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.
172Please respect copyright.ＰＥＮＡＮＡMLtStAbyzw
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:172Please respect copyright.ＰＥＮＡＮＡimvtkPWSmt
---------------http response with etag----------------172Please respect copyright.ＰＥＮＡＮＡNuJNrHPtCl
HTTP/1.1 200 OK172Please respect copyright.ＰＥＮＡＮＡiMGsu4kdTG
Date: Mon, 25 Nov 2024 10:56:12 GMT172Please respect copyright.ＰＥＮＡＮＡ02c6YzDuUB
accept-ranges: bytes172Please respect copyright.ＰＥＮＡＮＡeXVJ9VE3dn
ETag: "534231200_1732277116"172Please respect copyright.ＰＥＮＡＮＡ2mYsLVAObn
Content-Length: 534231200172Please respect copyright.ＰＥＮＡＮＡIVx5Urezfg
Content-Type: application/octet-stream172Please respect copyright.ＰＥＮＡＮＡwgt1KBKArP
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)172Please respect copyright.ＰＥＮＡＮＡ1tjpjEjmrJ
-----------------------------------------------------
172Please respect copyright.ＰＥＮＡＮＡ01KPRigsbm
Section B. if-none-match
The browser send back the etag to ask server if it has been changed by header like this:172Please respect copyright.ＰＥＮＡＮＡgFrGEakeKZ
-----http request for a cached resource------172Please respect copyright.ＰＥＮＡＮＡnpacPQpArM
GET /zen.iso HTTP/1.1172Please respect copyright.ＰＥＮＡＮＡT2vyXOfVd9
if-none-match: "534231200_1732277116"172Please respect copyright.ＰＥＮＡＮＡOxuatzAy66
---------------------------------------------
If the etag is unchanged, the server response like this:
1. for get and head requests:172Please respect copyright.ＰＥＮＡＮＡoZbQqQh9l6
-----------http response for unchanged resource------172Please respect copyright.ＰＥＮＡＮＡDomwlC94L4
HTTP/1.1 304 Not Modified
(http body canbe empty, the client will used its cache)172Please respect copyright.ＰＥＮＡＮＡKUhPl4QZVt
----------------------------------------------------
2. for other methods requests:172Please respect copyright.ＰＥＮＡＮＡ0rGXSWvdMv
-----------http response for unchanged resource------172Please respect copyright.ＰＥＮＡＮＡqdahkCypjH
HTTP/1.1 412 Precondition Failed
(http body canbe empty, the client will used its cache)172Please respect copyright.ＰＥＮＡＮＡjCYpYH9caR
----------------------------------------------------
172Please respect copyright.ＰＥＮＡＮＡtzn4G7tOCZ
If the etag changes, the server response 200 OK with the new file content like this:172Please respect copyright.ＰＥＮＡＮＡ5EakOPbTWI
---------------http response with etag----------------172Please respect copyright.ＰＥＮＡＮＡNkcZ1VXpcw
HTTP/1.1 200 OK172Please respect copyright.ＰＥＮＡＮＡgn8C9tojLj
Date: Mon, 25 Nov 2024 10:56:12 GMT172Please respect copyright.ＰＥＮＡＮＡvkHAttLNC5
accept-ranges: bytes172Please respect copyright.ＰＥＮＡＮＡv7vjyLWert
ETag: "534231200_1732277116"172Please respect copyright.ＰＥＮＡＮＡ0kvDTeeMlZ
Content-Length: 534231200172Please respect copyright.ＰＥＮＡＮＡ0FlOC8ttq2
Content-Type: application/octet-stream172Please respect copyright.ＰＥＮＡＮＡgg3jinrQCF
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)172Please respect copyright.ＰＥＮＡＮＡHbTsnWzdMq
-----------------------------------------------------
172Please respect copyright.ＰＥＮＡＮＡSpGMLN0L7E
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:172Please respect copyright.ＰＥＮＡＮＡ4h5orY3BNk
------resume download request-------172Please respect copyright.ＰＥＮＡＮＡQJ9Teaquoe
GET /zen.iso HTTP/1.1172Please respect copyright.ＰＥＮＡＮＡ8naZyOXVdH
Range: bytes=287718144-172Please respect copyright.ＰＥＮＡＮＡOrPnICJM9Z
If-Match: "534231200_1732277116"172Please respect copyright.ＰＥＮＡＮＡREOESWWnL8
-----------------------------------172Please respect copyright.ＰＥＮＡＮＡkUvKzBIdQR
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:172Please respect copyright.ＰＥＮＡＮＡgnNrt624Zq
