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.
109Please respect copyright.ＰＥＮＡＮＡEnhhyndDd4
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:109Please respect copyright.ＰＥＮＡＮＡMZ3iUsjaXh
---------------http response with etag----------------109Please respect copyright.ＰＥＮＡＮＡ7TYbFjHQ5z
HTTP/1.1 200 OK109Please respect copyright.ＰＥＮＡＮＡJQOgdPPATZ
Date: Mon, 25 Nov 2024 10:56:12 GMT109Please respect copyright.ＰＥＮＡＮＡbeFFrzVgXg
accept-ranges: bytes109Please respect copyright.ＰＥＮＡＮＡGLF7sG5KTS
ETag: "534231200_1732277116"109Please respect copyright.ＰＥＮＡＮＡrSYHMpdjCV
Content-Length: 534231200109Please respect copyright.ＰＥＮＡＮＡpyCMac9NkD
Content-Type: application/octet-stream109Please respect copyright.ＰＥＮＡＮＡgrzgmUQStw
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)109Please respect copyright.ＰＥＮＡＮＡpX4ZZvnYSE
-----------------------------------------------------
109Please respect copyright.ＰＥＮＡＮＡxpurEVkOLn
Section B. if-none-match
The browser send back the etag to ask server if it has been changed by header like this:109Please respect copyright.ＰＥＮＡＮＡWzZFmFbATG
-----http request for a cached resource------109Please respect copyright.ＰＥＮＡＮＡHCl1M5rylC
GET /zen.iso HTTP/1.1109Please respect copyright.ＰＥＮＡＮＡr6KlMJ4vrT
if-none-match: "534231200_1732277116"109Please respect copyright.ＰＥＮＡＮＡHNZMqKyZZH
---------------------------------------------
If the etag is unchanged, the server response like this:
1. for get and head requests:109Please respect copyright.ＰＥＮＡＮＡHAUi0hgQPU
-----------http response for unchanged resource------109Please respect copyright.ＰＥＮＡＮＡQxKUNlmJTM
HTTP/1.1 304 Not Modified
(http body canbe empty, the client will used its cache)109Please respect copyright.ＰＥＮＡＮＡ1QOzINoBPh
----------------------------------------------------
2. for other methods requests:109Please respect copyright.ＰＥＮＡＮＡjE8JCXS6MG
-----------http response for unchanged resource------109Please respect copyright.ＰＥＮＡＮＡZtJKXkhXGK
HTTP/1.1 412 Precondition Failed
(http body canbe empty, the client will used its cache)109Please respect copyright.ＰＥＮＡＮＡxbupjPOdZi
----------------------------------------------------
109Please respect copyright.ＰＥＮＡＮＡMSBtnw0hrZ
If the etag changes, the server response 200 OK with the new file content like this:109Please respect copyright.ＰＥＮＡＮＡmSKYCDkOXo
---------------http response with etag----------------109Please respect copyright.ＰＥＮＡＮＡaZAiarzYBm
HTTP/1.1 200 OK109Please respect copyright.ＰＥＮＡＮＡY8CuoOzuZt
Date: Mon, 25 Nov 2024 10:56:12 GMT109Please respect copyright.ＰＥＮＡＮＡQtVDiG98hb
accept-ranges: bytes109Please respect copyright.ＰＥＮＡＮＡpnfQS4Tlrj
ETag: "534231200_1732277116"109Please respect copyright.ＰＥＮＡＮＡ1uPAFhm0cD
Content-Length: 534231200109Please respect copyright.ＰＥＮＡＮＡY7CbyahwHB
Content-Type: application/octet-stream109Please respect copyright.ＰＥＮＡＮＡcWejLqzFdm
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)109Please respect copyright.ＰＥＮＡＮＡkdPKMcQd74
-----------------------------------------------------
109Please respect copyright.ＰＥＮＡＮＡzqA9GeHhYB
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:109Please respect copyright.ＰＥＮＡＮＡHBfhTYIyp7
------resume download request-------109Please respect copyright.ＰＥＮＡＮＡ4ROiryaM2v
GET /zen.iso HTTP/1.1109Please respect copyright.ＰＥＮＡＮＡrYDPD7f3NA
Range: bytes=287718144-109Please respect copyright.ＰＥＮＡＮＡVUJ98q8YPP
If-Match: "534231200_1732277116"109Please respect copyright.ＰＥＮＡＮＡcskZgAa7ct
-----------------------------------109Please respect copyright.ＰＥＮＡＮＡoRG0vdOkVO
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:109Please respect copyright.ＰＥＮＡＮＡJhTyfMgmjj
