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.
63Please respect copyright.ＰＥＮＡＮＡVhSeT7iUEp
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:63Please respect copyright.ＰＥＮＡＮＡprhCaYYqTE
---------------http response with etag----------------63Please respect copyright.ＰＥＮＡＮＡMiHSen9aKI
HTTP/1.1 200 OK63Please respect copyright.ＰＥＮＡＮＡDix5MyMn5G
Date: Mon, 25 Nov 2024 10:56:12 GMT63Please respect copyright.ＰＥＮＡＮＡL0ckVnIxh1
accept-ranges: bytes63Please respect copyright.ＰＥＮＡＮＡ0ytCR80YQI
ETag: "534231200_1732277116"63Please respect copyright.ＰＥＮＡＮＡL5rS8dDVZ3
Content-Length: 53423120063Please respect copyright.ＰＥＮＡＮＡpGr6g2Bw0g
Content-Type: application/octet-stream63Please respect copyright.ＰＥＮＡＮＡA5v9ZHx7Jk
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)63Please respect copyright.ＰＥＮＡＮＡRQHVbPl6la
-----------------------------------------------------
63Please respect copyright.ＰＥＮＡＮＡpdCUxI58Pq
Section B. if-none-match
The browser send back the etag to ask server if it has been changed by header like this:63Please respect copyright.ＰＥＮＡＮＡDvKpdPaxmK
-----http request for a cached resource------63Please respect copyright.ＰＥＮＡＮＡZUzvyXtJil
GET /zen.iso HTTP/1.163Please respect copyright.ＰＥＮＡＮＡRXvEgpfqSZ
if-none-match: "534231200_1732277116"63Please respect copyright.ＰＥＮＡＮＡtb0TaWMJuW
---------------------------------------------
If the etag is unchanged, the server response like this:
1. for get and head requests:63Please respect copyright.ＰＥＮＡＮＡZMGH1XjWVv
-----------http response for unchanged resource------63Please respect copyright.ＰＥＮＡＮＡ9UFpPG0uRm
HTTP/1.1 304 Not Modified
(http body canbe empty, the client will used its cache)63Please respect copyright.ＰＥＮＡＮＡjk9KTBaKWo
----------------------------------------------------
2. for other methods requests:63Please respect copyright.ＰＥＮＡＮＡsEF8gZydmc
-----------http response for unchanged resource------63Please respect copyright.ＰＥＮＡＮＡtUpu0ZaNDm
HTTP/1.1 412 Precondition Failed
(http body canbe empty, the client will used its cache)63Please respect copyright.ＰＥＮＡＮＡ8heqeytZLh
----------------------------------------------------
63Please respect copyright.ＰＥＮＡＮＡcYrtWPLwqO
If the etag changes, the server response 200 OK with the new file content like this:63Please respect copyright.ＰＥＮＡＮＡzOAQJvHPiy
---------------http response with etag----------------63Please respect copyright.ＰＥＮＡＮＡgBRyAhrXGJ
HTTP/1.1 200 OK63Please respect copyright.ＰＥＮＡＮＡ1UkqZtDjoh
Date: Mon, 25 Nov 2024 10:56:12 GMT63Please respect copyright.ＰＥＮＡＮＡrWgLXRvSzR
accept-ranges: bytes63Please respect copyright.ＰＥＮＡＮＡutWLZyrfjY
ETag: "534231200_1732277116"63Please respect copyright.ＰＥＮＡＮＡz1BMYXIyQv
Content-Length: 53423120063Please respect copyright.ＰＥＮＡＮＡN4fIaX5eJu
Content-Type: application/octet-stream63Please respect copyright.ＰＥＮＡＮＡmDTXOQ9Kwk
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)63Please respect copyright.ＰＥＮＡＮＡQ6wHNqgTra
-----------------------------------------------------
63Please respect copyright.ＰＥＮＡＮＡws5Masxwia
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:63Please respect copyright.ＰＥＮＡＮＡKeaxJ98hRZ
------resume download request-------63Please respect copyright.ＰＥＮＡＮＡF7eyzFYemu
GET /zen.iso HTTP/1.163Please respect copyright.ＰＥＮＡＮＡENqNS9J0WH
Range: bytes=287718144-63Please respect copyright.ＰＥＮＡＮＡ1YSFPZpHd3
If-Match: "534231200_1732277116"63Please respect copyright.ＰＥＮＡＮＡpRhxQE5iEn
-----------------------------------63Please respect copyright.ＰＥＮＡＮＡJcwtaf94Pa
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:63Please respect copyright.ＰＥＮＡＮＡaUeaTAWiAr
