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.
181Please respect copyright.ＰＥＮＡＮＡOB4uflTnke
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:181Please respect copyright.ＰＥＮＡＮＡeH6DpHPk2u
---------------http response with etag----------------181Please respect copyright.ＰＥＮＡＮＡBMjV8vBt4I
HTTP/1.1 200 OK181Please respect copyright.ＰＥＮＡＮＡ8tQCDqbrkD
Date: Mon, 25 Nov 2024 10:56:12 GMT181Please respect copyright.ＰＥＮＡＮＡKYrd2PwOO2
accept-ranges: bytes181Please respect copyright.ＰＥＮＡＮＡLs9SPLNYwY
ETag: "534231200_1732277116"181Please respect copyright.ＰＥＮＡＮＡJC5iyG25vp
Content-Length: 534231200181Please respect copyright.ＰＥＮＡＮＡgakD1Lqs5x
Content-Type: application/octet-stream181Please respect copyright.ＰＥＮＡＮＡbwOcp320Bt
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)181Please respect copyright.ＰＥＮＡＮＡasaEc3R6zZ
-----------------------------------------------------
181Please respect copyright.ＰＥＮＡＮＡ3RmCgZDJJc
Section B. if-none-match
The browser send back the etag to ask server if it has been changed by header like this:181Please respect copyright.ＰＥＮＡＮＡ9qVbYYvYHX
-----http request for a cached resource------181Please respect copyright.ＰＥＮＡＮＡ50pIAbKkCg
GET /zen.iso HTTP/1.1181Please respect copyright.ＰＥＮＡＮＡrIF0WZAl9D
if-none-match: "534231200_1732277116"181Please respect copyright.ＰＥＮＡＮＡC0ZAfItILs
---------------------------------------------
If the etag is unchanged, the server response like this:
1. for get and head requests:181Please respect copyright.ＰＥＮＡＮＡ2kpAjYlQQx
-----------http response for unchanged resource------181Please respect copyright.ＰＥＮＡＮＡx3L52hk0AF
HTTP/1.1 304 Not Modified
(http body canbe empty, the client will used its cache)181Please respect copyright.ＰＥＮＡＮＡ70XdQREXA6
----------------------------------------------------
2. for other methods requests:181Please respect copyright.ＰＥＮＡＮＡIPnQv01p2o
-----------http response for unchanged resource------181Please respect copyright.ＰＥＮＡＮＡa1CN2jxuqU
HTTP/1.1 412 Precondition Failed
(http body canbe empty, the client will used its cache)181Please respect copyright.ＰＥＮＡＮＡaWXIFm3C5W
----------------------------------------------------
181Please respect copyright.ＰＥＮＡＮＡGH4vfkbats
If the etag changes, the server response 200 OK with the new file content like this:181Please respect copyright.ＰＥＮＡＮＡwQpmeaqc9K
---------------http response with etag----------------181Please respect copyright.ＰＥＮＡＮＡFGbMRKpo2v
HTTP/1.1 200 OK181Please respect copyright.ＰＥＮＡＮＡ7BgLfsrYDo
Date: Mon, 25 Nov 2024 10:56:12 GMT181Please respect copyright.ＰＥＮＡＮＡGCIeff5Hnw
accept-ranges: bytes181Please respect copyright.ＰＥＮＡＮＡh6tnt5M7Q7
ETag: "534231200_1732277116"181Please respect copyright.ＰＥＮＡＮＡ4faIceJQwW
Content-Length: 534231200181Please respect copyright.ＰＥＮＡＮＡrwZgf7aZrP
Content-Type: application/octet-stream181Please respect copyright.ＰＥＮＡＮＡhyPa2VaqeT
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)181Please respect copyright.ＰＥＮＡＮＡRraH3uX61U
-----------------------------------------------------
181Please respect copyright.ＰＥＮＡＮＡbfRni5gQ3G
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:181Please respect copyright.ＰＥＮＡＮＡwCsIxk6wNE
------resume download request-------181Please respect copyright.ＰＥＮＡＮＡBdadeVT0RL
GET /zen.iso HTTP/1.1181Please respect copyright.ＰＥＮＡＮＡ6093JcpVqj
Range: bytes=287718144-181Please respect copyright.ＰＥＮＡＮＡl8rY3nQhpV
If-Match: "534231200_1732277116"181Please respect copyright.ＰＥＮＡＮＡ46CuVQV0FT
-----------------------------------181Please respect copyright.ＰＥＮＡＮＡ7vtKSbr3BF
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:181Please respect copyright.ＰＥＮＡＮＡb9hw8mEp2O