-------------range response-------------------109Please respect copyright.ＰＥＮＡＮＡJpjPpOrBCv
HTTP/1.1 206 Partial Content109Please respect copyright.ＰＥＮＡＮＡD4HQVgkO80
accept-ranges: bytes109Please respect copyright.ＰＥＮＡＮＡW09LpkQ9o7
content-range: bytes 287718144-534231199/534231200109Please respect copyright.ＰＥＮＡＮＡm7co8o55JO
ETag: "534231200_1732277116"109Please respect copyright.ＰＥＮＡＮＡUNVyTJYTrZ
Content-Length: 246513056109Please respect copyright.ＰＥＮＡＮＡ3fsrhmJqPy
Content-Type: application/octet-stream109Please respect copyright.ＰＥＮＡＮＡWM4snZYc2W
Content-Disposition: attachment; filename="zen.iso"
(http body is the remain file content)109Please respect copyright.ＰＥＮＡＮＡEtCsWSbNuw
-------------------------------------------
If the etag changes, response 412 like this:109Please respect copyright.ＰＥＮＡＮＡdq4miQNwrn
-----------http response for changed resource------109Please respect copyright.ＰＥＮＡＮＡgTeYlmdCCL
HTTP/1.1 412 Precondition Failed
(http body canbe empty, the client will stop downloading the resumed file)109Please respect copyright.ＰＥＮＡＮＡUTPWFnGsIZ
---------------------------------------------------
109Please respect copyright.ＰＥＮＡＮＡmM3P0UAECc
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---109Please respect copyright.ＰＥＮＡＮＡiiL657BVvz
GET /zen.iso HTTP/1.1109Please respect copyright.ＰＥＮＡＮＡ4wTVTFcy76
Range: bytes=287718144-109Please respect copyright.ＰＥＮＡＮＡdEh0HWsjDj
If-Range: "534231200_1732277116"109Please respect copyright.ＰＥＮＡＮＡ52QvkBIm4V
------------------------------------109Please respect copyright.ＰＥＮＡＮＡN3TaSfSNCx
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:109Please respect copyright.ＰＥＮＡＮＡb8ALaQrKFP
-------------range response-------------------109Please respect copyright.ＰＥＮＡＮＡCxtTRqXGVQ
HTTP/1.1 206 Partial Content109Please respect copyright.ＰＥＮＡＮＡSx5QXhSd5y
accept-ranges: bytes109Please respect copyright.ＰＥＮＡＮＡ40fFzkMoas
content-range: bytes 287718144-534231199/534231200109Please respect copyright.ＰＥＮＡＮＡesVjgirYUE
ETag: "534231200_1732277116"109Please respect copyright.ＰＥＮＡＮＡBWMP3mXFMW
Content-Length: 246513056109Please respect copyright.ＰＥＮＡＮＡcMwCAXEh73
Content-Type: application/octet-stream109Please respect copyright.ＰＥＮＡＮＡF5mPhk1gkW
Content-Disposition: attachment; filename="zen.iso"
(http body is the remain file content)109Please respect copyright.ＰＥＮＡＮＡMDWBsNtBFC
-------------------------------------------
If the etag changes, response 200 from beginning like this:109Please respect copyright.ＰＥＮＡＮＡNNRIuzR1SE
-----------http response from the beginning------109Please respect copyright.ＰＥＮＡＮＡFlULx1Kzcb
HTTP/1.1 200 OK109Please respect copyright.ＰＥＮＡＮＡJrh3Jff0jY
Date: Mon, 25 Nov 2024 10:56:12 GMT109Please respect copyright.ＰＥＮＡＮＡlwFlmNxmmn
accept-ranges: bytes109Please respect copyright.ＰＥＮＡＮＡVdKAlqodxT
ETag: "534231200_1732277116"109Please respect copyright.ＰＥＮＡＮＡ2giU0gbOll
Content-Length: 534231200109Please respect copyright.ＰＥＮＡＮＡ8OAj89FzeC
Content-Type: application/octet-stream109Please respect copyright.ＰＥＮＡＮＡMGg8F2kA6Q
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)109Please respect copyright.ＰＥＮＡＮＡ17DvgMARzZ
---------------------------------------------------
109Please respect copyright.ＰＥＮＡＮＡw8Dbl9bu7M
Section X. Thanks
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/If-None-Match109Please respect copyright.ＰＥＮＡＮＡyG4PVn81XM
https://blog.csdn.net/catoop/article/details/134174653109Please respect copyright.ＰＥＮＡＮＡJYIBWppNqd
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]109Please respect copyright.ＰＥＮＡＮＡCOy0aTOYfk
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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