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.
189Please respect copyright.ＰＥＮＡＮＡPBdICeybPw
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:189Please respect copyright.ＰＥＮＡＮＡeyywpXJH92
---------------http response with etag----------------189Please respect copyright.ＰＥＮＡＮＡtrfLex4Q89
HTTP/1.1 200 OK189Please respect copyright.ＰＥＮＡＮＡOewzAvosQi
Date: Mon, 25 Nov 2024 10:56:12 GMT189Please respect copyright.ＰＥＮＡＮＡ6uJATDIA2s
accept-ranges: bytes189Please respect copyright.ＰＥＮＡＮＡBX4r1X6z5d
ETag: "534231200_1732277116"189Please respect copyright.ＰＥＮＡＮＡ8fv1dqFrBt
Content-Length: 534231200189Please respect copyright.ＰＥＮＡＮＡEIyOISSuQn
Content-Type: application/octet-stream189Please respect copyright.ＰＥＮＡＮＡUAuY5HUBCv
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)189Please respect copyright.ＰＥＮＡＮＡE8YJGzDNMu
-----------------------------------------------------
189Please respect copyright.ＰＥＮＡＮＡUEImKrdZ9C
Section B. if-none-match
The browser send back the etag to ask server if it has been changed by header like this:189Please respect copyright.ＰＥＮＡＮＡSBW9KOTgV7
-----http request for a cached resource------189Please respect copyright.ＰＥＮＡＮＡ2241b8CdcH
GET /zen.iso HTTP/1.1189Please respect copyright.ＰＥＮＡＮＡYOCJOoj6no
if-none-match: "534231200_1732277116"189Please respect copyright.ＰＥＮＡＮＡFUE9hp3IGz
---------------------------------------------
If the etag is unchanged, the server response like this:
1. for get and head requests:189Please respect copyright.ＰＥＮＡＮＡDYHg3whI86
-----------http response for unchanged resource------189Please respect copyright.ＰＥＮＡＮＡzZa5HL6vgn
HTTP/1.1 304 Not Modified
(http body canbe empty, the client will used its cache)189Please respect copyright.ＰＥＮＡＮＡJ4QFQouTOU
----------------------------------------------------
2. for other methods requests:189Please respect copyright.ＰＥＮＡＮＡ03kxWP3J1o
-----------http response for unchanged resource------189Please respect copyright.ＰＥＮＡＮＡiSLOad0bkY
HTTP/1.1 412 Precondition Failed
(http body canbe empty, the client will used its cache)189Please respect copyright.ＰＥＮＡＮＡLTffFfiwQi
----------------------------------------------------
189Please respect copyright.ＰＥＮＡＮＡvyHJ4GHb6j
If the etag changes, the server response 200 OK with the new file content like this:189Please respect copyright.ＰＥＮＡＮＡ5tPlsFMCdw
---------------http response with etag----------------189Please respect copyright.ＰＥＮＡＮＡBiC1vLY0mD
HTTP/1.1 200 OK189Please respect copyright.ＰＥＮＡＮＡnarrqcx6c9
Date: Mon, 25 Nov 2024 10:56:12 GMT189Please respect copyright.ＰＥＮＡＮＡ5oW2UsBlZe
accept-ranges: bytes189Please respect copyright.ＰＥＮＡＮＡAkx4NIt8qS
ETag: "534231200_1732277116"189Please respect copyright.ＰＥＮＡＮＡNMRNrWkqkU
Content-Length: 534231200189Please respect copyright.ＰＥＮＡＮＡQ9VelOc091
Content-Type: application/octet-stream189Please respect copyright.ＰＥＮＡＮＡzwZg33jZsG
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)189Please respect copyright.ＰＥＮＡＮＡLTBZxY80O6
-----------------------------------------------------
189Please respect copyright.ＰＥＮＡＮＡvHSZT1vAqp
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:189Please respect copyright.ＰＥＮＡＮＡoCkbg2wNb0
------resume download request-------189Please respect copyright.ＰＥＮＡＮＡNNQXRU0BS1
GET /zen.iso HTTP/1.1189Please respect copyright.ＰＥＮＡＮＡzRuPrpSGun
Range: bytes=287718144-189Please respect copyright.ＰＥＮＡＮＡyurzPqFiX1
If-Match: "534231200_1732277116"189Please respect copyright.ＰＥＮＡＮＡBbD6OXO8BI
-----------------------------------189Please respect copyright.ＰＥＮＡＮＡPs57q7jAn2
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:189Please respect copyright.ＰＥＮＡＮＡs4WwmG0TZ8