-------------range response-------------------181Please respect copyright.ＰＥＮＡＮＡYVtB7Sff26
HTTP/1.1 206 Partial Content181Please respect copyright.ＰＥＮＡＮＡgwzQcWoIpp
accept-ranges: bytes181Please respect copyright.ＰＥＮＡＮＡcala7mru4x
content-range: bytes 287718144-534231199/534231200181Please respect copyright.ＰＥＮＡＮＡDuwvRimyPj
ETag: "534231200_1732277116"181Please respect copyright.ＰＥＮＡＮＡxeRG6ZW0oY
Content-Length: 246513056181Please respect copyright.ＰＥＮＡＮＡ4n9CQu7cMy
Content-Type: application/octet-stream181Please respect copyright.ＰＥＮＡＮＡUDkrPG5nvJ
Content-Disposition: attachment; filename="zen.iso"
(http body is the remain file content)181Please respect copyright.ＰＥＮＡＮＡgOxLOXiYtk
-------------------------------------------
If the etag changes, response 412 like this:181Please respect copyright.ＰＥＮＡＮＡGG03lnIRJX
-----------http response for changed resource------181Please respect copyright.ＰＥＮＡＮＡtXtdolXC6X
HTTP/1.1 412 Precondition Failed
(http body canbe empty, the client will stop downloading the resumed file)181Please respect copyright.ＰＥＮＡＮＡSj67gBczoS
---------------------------------------------------
181Please respect copyright.ＰＥＮＡＮＡpLFySYNfpQ
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---181Please respect copyright.ＰＥＮＡＮＡD61LYCr87D
GET /zen.iso HTTP/1.1181Please respect copyright.ＰＥＮＡＮＡWdzrGZaRHD
Range: bytes=287718144-181Please respect copyright.ＰＥＮＡＮＡTcGK5B9il1
If-Range: "534231200_1732277116"181Please respect copyright.ＰＥＮＡＮＡYktYPkPb0o
------------------------------------181Please respect copyright.ＰＥＮＡＮＡlor48XWUJr
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:181Please respect copyright.ＰＥＮＡＮＡZxu85xXucG
-------------range response-------------------181Please respect copyright.ＰＥＮＡＮＡhzv2ZZrkTJ
HTTP/1.1 206 Partial Content181Please respect copyright.ＰＥＮＡＮＡOse1DDqGMb
accept-ranges: bytes181Please respect copyright.ＰＥＮＡＮＡiihlPlXD6D
content-range: bytes 287718144-534231199/534231200181Please respect copyright.ＰＥＮＡＮＡ4d4n4OOs9k
ETag: "534231200_1732277116"181Please respect copyright.ＰＥＮＡＮＡjzjkqJYpuA
Content-Length: 246513056181Please respect copyright.ＰＥＮＡＮＡi1mV6DwoOH
Content-Type: application/octet-stream181Please respect copyright.ＰＥＮＡＮＡvzYFzTXBDd
Content-Disposition: attachment; filename="zen.iso"
(http body is the remain file content)181Please respect copyright.ＰＥＮＡＮＡYVQgWDwJTF
-------------------------------------------
If the etag changes, response 200 from beginning like this:181Please respect copyright.ＰＥＮＡＮＡQ8OCYQChTv
-----------http response from the beginning------181Please respect copyright.ＰＥＮＡＮＡaf0VRnhqE3
HTTP/1.1 200 OK181Please respect copyright.ＰＥＮＡＮＡrSSVtpJacM
Date: Mon, 25 Nov 2024 10:56:12 GMT181Please respect copyright.ＰＥＮＡＮＡYB3F3fpO5B
accept-ranges: bytes181Please respect copyright.ＰＥＮＡＮＡs2E562INRU
ETag: "534231200_1732277116"181Please respect copyright.ＰＥＮＡＮＡ2fDVgGr5T8
Content-Length: 534231200181Please respect copyright.ＰＥＮＡＮＡ46mXSbsUfF
Content-Type: application/octet-stream181Please respect copyright.ＰＥＮＡＮＡCP69U1xu6o
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)181Please respect copyright.ＰＥＮＡＮＡD1D4jihYjJ
---------------------------------------------------
181Please respect copyright.ＰＥＮＡＮＡDXpX13W1xN
Section X. Thanks
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/If-None-Match181Please respect copyright.ＰＥＮＡＮＡPVOYfU5vK1
https://blog.csdn.net/catoop/article/details/134174653181Please respect copyright.ＰＥＮＡＮＡXzHtBFXHqf
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]181Please respect copyright.ＰＥＮＡＮＡ7mnYoC4laS
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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