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.
149Please respect copyright.ＰＥＮＡＮＡbXdm3efHzN
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:149Please respect copyright.ＰＥＮＡＮＡGIeLz5IY4d
---------------http response with etag----------------149Please respect copyright.ＰＥＮＡＮＡ3xqYlUGi2h
HTTP/1.1 200 OK149Please respect copyright.ＰＥＮＡＮＡPPbcbCRgtt
Date: Mon, 25 Nov 2024 10:56:12 GMT149Please respect copyright.ＰＥＮＡＮＡQVlB9V0aCt
accept-ranges: bytes149Please respect copyright.ＰＥＮＡＮＡxmVExbLyni
ETag: "534231200_1732277116"149Please respect copyright.ＰＥＮＡＮＡN4GOzFIddd
Content-Length: 534231200149Please respect copyright.ＰＥＮＡＮＡeYjTrfVvR9
Content-Type: application/octet-stream149Please respect copyright.ＰＥＮＡＮＡdsSPDDrlDU
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)149Please respect copyright.ＰＥＮＡＮＡUvDjHF9Uuj
-----------------------------------------------------
149Please respect copyright.ＰＥＮＡＮＡoVUoBxewkG
Section B. if-none-match
The browser send back the etag to ask server if it has been changed by header like this:149Please respect copyright.ＰＥＮＡＮＡdR3jVdboAN
-----http request for a cached resource------149Please respect copyright.ＰＥＮＡＮＡB5bbHP59Te
GET /zen.iso HTTP/1.1149Please respect copyright.ＰＥＮＡＮＡ5MqzIKOwZr
if-none-match: "534231200_1732277116"149Please respect copyright.ＰＥＮＡＮＡkd05Y5yR0M
---------------------------------------------
If the etag is unchanged, the server response like this:
1. for get and head requests:149Please respect copyright.ＰＥＮＡＮＡSaDOb38z4H
-----------http response for unchanged resource------149Please respect copyright.ＰＥＮＡＮＡub6lYZEb6y
HTTP/1.1 304 Not Modified
(http body canbe empty, the client will used its cache)149Please respect copyright.ＰＥＮＡＮＡMifrFFz5GC
----------------------------------------------------
2. for other methods requests:149Please respect copyright.ＰＥＮＡＮＡYRGwuJ0A13
-----------http response for unchanged resource------149Please respect copyright.ＰＥＮＡＮＡ0sfLo2ImuD
HTTP/1.1 412 Precondition Failed
(http body canbe empty, the client will used its cache)149Please respect copyright.ＰＥＮＡＮＡ7a13yGxhr7
----------------------------------------------------
149Please respect copyright.ＰＥＮＡＮＡZZ6GWZVYZa
If the etag changes, the server response 200 OK with the new file content like this:149Please respect copyright.ＰＥＮＡＮＡ1EK6VGPx91
---------------http response with etag----------------149Please respect copyright.ＰＥＮＡＮＡ80bCFy7b8t
HTTP/1.1 200 OK149Please respect copyright.ＰＥＮＡＮＡhGn0M3Hnu8
Date: Mon, 25 Nov 2024 10:56:12 GMT149Please respect copyright.ＰＥＮＡＮＡYGlq7EjPY5
accept-ranges: bytes149Please respect copyright.ＰＥＮＡＮＡXeOS4HDitk
ETag: "534231200_1732277116"149Please respect copyright.ＰＥＮＡＮＡlHT9CSAs6J
Content-Length: 534231200149Please respect copyright.ＰＥＮＡＮＡvmFym6bsRN
Content-Type: application/octet-stream149Please respect copyright.ＰＥＮＡＮＡxBX7q1ADTS
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)149Please respect copyright.ＰＥＮＡＮＡkAOEIOD0X9
-----------------------------------------------------
149Please respect copyright.ＰＥＮＡＮＡU5zcoaLyvc
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:149Please respect copyright.ＰＥＮＡＮＡlNKUbXgK96
------resume download request-------149Please respect copyright.ＰＥＮＡＮＡDY8781qrDM
GET /zen.iso HTTP/1.1149Please respect copyright.ＰＥＮＡＮＡTrZMZmTm9Y
Range: bytes=287718144-149Please respect copyright.ＰＥＮＡＮＡckvgrFd4l5
If-Match: "534231200_1732277116"149Please respect copyright.ＰＥＮＡＮＡKMkGyg6yOS
-----------------------------------149Please respect copyright.ＰＥＮＡＮＡGokHs4U4EH
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:149Please respect copyright.ＰＥＮＡＮＡsjRSQQBA1w
