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.
35Please respect copyright.ＰＥＮＡＮＡndWNCijLUM
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:35Please respect copyright.ＰＥＮＡＮＡTuKF5sc3My
---------------http response with etag----------------35Please respect copyright.ＰＥＮＡＮＡb7vZzTccX2
HTTP/1.1 200 OK35Please respect copyright.ＰＥＮＡＮＡ3CuiCAEVcS
Date: Mon, 25 Nov 2024 10:56:12 GMT35Please respect copyright.ＰＥＮＡＮＡG6Ble6XghJ
accept-ranges: bytes35Please respect copyright.ＰＥＮＡＮＡ5zVj1JSm8g
ETag: "534231200_1732277116"35Please respect copyright.ＰＥＮＡＮＡsz9lxgmJ8G
Content-Length: 53423120035Please respect copyright.ＰＥＮＡＮＡr1yCcw0xxq
Content-Type: application/octet-stream35Please respect copyright.ＰＥＮＡＮＡSA0RQPAcOU
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)35Please respect copyright.ＰＥＮＡＮＡHs6haRV4Dr
-----------------------------------------------------
35Please respect copyright.ＰＥＮＡＮＡWtDxdoaVSp
Section B. if-none-match
The browser send back the etag to ask server if it has been changed by header like this:35Please respect copyright.ＰＥＮＡＮＡu0CTbRjeNH
-----http request for a cached resource------35Please respect copyright.ＰＥＮＡＮＡfp24a709zW
GET /zen.iso HTTP/1.135Please respect copyright.ＰＥＮＡＮＡHIfPbS6GpU
if-none-match: "534231200_1732277116"35Please respect copyright.ＰＥＮＡＮＡ4nZnsPk5Y8
---------------------------------------------
If the etag is unchanged, the server response like this:
1. for get and head requests:35Please respect copyright.ＰＥＮＡＮＡxpnVpI8Ydv
-----------http response for unchanged resource------35Please respect copyright.ＰＥＮＡＮＡ5e6iXXZgh8
HTTP/1.1 304 Not Modified
(http body canbe empty, the client will used its cache)35Please respect copyright.ＰＥＮＡＮＡBic6QD0KRB
----------------------------------------------------
2. for other methods requests:35Please respect copyright.ＰＥＮＡＮＡhTuxiRLCYB
-----------http response for unchanged resource------35Please respect copyright.ＰＥＮＡＮＡq2zbN6qUZY
HTTP/1.1 412 Precondition Failed
(http body canbe empty, the client will used its cache)35Please respect copyright.ＰＥＮＡＮＡ1DNTK8tUiV
----------------------------------------------------
35Please respect copyright.ＰＥＮＡＮＡu1qAUSfhcq
If the etag changes, the server response 200 OK with the new file content like this:35Please respect copyright.ＰＥＮＡＮＡqr14aIIew6
---------------http response with etag----------------35Please respect copyright.ＰＥＮＡＮＡt5M16Mrfpc
HTTP/1.1 200 OK35Please respect copyright.ＰＥＮＡＮＡNyjBly1C8E
Date: Mon, 25 Nov 2024 10:56:12 GMT35Please respect copyright.ＰＥＮＡＮＡ0441Zu1Tc2
accept-ranges: bytes35Please respect copyright.ＰＥＮＡＮＡmYtMywpIo5
ETag: "534231200_1732277116"35Please respect copyright.ＰＥＮＡＮＡZSuYYmhWQW
Content-Length: 53423120035Please respect copyright.ＰＥＮＡＮＡx1TxKIx9kg
Content-Type: application/octet-stream35Please respect copyright.ＰＥＮＡＮＡAGydmzFgLb
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)35Please respect copyright.ＰＥＮＡＮＡjF0h4l9ns2
-----------------------------------------------------
35Please respect copyright.ＰＥＮＡＮＡacK2ew39YC
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:35Please respect copyright.ＰＥＮＡＮＡpHeOpojFL5
------resume download request-------35Please respect copyright.ＰＥＮＡＮＡuHsvXmvgAl
GET /zen.iso HTTP/1.135Please respect copyright.ＰＥＮＡＮＡ6wkg4rFsBc
Range: bytes=287718144-35Please respect copyright.ＰＥＮＡＮＡ4ZpjDxALpP
If-Match: "534231200_1732277116"35Please respect copyright.ＰＥＮＡＮＡWYG0ERa67Y
-----------------------------------35Please respect copyright.ＰＥＮＡＮＡzs1Yhi9oFo
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:35Please respect copyright.ＰＥＮＡＮＡNdsOT81dly