-------------range response-------------------172Please respect copyright.ＰＥＮＡＮＡr4VleuGhNz
HTTP/1.1 206 Partial Content172Please respect copyright.ＰＥＮＡＮＡONIcYl3HHh
accept-ranges: bytes172Please respect copyright.ＰＥＮＡＮＡG18zwe3ulE
content-range: bytes 287718144-534231199/534231200172Please respect copyright.ＰＥＮＡＮＡnZ6Uu8xHC4
ETag: "534231200_1732277116"172Please respect copyright.ＰＥＮＡＮＡyqvX2hz2Wz
Content-Length: 246513056172Please respect copyright.ＰＥＮＡＮＡ8Vx7UOIjFu
Content-Type: application/octet-stream172Please respect copyright.ＰＥＮＡＮＡ3aMWLp5wrY
Content-Disposition: attachment; filename="zen.iso"
(http body is the remain file content)172Please respect copyright.ＰＥＮＡＮＡFKWjP9Iy9I
-------------------------------------------
If the etag changes, response 412 like this:172Please respect copyright.ＰＥＮＡＮＡAzHz5VqyXL
-----------http response for changed resource------172Please respect copyright.ＰＥＮＡＮＡbJ3a9WWTM7
HTTP/1.1 412 Precondition Failed
(http body canbe empty, the client will stop downloading the resumed file)172Please respect copyright.ＰＥＮＡＮＡYYJaciOjoc
---------------------------------------------------
172Please respect copyright.ＰＥＮＡＮＡ5fwWa9tgeW
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---172Please respect copyright.ＰＥＮＡＮＡxjVNuFgvvS
GET /zen.iso HTTP/1.1172Please respect copyright.ＰＥＮＡＮＡ2RnBkdtkbZ
Range: bytes=287718144-172Please respect copyright.ＰＥＮＡＮＡUrAPGZ55iw
If-Range: "534231200_1732277116"172Please respect copyright.ＰＥＮＡＮＡnPo9et1J7h
------------------------------------172Please respect copyright.ＰＥＮＡＮＡdLbBCWtKu9
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:172Please respect copyright.ＰＥＮＡＮＡeh0iY0pvSE
-------------range response-------------------172Please respect copyright.ＰＥＮＡＮＡ5rJxNrEkvC
HTTP/1.1 206 Partial Content172Please respect copyright.ＰＥＮＡＮＡVddcZTy9he
accept-ranges: bytes172Please respect copyright.ＰＥＮＡＮＡOe8BvaxvxN
content-range: bytes 287718144-534231199/534231200172Please respect copyright.ＰＥＮＡＮＡjHbtlxQZke
ETag: "534231200_1732277116"172Please respect copyright.ＰＥＮＡＮＡgdTq9sCYZ5
Content-Length: 246513056172Please respect copyright.ＰＥＮＡＮＡIl22Jgw1UI
Content-Type: application/octet-stream172Please respect copyright.ＰＥＮＡＮＡxBMFYsHFMF
Content-Disposition: attachment; filename="zen.iso"
(http body is the remain file content)172Please respect copyright.ＰＥＮＡＮＡ4AHzKBro5U
-------------------------------------------
If the etag changes, response 200 from beginning like this:172Please respect copyright.ＰＥＮＡＮＡHd3mO37J0B
-----------http response from the beginning------172Please respect copyright.ＰＥＮＡＮＡFZPS901hbV
HTTP/1.1 200 OK172Please respect copyright.ＰＥＮＡＮＡoyGcOgWURZ
Date: Mon, 25 Nov 2024 10:56:12 GMT172Please respect copyright.ＰＥＮＡＮＡPx6ZJQSFcd
accept-ranges: bytes172Please respect copyright.ＰＥＮＡＮＡHTgIe9FTqv
ETag: "534231200_1732277116"172Please respect copyright.ＰＥＮＡＮＡKceeqNvexB
Content-Length: 534231200172Please respect copyright.ＰＥＮＡＮＡitY0A0UIfd
Content-Type: application/octet-stream172Please respect copyright.ＰＥＮＡＮＡh6mhtr6KVv
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)172Please respect copyright.ＰＥＮＡＮＡNk1rDyXLCB
---------------------------------------------------
172Please respect copyright.ＰＥＮＡＮＡ5yEoXn0Lbh
Section X. Thanks
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/If-None-Match172Please respect copyright.ＰＥＮＡＮＡoVZPGdWgjW
https://blog.csdn.net/catoop/article/details/134174653172Please respect copyright.ＰＥＮＡＮＡqu3ZeEgLKo
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]172Please respect copyright.ＰＥＮＡＮＡjIxwEyejJI
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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