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.
82Please respect copyright.ＰＥＮＡＮＡL7vEXuubmU
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:82Please respect copyright.ＰＥＮＡＮＡNMLvGSpDMZ
---------------http response with etag----------------82Please respect copyright.ＰＥＮＡＮＡWZtDnpApYY
HTTP/1.1 200 OK82Please respect copyright.ＰＥＮＡＮＡapAHqDtjUO
Date: Mon, 25 Nov 2024 10:56:12 GMT82Please respect copyright.ＰＥＮＡＮＡPy35LYT24D
accept-ranges: bytes82Please respect copyright.ＰＥＮＡＮＡ9x4R0bKzAz
ETag: "534231200_1732277116"82Please respect copyright.ＰＥＮＡＮＡawFZOyobf5
Content-Length: 53423120082Please respect copyright.ＰＥＮＡＮＡrbfuFY3WEU
Content-Type: application/octet-stream82Please respect copyright.ＰＥＮＡＮＡyZahWdbXuy
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)82Please respect copyright.ＰＥＮＡＮＡdruN11Z6hc
-----------------------------------------------------
82Please respect copyright.ＰＥＮＡＮＡ2c1DUVFJ7G
Section B. if-none-match
The browser send back the etag to ask server if it has been changed by header like this:82Please respect copyright.ＰＥＮＡＮＡpu2cUVpnfM
-----http request for a cached resource------82Please respect copyright.ＰＥＮＡＮＡirYjB3Xmkn
GET /zen.iso HTTP/1.182Please respect copyright.ＰＥＮＡＮＡaBKrh0qBqe
if-none-match: "534231200_1732277116"82Please respect copyright.ＰＥＮＡＮＡmV32RmyQJH
---------------------------------------------
If the etag is unchanged, the server response like this:
1. for get and head requests:82Please respect copyright.ＰＥＮＡＮＡBRyU47OXrZ
-----------http response for unchanged resource------82Please respect copyright.ＰＥＮＡＮＡ6HEL8X1J2P
HTTP/1.1 304 Not Modified
(http body canbe empty, the client will used its cache)82Please respect copyright.ＰＥＮＡＮＡVTl9rwJymM
----------------------------------------------------
2. for other methods requests:82Please respect copyright.ＰＥＮＡＮＡolA73ibpTG
-----------http response for unchanged resource------82Please respect copyright.ＰＥＮＡＮＡ5zsgKQv1gz
HTTP/1.1 412 Precondition Failed
(http body canbe empty, the client will used its cache)82Please respect copyright.ＰＥＮＡＮＡaX0lxSlDMx
----------------------------------------------------
82Please respect copyright.ＰＥＮＡＮＡzEcQTNLmaW
If the etag changes, the server response 200 OK with the new file content like this:82Please respect copyright.ＰＥＮＡＮＡghjbPFkjtW
---------------http response with etag----------------82Please respect copyright.ＰＥＮＡＮＡoCWXLeIKIB
HTTP/1.1 200 OK82Please respect copyright.ＰＥＮＡＮＡkNaTu8ZN56
Date: Mon, 25 Nov 2024 10:56:12 GMT82Please respect copyright.ＰＥＮＡＮＡGOkwftsmIh
accept-ranges: bytes82Please respect copyright.ＰＥＮＡＮＡjyFFzBEL7y
ETag: "534231200_1732277116"82Please respect copyright.ＰＥＮＡＮＡQsS7StmBCi
Content-Length: 53423120082Please respect copyright.ＰＥＮＡＮＡOUT77kB0Hw
Content-Type: application/octet-stream82Please respect copyright.ＰＥＮＡＮＡDXc4FnxKeP
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)82Please respect copyright.ＰＥＮＡＮＡP3bN9YKjQ1
-----------------------------------------------------
82Please respect copyright.ＰＥＮＡＮＡJ8McuBZwyV
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:82Please respect copyright.ＰＥＮＡＮＡBjsv8gbeKG
------resume download request-------82Please respect copyright.ＰＥＮＡＮＡZ8MW2CYEXq
GET /zen.iso HTTP/1.182Please respect copyright.ＰＥＮＡＮＡlXjrMOncAj
Range: bytes=287718144-82Please respect copyright.ＰＥＮＡＮＡepNsUDxcoh
If-Match: "534231200_1732277116"82Please respect copyright.ＰＥＮＡＮＡyzQMI7H0al
-----------------------------------82Please respect copyright.ＰＥＮＡＮＡ4sgb3BFXmA
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:82Please respect copyright.ＰＥＮＡＮＡirFlFbvogK