-------------range response-------------------35Please respect copyright.ＰＥＮＡＮＡeLXHrkoFlZ
HTTP/1.1 206 Partial Content35Please respect copyright.ＰＥＮＡＮＡYLXUW4Al8w
accept-ranges: bytes35Please respect copyright.ＰＥＮＡＮＡKDCVDQ0lOR
content-range: bytes 287718144-534231199/53423120035Please respect copyright.ＰＥＮＡＮＡNnVEIWlMCo
ETag: "534231200_1732277116"35Please respect copyright.ＰＥＮＡＮＡMPg0g2GsaV
Content-Length: 24651305635Please respect copyright.ＰＥＮＡＮＡhAd5Cpnk4f
Content-Type: application/octet-stream35Please respect copyright.ＰＥＮＡＮＡNCzXxGObHJ
Content-Disposition: attachment; filename="zen.iso"
(http body is the remain file content)35Please respect copyright.ＰＥＮＡＮＡztma7bL2uk
-------------------------------------------
If the etag changes, response 412 like this:35Please respect copyright.ＰＥＮＡＮＡ0UAlA0yc23
-----------http response for changed resource------35Please respect copyright.ＰＥＮＡＮＡjSr6C1mDz7
HTTP/1.1 412 Precondition Failed
(http body canbe empty, the client will stop downloading the resumed file)35Please respect copyright.ＰＥＮＡＮＡYrV2LxFcp5
---------------------------------------------------
35Please respect copyright.ＰＥＮＡＮＡjd7wvwDKOQ
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---35Please respect copyright.ＰＥＮＡＮＡzLDALNiGJk
GET /zen.iso HTTP/1.135Please respect copyright.ＰＥＮＡＮＡlV3F7XQyeu
Range: bytes=287718144-35Please respect copyright.ＰＥＮＡＮＡUjRst2p0Di
If-Range: "534231200_1732277116"35Please respect copyright.ＰＥＮＡＮＡi0nokQtDWX
------------------------------------35Please respect copyright.ＰＥＮＡＮＡPNDCYzSSE0
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:35Please respect copyright.ＰＥＮＡＮＡfOvNwkbC7J
-------------range response-------------------35Please respect copyright.ＰＥＮＡＮＡBxt6JJU5he
HTTP/1.1 206 Partial Content35Please respect copyright.ＰＥＮＡＮＡfjBlGhES40
accept-ranges: bytes35Please respect copyright.ＰＥＮＡＮＡ2cAD4wdtcV
content-range: bytes 287718144-534231199/53423120035Please respect copyright.ＰＥＮＡＮＡ1qFTjiBjBb
ETag: "534231200_1732277116"35Please respect copyright.ＰＥＮＡＮＡO7clJHpYC6
Content-Length: 24651305635Please respect copyright.ＰＥＮＡＮＡNytDzdrVT5
Content-Type: application/octet-stream35Please respect copyright.ＰＥＮＡＮＡUUNkRBU1Z9
Content-Disposition: attachment; filename="zen.iso"
(http body is the remain file content)35Please respect copyright.ＰＥＮＡＮＡwhba6PkZay
-------------------------------------------
If the etag changes, response 200 from beginning like this:35Please respect copyright.ＰＥＮＡＮＡkLAuhff7Uq
-----------http response from the beginning------35Please respect copyright.ＰＥＮＡＮＡDfYXpztnL8
HTTP/1.1 200 OK35Please respect copyright.ＰＥＮＡＮＡGLbFiTwTCE
Date: Mon, 25 Nov 2024 10:56:12 GMT35Please respect copyright.ＰＥＮＡＮＡPCWdAs7xcq
accept-ranges: bytes35Please respect copyright.ＰＥＮＡＮＡUe8ZuMzA4z
ETag: "534231200_1732277116"35Please respect copyright.ＰＥＮＡＮＡ2Naey5E78B
Content-Length: 53423120035Please respect copyright.ＰＥＮＡＮＡoZ1R7fsy3C
Content-Type: application/octet-stream35Please respect copyright.ＰＥＮＡＮＡEm7blLocbv
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)35Please respect copyright.ＰＥＮＡＮＡp8H0w6l7L7
---------------------------------------------------
35Please respect copyright.ＰＥＮＡＮＡ9PUaHJfFkK
Section X. Thanks
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/If-None-Match35Please respect copyright.ＰＥＮＡＮＡHQtI98JFqd
https://blog.csdn.net/catoop/article/details/13417465335Please respect copyright.ＰＥＮＡＮＡW8HM1wF8Wr
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.com35Please respect copyright.ＰＥＮＡＮＡJrX2EIflys
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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