-------------range response-------------------63Please respect copyright.ＰＥＮＡＮＡHn4k6zM3LQ
HTTP/1.1 206 Partial Content63Please respect copyright.ＰＥＮＡＮＡLo0n4qblFI
accept-ranges: bytes63Please respect copyright.ＰＥＮＡＮＡpW214xWcpS
content-range: bytes 287718144-534231199/53423120063Please respect copyright.ＰＥＮＡＮＡ7JBtSf5m9x
ETag: "534231200_1732277116"63Please respect copyright.ＰＥＮＡＮＡxRvpeoKAtQ
Content-Length: 24651305663Please respect copyright.ＰＥＮＡＮＡ1zD67ikWIb
Content-Type: application/octet-stream63Please respect copyright.ＰＥＮＡＮＡsJwLTQSH3D
Content-Disposition: attachment; filename="zen.iso"
(http body is the remain file content)63Please respect copyright.ＰＥＮＡＮＡrUiBtU6btc
-------------------------------------------
If the etag changes, response 412 like this:63Please respect copyright.ＰＥＮＡＮＡN6QB9A68c3
-----------http response for changed resource------63Please respect copyright.ＰＥＮＡＮＡtWh5KgzG0M
HTTP/1.1 412 Precondition Failed
(http body canbe empty, the client will stop downloading the resumed file)63Please respect copyright.ＰＥＮＡＮＡrBU5wJ9sRN
---------------------------------------------------
63Please respect copyright.ＰＥＮＡＮＡdHuUTIlBmF
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---63Please respect copyright.ＰＥＮＡＮＡpmee7rpmty
GET /zen.iso HTTP/1.163Please respect copyright.ＰＥＮＡＮＡaXuQ3ABbEt
Range: bytes=287718144-63Please respect copyright.ＰＥＮＡＮＡdgirhHesLG
If-Range: "534231200_1732277116"63Please respect copyright.ＰＥＮＡＮＡPMOqh4Up6R
------------------------------------63Please respect copyright.ＰＥＮＡＮＡ8US0vIeuxL
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:63Please respect copyright.ＰＥＮＡＮＡ0mOYW2CT5C
-------------range response-------------------63Please respect copyright.ＰＥＮＡＮＡDgVlbfcRnR
HTTP/1.1 206 Partial Content63Please respect copyright.ＰＥＮＡＮＡtPbfKnKbh5
accept-ranges: bytes63Please respect copyright.ＰＥＮＡＮＡJ8KdfqLhlv
content-range: bytes 287718144-534231199/53423120063Please respect copyright.ＰＥＮＡＮＡPKBLCSlEPN
ETag: "534231200_1732277116"63Please respect copyright.ＰＥＮＡＮＡCoyFRhIqsf
Content-Length: 24651305663Please respect copyright.ＰＥＮＡＮＡ9wo93kkIwO
Content-Type: application/octet-stream63Please respect copyright.ＰＥＮＡＮＡTPFcKBA5PR
Content-Disposition: attachment; filename="zen.iso"
(http body is the remain file content)63Please respect copyright.ＰＥＮＡＮＡLwJDOgfxVb
-------------------------------------------
If the etag changes, response 200 from beginning like this:63Please respect copyright.ＰＥＮＡＮＡQiVEOS29yU
-----------http response from the beginning------63Please respect copyright.ＰＥＮＡＮＡhSYuRL3Jjf
HTTP/1.1 200 OK63Please respect copyright.ＰＥＮＡＮＡeX0fGBmpdq
Date: Mon, 25 Nov 2024 10:56:12 GMT63Please respect copyright.ＰＥＮＡＮＡQ9hFSdRkcA
accept-ranges: bytes63Please respect copyright.ＰＥＮＡＮＡdscSmQgwWP
ETag: "534231200_1732277116"63Please respect copyright.ＰＥＮＡＮＡI5L7W1MTfx
Content-Length: 53423120063Please respect copyright.ＰＥＮＡＮＡ5JAaWDXwvY
Content-Type: application/octet-stream63Please respect copyright.ＰＥＮＡＮＡE5JaGdZFRo
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)63Please respect copyright.ＰＥＮＡＮＡz9jFwkw3Ru
---------------------------------------------------
63Please respect copyright.ＰＥＮＡＮＡa0LXIFH3Lh
Section X. Thanks
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/If-None-Match63Please respect copyright.ＰＥＮＡＮＡ375SvUi0WE
https://blog.csdn.net/catoop/article/details/13417465363Please respect copyright.ＰＥＮＡＮＡqrKeWM8fPs
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: si.jinmin@gmail.com63Please respect copyright.ＰＥＮＡＮＡq5SYZkxG8J
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: si.jinmin@gmail.com
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