-------------range response-------------------82Please respect copyright.ＰＥＮＡＮＡeTmdHA84p4
HTTP/1.1 206 Partial Content82Please respect copyright.ＰＥＮＡＮＡn5CONpCfFl
accept-ranges: bytes82Please respect copyright.ＰＥＮＡＮＡkP72sXbV2s
content-range: bytes 287718144-534231199/53423120082Please respect copyright.ＰＥＮＡＮＡPfDelgsRl7
ETag: "534231200_1732277116"82Please respect copyright.ＰＥＮＡＮＡaBafKtc9UE
Content-Length: 24651305682Please respect copyright.ＰＥＮＡＮＡUStLHo1pPs
Content-Type: application/octet-stream82Please respect copyright.ＰＥＮＡＮＡ3D7EJR4Fqz
Content-Disposition: attachment; filename="zen.iso"
(http body is the remain file content)82Please respect copyright.ＰＥＮＡＮＡFT0ezSou9C
-------------------------------------------
If the etag changes, response 412 like this:82Please respect copyright.ＰＥＮＡＮＡuXS1pCEH6J
-----------http response for changed resource------82Please respect copyright.ＰＥＮＡＮＡhq4NC0NuPu
HTTP/1.1 412 Precondition Failed
(http body canbe empty, the client will stop downloading the resumed file)82Please respect copyright.ＰＥＮＡＮＡaW9O0v7HQ3
---------------------------------------------------
82Please respect copyright.ＰＥＮＡＮＡWJyi4cR5f2
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---82Please respect copyright.ＰＥＮＡＮＡHKVF9bUlJB
GET /zen.iso HTTP/1.182Please respect copyright.ＰＥＮＡＮＡX6bdrz79ZE
Range: bytes=287718144-82Please respect copyright.ＰＥＮＡＮＡoWqFf3bMKO
If-Range: "534231200_1732277116"82Please respect copyright.ＰＥＮＡＮＡbLdaOOtSAV
------------------------------------82Please respect copyright.ＰＥＮＡＮＡNUODoq2WLx
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:82Please respect copyright.ＰＥＮＡＮＡ1gJi5VLyxs
-------------range response-------------------82Please respect copyright.ＰＥＮＡＮＡ6fZUz5Ubxl
HTTP/1.1 206 Partial Content82Please respect copyright.ＰＥＮＡＮＡsujG3K7cA2
accept-ranges: bytes82Please respect copyright.ＰＥＮＡＮＡXCMeQZWkzQ
content-range: bytes 287718144-534231199/53423120082Please respect copyright.ＰＥＮＡＮＡCsYBNsnaLA
ETag: "534231200_1732277116"82Please respect copyright.ＰＥＮＡＮＡ0apXpYj8BQ
Content-Length: 24651305682Please respect copyright.ＰＥＮＡＮＡmb0Yymvgim
Content-Type: application/octet-stream82Please respect copyright.ＰＥＮＡＮＡTVStdRpetJ
Content-Disposition: attachment; filename="zen.iso"
(http body is the remain file content)82Please respect copyright.ＰＥＮＡＮＡs46RjzYRCq
-------------------------------------------
If the etag changes, response 200 from beginning like this:82Please respect copyright.ＰＥＮＡＮＡlNRqV6Ande
-----------http response from the beginning------82Please respect copyright.ＰＥＮＡＮＡ03uaFZICQn
HTTP/1.1 200 OK82Please respect copyright.ＰＥＮＡＮＡ8MJK4nysx3
Date: Mon, 25 Nov 2024 10:56:12 GMT82Please respect copyright.ＰＥＮＡＮＡc6uuZ61ujo
accept-ranges: bytes82Please respect copyright.ＰＥＮＡＮＡbcm0ocBaDS
ETag: "534231200_1732277116"82Please respect copyright.ＰＥＮＡＮＡVVV39Cfj86
Content-Length: 53423120082Please respect copyright.ＰＥＮＡＮＡGMwAcgJWSW
Content-Type: application/octet-stream82Please respect copyright.ＰＥＮＡＮＡqEoxSvft6p
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)82Please respect copyright.ＰＥＮＡＮＡGHhiBR43EA
---------------------------------------------------
82Please respect copyright.ＰＥＮＡＮＡlKVZThoMQA
Section X. Thanks
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/If-None-Match82Please respect copyright.ＰＥＮＡＮＡ20u3Fq7jbu
https://blog.csdn.net/catoop/article/details/13417465382Please respect copyright.ＰＥＮＡＮＡZH0499MvBk
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]82Please respect copyright.ＰＥＮＡＮＡaReuvLOZa9
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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