-------------range response-------------------149Please respect copyright.ＰＥＮＡＮＡJd44gmtCiC
HTTP/1.1 206 Partial Content149Please respect copyright.ＰＥＮＡＮＡu5LJSQtD3R
accept-ranges: bytes149Please respect copyright.ＰＥＮＡＮＡQhxoRIrkYV
content-range: bytes 287718144-534231199/534231200149Please respect copyright.ＰＥＮＡＮＡhMU0DEGIh5
ETag: "534231200_1732277116"149Please respect copyright.ＰＥＮＡＮＡ25Ze2aRT7f
Content-Length: 246513056149Please respect copyright.ＰＥＮＡＮＡlfxfbWFzbp
Content-Type: application/octet-stream149Please respect copyright.ＰＥＮＡＮＡE2VwGwH8iA
Content-Disposition: attachment; filename="zen.iso"
(http body is the remain file content)149Please respect copyright.ＰＥＮＡＮＡdWepgbRr8t
-------------------------------------------
If the etag changes, response 412 like this:149Please respect copyright.ＰＥＮＡＮＡKcbjW718By
-----------http response for changed resource------149Please respect copyright.ＰＥＮＡＮＡFWEY2rOiT8
HTTP/1.1 412 Precondition Failed
(http body canbe empty, the client will stop downloading the resumed file)149Please respect copyright.ＰＥＮＡＮＡpU1TRLrWGs
---------------------------------------------------
149Please respect copyright.ＰＥＮＡＮＡ7O4wHfQ7vl
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---149Please respect copyright.ＰＥＮＡＮＡetjDcIydIM
GET /zen.iso HTTP/1.1149Please respect copyright.ＰＥＮＡＮＡlyylBOZvH6
Range: bytes=287718144-149Please respect copyright.ＰＥＮＡＮＡHrN8vEaoXM
If-Range: "534231200_1732277116"149Please respect copyright.ＰＥＮＡＮＡC9PRcV9K0L
------------------------------------149Please respect copyright.ＰＥＮＡＮＡhnl8JYSFio
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:149Please respect copyright.ＰＥＮＡＮＡE2nALJuyBF
-------------range response-------------------149Please respect copyright.ＰＥＮＡＮＡP9PKcqkiAV
HTTP/1.1 206 Partial Content149Please respect copyright.ＰＥＮＡＮＡrMOfzxIHxp
accept-ranges: bytes149Please respect copyright.ＰＥＮＡＮＡkhhN0k0nlW
content-range: bytes 287718144-534231199/534231200149Please respect copyright.ＰＥＮＡＮＡN2pBMq0gge
ETag: "534231200_1732277116"149Please respect copyright.ＰＥＮＡＮＡCjG5DWa2xE
Content-Length: 246513056149Please respect copyright.ＰＥＮＡＮＡlJY4uU1jLm
Content-Type: application/octet-stream149Please respect copyright.ＰＥＮＡＮＡdjEREkYsOg
Content-Disposition: attachment; filename="zen.iso"
(http body is the remain file content)149Please respect copyright.ＰＥＮＡＮＡ1Zn4qQ4FOq
-------------------------------------------
If the etag changes, response 200 from beginning like this:149Please respect copyright.ＰＥＮＡＮＡ5NARSoUtnj
-----------http response from the beginning------149Please respect copyright.ＰＥＮＡＮＡuT1U6dPRn4
HTTP/1.1 200 OK149Please respect copyright.ＰＥＮＡＮＡFEE9MeCRH1
Date: Mon, 25 Nov 2024 10:56:12 GMT149Please respect copyright.ＰＥＮＡＮＡh1rYKdNXB5
accept-ranges: bytes149Please respect copyright.ＰＥＮＡＮＡgelMcDS9f6
ETag: "534231200_1732277116"149Please respect copyright.ＰＥＮＡＮＡFHFVrIo8OO
Content-Length: 534231200149Please respect copyright.ＰＥＮＡＮＡu1N9vUg04z
Content-Type: application/octet-stream149Please respect copyright.ＰＥＮＡＮＡ29uwcReNiW
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)149Please respect copyright.ＰＥＮＡＮＡGbiiIssCdE
---------------------------------------------------
149Please respect copyright.ＰＥＮＡＮＡUzgVyPVUjY
Section X. Thanks
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/If-None-Match149Please respect copyright.ＰＥＮＡＮＡTzlzw8IMmf
https://blog.csdn.net/catoop/article/details/134174653149Please respect copyright.ＰＥＮＡＮＡxSNIBA9zyl
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]149Please respect copyright.ＰＥＮＡＮＡWKUfaipcKk
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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