canbe empty, the client will used its cache)146Please respect copyright.ＰＥＮＡＮＡyBFLUbFZUf
----------------------------------------------------
2. for other methods requests:146Please respect copyright.ＰＥＮＡＮＡx0b30mgNoQ
-----------http response for unchanged resource------146Please respect copyright.ＰＥＮＡＮＡt2pA9C14h5
HTTP/1.1 412 Precondition Failed
(http body canbe empty, the client will used its cache)146Please respect copyright.ＰＥＮＡＮＡrOA1Ir7hBq
----------------------------------------------------
146Please respect copyright.ＰＥＮＡＮＡcWtuo2RXQH
If the etag changes, the server response 200 OK with the new file content like this:146Please respect copyright.ＰＥＮＡＮＡIHxVwLEATN
---------------http response with etag----------------146Please respect copyright.ＰＥＮＡＮＡX9eBcCB3Sv
HTTP/1.1 200 OK146Please respect copyright.ＰＥＮＡＮＡAFeRLUVDjJ
Date: Mon, 25 Nov 2024 10:56:12 GMT146Please respect copyright.ＰＥＮＡＮＡM3Sev74uLE
accept-ranges: bytes146Please respect copyright.ＰＥＮＡＮＡLWgYzOBGMx
ETag: "534231200_1732277116"146Please respect copyright.ＰＥＮＡＮＡDPNJ57xKc3
Content-Length: 534231200146Please respect copyright.ＰＥＮＡＮＡWGju2ma35K
Content-Type: application/octet-stream146Please respect copyright.ＰＥＮＡＮＡdV3wPpsQgp
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)146Please respect copyright.ＰＥＮＡＮＡfgsAzKQNmd
-----------------------------------------------------
146Please respect copyright.ＰＥＮＡＮＡhwnHN1RXXv
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:146Please respect copyright.ＰＥＮＡＮＡLNVQTQ3mFR
------resume download request-------146Please respect copyright.ＰＥＮＡＮＡ8FqUR7Z4JW
GET /zen.iso HTTP/1.1146Please respect copyright.ＰＥＮＡＮＡ4rYjPDig9c
Range: bytes=287718144-146Please respect copyright.ＰＥＮＡＮＡx07n9kbhq5
If-Match: "534231200_1732277116"146Please respect copyright.ＰＥＮＡＮＡpO74SiMge9
-----------------------------------146Please respect copyright.ＰＥＮＡＮＡeQzh7mKgS7
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:146Please respect copyright.ＰＥＮＡＮＡf3XFbEiU66
-------------range response-------------------146Please respect copyright.ＰＥＮＡＮＡdGR2kk3TD4
HTTP/1.1 206 Partial Content146Please respect copyright.ＰＥＮＡＮＡrBTXSV8k4c
accept-ranges: bytes146Please respect copyright.ＰＥＮＡＮＡZoNEBvTftV
content-range: bytes 287718144-534231199/534231200146Please respect copyright.ＰＥＮＡＮＡ2e9IsZ6MQ9
ETag: "534231200_1732277116"146Please respect copyright.ＰＥＮＡＮＡB4M5MijGlh
Content-Length: 246513056146Please respect copyright.ＰＥＮＡＮＡb3W1Nasm5B
Content-Type: application/octet-stream146Please respect copyright.ＰＥＮＡＮＡqPBBUBTRcb
Content-Disposition: attachment; filename="zen.iso"
(http body is the remain file content)146Please respect copyright.ＰＥＮＡＮＡTlZgieTWTt
-------------------------------------------
If the etag changes, response 412 like this:146Please respect copyright.ＰＥＮＡＮＡXoRodHs0C3
-----------http response for changed resource------146Please respect copyright.ＰＥＮＡＮＡpYveXxhDi8
HTTP/1.1 412 Precondition Failed
(http body canbe empty, the client will stop downloading the resumed file)146Please respect copyright.ＰＥＮＡＮＡhV2pfIE35B
---------------------------------------------------
146Please respect copyright.ＰＥＮＡＮＡ1WCvSVfDmt
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---146Please respect copyright.ＰＥＮＡＮＡmUR4z3JZ8c
GET /zen.iso HTTP/1.1146Please respect copyright.ＰＥＮＡＮＡm54FIwMQgN
Range: bytes=287718144-146Please respect copyright.ＰＥＮＡＮＡAMbQgRWluP
If-Range: "534231200_1732277116"146Please respect copyright.ＰＥＮＡＮＡ4iejUrwHdy
------------------------------------146Please respect copyright.ＰＥＮＡＮＡUWHxNzpWmH
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:146Please respect copyright.ＰＥＮＡＮＡpKT9ax0VmM
-------------range response-------------------146Please respect copyright.ＰＥＮＡＮＡoUtWZAYopz
HTTP/1.1 206 Partial Content146Please respect copyright.ＰＥＮＡＮＡ7JnqZUbnHp
accept-ranges: bytes146Please respect copyright.ＰＥＮＡＮＡMiY67h44zY
content-range: bytes 287718144-534231199/534231200146Please respect copyright.ＰＥＮＡＮＡpcVWz7MbHT
ETag: "534231200_1732277116"146Please respect copyright.ＰＥＮＡＮＡqv2wiDOfB7
Content-Length: 246513056146Please respect copyright.ＰＥＮＡＮＡ3fu7lSkmOF
Content-Type: application/octet-stream146Please respect copyright.ＰＥＮＡＮＡJw0y6f9v57
Content-Disposition: attachment; filename="zen.iso"
(http body is the remain file content)146Please respect copyright.ＰＥＮＡＮＡxprjGPuzC3
-------------------------------------------
If the etag changes, response 200 from beginning like this:146Please respect copyright.ＰＥＮＡＮＡsKFL56dcAw
-----------http response from the beginning------146Please respect copyright.ＰＥＮＡＮＡcQvnE3QvhX
HTTP/1.1 200 OK146Please respect copyright.ＰＥＮＡＮＡiJQZvBKkUw
Date: Mon, 25 Nov 2024 10:56:12 GMT146Please respect copyright.ＰＥＮＡＮＡzWvjwbP2Hi
accept-ranges: bytes146Please respect copyright.ＰＥＮＡＮＡIjH2lAkcP6
ETag: "534231200_1732277116"146Please respect copyright.ＰＥＮＡＮＡrFT4vCASWE
Content-Length: 534231200146Please respect copyright.ＰＥＮＡＮＡCu3kFYTbK9
Content-Type: application/octet-stream146Please respect copyright.ＰＥＮＡＮＡ1lkWaUZ7Nj
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)146Please respect copyright.ＰＥＮＡＮＡmzha6ridU9
---------------------------------------------------
146Please respect copyright.ＰＥＮＡＮＡFpJNnrq7X2
Section X. Thanks
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/If-None-Match146Please respect copyright.ＰＥＮＡＮＡyAXqr9DOH4
https://blog.csdn.net/catoop/article/details/134174653146Please respect copyright.ＰＥＮＡＮＡdl1J7LgdLC
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]146Please respect copyright.ＰＥＮＡＮＡGlQ7tslQQ4
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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