-------------range response-------------------189Please respect copyright.ＰＥＮＡＮＡPp1RHb1T5b
HTTP/1.1 206 Partial Content189Please respect copyright.ＰＥＮＡＮＡ1gNt1soaS8
accept-ranges: bytes189Please respect copyright.ＰＥＮＡＮＡ4CAC1SlWCE
content-range: bytes 287718144-534231199/534231200189Please respect copyright.ＰＥＮＡＮＡdxcwcgLyMe
ETag: "534231200_1732277116"189Please respect copyright.ＰＥＮＡＮＡTufgVxIwoG
Content-Length: 246513056189Please respect copyright.ＰＥＮＡＮＡjbR8DZj1Ke
Content-Type: application/octet-stream189Please respect copyright.ＰＥＮＡＮＡo9eAh2mREL
Content-Disposition: attachment; filename="zen.iso"
(http body is the remain file content)189Please respect copyright.ＰＥＮＡＮＡbipN0zPJyE
-------------------------------------------
If the etag changes, response 412 like this:189Please respect copyright.ＰＥＮＡＮＡKu8GEK9KWA
-----------http response for changed resource------189Please respect copyright.ＰＥＮＡＮＡplKVxCnzr8
HTTP/1.1 412 Precondition Failed
(http body canbe empty, the client will stop downloading the resumed file)189Please respect copyright.ＰＥＮＡＮＡm3aDF89fsC
---------------------------------------------------
189Please respect copyright.ＰＥＮＡＮＡcrIoKat7kb
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---189Please respect copyright.ＰＥＮＡＮＡJmhKRcyATt
GET /zen.iso HTTP/1.1189Please respect copyright.ＰＥＮＡＮＡllLgG2ybty
Range: bytes=287718144-189Please respect copyright.ＰＥＮＡＮＡYqITQlobP4
If-Range: "534231200_1732277116"189Please respect copyright.ＰＥＮＡＮＡyS4hPs3RV0
------------------------------------189Please respect copyright.ＰＥＮＡＮＡRiUOPS7EIV
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:189Please respect copyright.ＰＥＮＡＮＡTW7BdhO41F
-------------range response-------------------189Please respect copyright.ＰＥＮＡＮＡDjVTHzCGoW
HTTP/1.1 206 Partial Content189Please respect copyright.ＰＥＮＡＮＡ0LazTN2tZT
accept-ranges: bytes189Please respect copyright.ＰＥＮＡＮＡcu7gMRNAhK
content-range: bytes 287718144-534231199/534231200189Please respect copyright.ＰＥＮＡＮＡqsNiTgzH4D
ETag: "534231200_1732277116"189Please respect copyright.ＰＥＮＡＮＡ322DTf4YE4
Content-Length: 246513056189Please respect copyright.ＰＥＮＡＮＡmRLb1Cc9xi
Content-Type: application/octet-stream189Please respect copyright.ＰＥＮＡＮＡ3wNAXvkOLb
Content-Disposition: attachment; filename="zen.iso"
(http body is the remain file content)189Please respect copyright.ＰＥＮＡＮＡeTBFRfUZ28
-------------------------------------------
If the etag changes, response 200 from beginning like this:189Please respect copyright.ＰＥＮＡＮＡ11bo7zgXAz
-----------http response from the beginning------189Please respect copyright.ＰＥＮＡＮＡoJd5uUCNgA
HTTP/1.1 200 OK189Please respect copyright.ＰＥＮＡＮＡt414iJ4aC3
Date: Mon, 25 Nov 2024 10:56:12 GMT189Please respect copyright.ＰＥＮＡＮＡo93i5rnB4X
accept-ranges: bytes189Please respect copyright.ＰＥＮＡＮＡt8GJeaeS2g
ETag: "534231200_1732277116"189Please respect copyright.ＰＥＮＡＮＡRG80yLLZhu
Content-Length: 534231200189Please respect copyright.ＰＥＮＡＮＡbeFARLAjr7
Content-Type: application/octet-stream189Please respect copyright.ＰＥＮＡＮＡRPkhVs118y
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)189Please respect copyright.ＰＥＮＡＮＡC5xk7Xa6zT
---------------------------------------------------
189Please respect copyright.ＰＥＮＡＮＡyufg8XE25c
Section X. Thanks
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/If-None-Match189Please respect copyright.ＰＥＮＡＮＡA8HHKeH7kt
https://blog.csdn.net/catoop/article/details/134174653189Please respect copyright.ＰＥＮＡＮＡjtO5x5shCP
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]189Please respect copyright.ＰＥＮＡＮＡDO3vNizRa4
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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