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.
24Please respect copyright.ＰＥＮＡＮＡZ77HBr3Zm6
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:24Please respect copyright.ＰＥＮＡＮＡJVWzCSZGJ5
---------------http response with etag----------------24Please respect copyright.ＰＥＮＡＮＡB6P0PPvNSY
HTTP/1.1 200 OK24Please respect copyright.ＰＥＮＡＮＡVES2AzwG1I
Date: Mon, 25 Nov 2024 10:56:12 GMT24Please respect copyright.ＰＥＮＡＮＡI9ZRYlmaIA
accept-ranges: bytes24Please respect copyright.ＰＥＮＡＮＡNLwxYdLHsK
ETag: "534231200_1732277116"24Please respect copyright.ＰＥＮＡＮＡsel1RIokdE
Content-Length: 53423120024Please respect copyright.ＰＥＮＡＮＡgROiAXWUum
Content-Type: application/octet-stream24Please respect copyright.ＰＥＮＡＮＡLvTmwn0mqb
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)24Please respect copyright.ＰＥＮＡＮＡ0R3wKArUJY
-----------------------------------------------------
24Please respect copyright.ＰＥＮＡＮＡqqcqycaW7z
Section B. if-none-match
The browser send back the etag to ask server if it has been changed by header like this:24Please respect copyright.ＰＥＮＡＮＡBdMTLNC1vD
-----http request for a cached resource------24Please respect copyright.ＰＥＮＡＮＡqweZIZSOAd
GET /zen.iso HTTP/1.124Please respect copyright.ＰＥＮＡＮＡxvJQOx91Um
if-none-match: "534231200_1732277116"24Please respect copyright.ＰＥＮＡＮＡfPfNEfEuks
---------------------------------------------
If the etag is unchanged, the server response like this:
1. for get and head requests:24Please respect copyright.ＰＥＮＡＮＡBZTkBnAqIm
-----------http response for unchanged resource------24Please respect copyright.ＰＥＮＡＮＡgPmlf3H21v
HTTP/1.1 304 Not Modified
(http body canbe empty, the client will used its cache)24Please respect copyright.ＰＥＮＡＮＡwAMaZ4Y4nu
----------------------------------------------------
2. for other methods requests:24Please respect copyright.ＰＥＮＡＮＡDfD8BOOu0F
-----------http response for unchanged resource------24Please respect copyright.ＰＥＮＡＮＡe7NKPgZJGW
HTTP/1.1 412 Precondition Failed
(http body canbe empty, the client will used its cache)24Please respect copyright.ＰＥＮＡＮＡ0cADXNb2r9
----------------------------------------------------
24Please respect copyright.ＰＥＮＡＮＡA4zgn4si3z
If the etag changes, the server response 200 OK with the new file content like this:24Please respect copyright.ＰＥＮＡＮＡKtyPRZzWj8
---------------http response with etag----------------24Please respect copyright.ＰＥＮＡＮＡVGfFm9ZLxL
HTTP/1.1 200 OK24Please respect copyright.ＰＥＮＡＮＡVTxmMRagD2
Date: Mon, 25 Nov 2024 10:56:12 GMT24Please respect copyright.ＰＥＮＡＮＡ4MqBgWHSlT
accept-ranges: bytes24Please respect copyright.ＰＥＮＡＮＡh5L2gmZ4CF
ETag: "534231200_1732277116"24Please respect copyright.ＰＥＮＡＮＡaY63XwjxQm
Content-Length: 53423120024Please respect copyright.ＰＥＮＡＮＡnnvDVciMRn
Content-Type: application/octet-stream24Please respect copyright.ＰＥＮＡＮＡAkNqLkllat
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)24Please respect copyright.ＰＥＮＡＮＡM68TxiHUto
-----------------------------------------------------
24Please respect copyright.ＰＥＮＡＮＡGLIpEfghZR
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:24Please respect copyright.ＰＥＮＡＮＡmBFDp3lb9M
------resume download request-------24Please respect copyright.ＰＥＮＡＮＡtXhCdllAu2
GET /zen.iso HTTP/1.124Please respect copyright.ＰＥＮＡＮＡb1dJia5k7O
Range: bytes=287718144-24Please respect copyright.ＰＥＮＡＮＡD1eabdpl6k
If-Match: "534231200_1732277116"24Please respect copyright.ＰＥＮＡＮＡ3bODJ7pOkM
-----------------------------------24Please respect copyright.ＰＥＮＡＮＡZ1sUnLPvLd
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:24Please respect copyright.ＰＥＮＡＮＡvuGx1CP64z
-------------range response-------------------24Please respect copyright.ＰＥＮＡＮＡlNUDGjBv5R
HTTP/1.1 206 Partial Content24Please respect copyright.ＰＥＮＡＮＡs98eePtfIv
accept-ranges: bytes24Please respect copyright.ＰＥＮＡＮＡtvCjSX737H
content-range: bytes 287718144-534231199/53423120024Please respect copyright.ＰＥＮＡＮＡDjLAYvyzKh
ETag: "534231200_1732277116"24Please respect copyright.ＰＥＮＡＮＡrarlI8a5qO
Content-Length: 24651305624Please respect copyright.ＰＥＮＡＮＡvoKy3vbyet
Content-Type: application/octet-stream24Please respect copyright.ＰＥＮＡＮＡQ3Yr8iKKZP
Content-Disposition: attachment; filename="zen.iso"
(http body is the remain file content)24Please respect copyright.ＰＥＮＡＮＡ0FxopePJkb
-------------------------------------------
If the etag changes, response 412 like this:24Please respect copyright.ＰＥＮＡＮＡt2UHB12Y7z
-----------http response for changed resource------24Please respect copyright.ＰＥＮＡＮＡvGPirKJWGA
HTTP/1.1 412 Precondition Failed
(http body canbe empty, the client will stop downloading the resumed file)24Please respect copyright.ＰＥＮＡＮＡZV7EopZ3wJ
---------------------------------------------------
24Please respect copyright.ＰＥＮＡＮＡ7rLqoEDNHM
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---24Please respect copyright.ＰＥＮＡＮＡGMZfspWMgL
GET /zen.iso HTTP/1.124Please respect copyright.ＰＥＮＡＮＡiClecHykJC
Range: bytes=287718144-24Please respect copyright.ＰＥＮＡＮＡeb87CDYcUy
If-Range: "534231200_1732277116"24Please respect copyright.ＰＥＮＡＮＡgzziHcgYDZ
------------------------------------24Please respect copyright.ＰＥＮＡＮＡygmtVQSsCF
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:24Please respect copyright.ＰＥＮＡＮＡEfrJzngEuR
-------------range response-------------------24Please respect copyright.ＰＥＮＡＮＡbkgosFJsQA
HTTP/1.1 206 Partial Content24Please respect copyright.ＰＥＮＡＮＡxd9uERBS1n
accept-ranges: bytes24Please respect copyright.ＰＥＮＡＮＡuGxcOdyYGx
content-range: bytes 287718144-534231199/53423120024Please respect copyright.ＰＥＮＡＮＡD95Fb9CNyb
ETag: "534231200_1732277116"24Please respect copyright.ＰＥＮＡＮＡxm2OM6EYFO
Content-Length: 24651305624Please respect copyright.ＰＥＮＡＮＡAzlAEytNY6
Content-Type: application/octet-stream24Please respect copyright.ＰＥＮＡＮＡR3EfzM787P
Content-Disposition: attachment; filename="zen.iso"
(http body is the remain file content)24Please respect copyright.ＰＥＮＡＮＡWcK9DWefmu
-------------------------------------------
If the etag changes, response 200 from beginning like this:24Please respect copyright.ＰＥＮＡＮＡYqboCLGvGu
-----------http response from the beginning------24Please respect copyright.ＰＥＮＡＮＡuUffM1W0Ea
HTTP/1.1 200 OK24Please respect copyright.ＰＥＮＡＮＡ1cJgSEDIoe
Date: Mon, 25 Nov 2024 10:56:12 GMT24Please respect copyright.ＰＥＮＡＮＡXNV2D14VOX
accept-ranges: bytes24Please respect copyright.ＰＥＮＡＮＡd3yDkIAKnp
ETag: "534231200_1732277116"24Please respect copyright.ＰＥＮＡＮＡa0Jr3l9dVS
Content-Length: 53423120024Please respect copyright.ＰＥＮＡＮＡQjyW46Tes0
Content-Type: application/octet-stream24Please respect copyright.ＰＥＮＡＮＡZDI8HVVtuZ
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)24Please respect copyright.ＰＥＮＡＮＡJMweFSp9Dl
---------------------------------------------------
24Please respect copyright.ＰＥＮＡＮＡ3jEesV62Zt
Section X. Thanks
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/If-None-Match24Please respect copyright.ＰＥＮＡＮＡEG1Ogf2qrA
https://blog.csdn.net/catoop/article/details/13417465324Please respect copyright.ＰＥＮＡＮＡ3S7hv6SSfa
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.com24Please respect copyright.ＰＥＮＡＮＡjw5fQhkj91
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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