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.
209Please respect copyright.ＰＥＮＡＮＡwbs0sU90EB
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:209Please respect copyright.ＰＥＮＡＮＡxyhgPNZOpn
---------------http response with etag----------------209Please respect copyright.ＰＥＮＡＮＡtOSjcg3Ajt
HTTP/1.1 200 OK209Please respect copyright.ＰＥＮＡＮＡGa9vdj3Hnu
Date: Mon, 25 Nov 2024 10:56:12 GMT209Please respect copyright.ＰＥＮＡＮＡwktNzi9CH7
accept-ranges: bytes209Please respect copyright.ＰＥＮＡＮＡpt2dIlIs1S
ETag: "534231200_1732277116"209Please respect copyright.ＰＥＮＡＮＡItC2JOK3xT
Content-Length: 534231200209Please respect copyright.ＰＥＮＡＮＡGcPtwjOgkP
Content-Type: application/octet-stream209Please respect copyright.ＰＥＮＡＮＡGAfXekd1Gp
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)209Please respect copyright.ＰＥＮＡＮＡkdpoS9etCE
-----------------------------------------------------
209Please respect copyright.ＰＥＮＡＮＡnMFi77onYT
Section B. if-none-match
The browser send back the etag to ask server if it has been changed by header like this:209Please respect copyright.ＰＥＮＡＮＡ3X1O72ZET7
-----http request for a cached resource------209Please respect copyright.ＰＥＮＡＮＡxPdR7FNX0h
GET /zen.iso HTTP/1.1209Please respect copyright.ＰＥＮＡＮＡQJijLg8z3v
if-none-match: "534231200_1732277116"209Please respect copyright.ＰＥＮＡＮＡvTbK922njm
---------------------------------------------
If the etag is unchanged, the server response like this:
1. for get and head requests:209Please respect copyright.ＰＥＮＡＮＡP3laAdmPfp
-----------http response for unchanged resource------209Please respect copyright.ＰＥＮＡＮＡvEeh5aCGn0
HTTP/1.1 304 Not Modified
(http body canbe empty, the client will used its cache)209Please respect copyright.ＰＥＮＡＮＡmqzEeHmIgN
----------------------------------------------------
2. for other methods requests:209Please respect copyright.ＰＥＮＡＮＡk5ZwjDG1gC
-----------http response for unchanged resource------209Please respect copyright.ＰＥＮＡＮＡUTvTgpet7h
HTTP/1.1 412 Precondition Failed
(http body canbe empty, the client will used its cache)209Please respect copyright.ＰＥＮＡＮＡdOsOKtoC5e
----------------------------------------------------
209Please respect copyright.ＰＥＮＡＮＡWAVf62jUmq
If the etag changes, the server response 200 OK with the new file content like this:209Please respect copyright.ＰＥＮＡＮＡ4duLjtEA0c
---------------http response with etag----------------209Please respect copyright.ＰＥＮＡＮＡJd3koqdNou
HTTP/1.1 200 OK209Please respect copyright.ＰＥＮＡＮＡwTS90HR4Vw
Date: Mon, 25 Nov 2024 10:56:12 GMT209Please respect copyright.ＰＥＮＡＮＡPSmPqVhHtM
accept-ranges: bytes209Please respect copyright.ＰＥＮＡＮＡFMobhNSdMc
ETag: "534231200_1732277116"209Please respect copyright.ＰＥＮＡＮＡVwqJnMFiIa
Content-Length: 534231200209Please respect copyright.ＰＥＮＡＮＡMOCoRpGm5g
Content-Type: application/octet-stream209Please respect copyright.ＰＥＮＡＮＡIvG6fpcIc1
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)209Please respect copyright.ＰＥＮＡＮＡlGg1UhBmHn
-----------------------------------------------------
209Please respect copyright.ＰＥＮＡＮＡ5vJFPn9jXA
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:209Please respect copyright.ＰＥＮＡＮＡXmYJb8ffPB
------resume download request-------209Please respect copyright.ＰＥＮＡＮＡnFrEX1wInJ
GET /zen.iso HTTP/1.1209Please respect copyright.ＰＥＮＡＮＡEMkAfFdtok
Range: bytes=287718144-209Please respect copyright.ＰＥＮＡＮＡtE3qHOQmad
If-Match: "534231200_1732277116"209Please respect copyright.ＰＥＮＡＮＡZBPwJXybGS
-----------------------------------209Please respect copyright.ＰＥＮＡＮＡxl6ajPvUql
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:209Please respect copyright.ＰＥＮＡＮＡ4sFtBC8Ytt
-------------range response-------------------209Please respect copyright.ＰＥＮＡＮＡBSgLBHYrhG
HTTP/1.1 206 Partial Content209Please respect copyright.ＰＥＮＡＮＡuSLFFfpsRm
accept-ranges: bytes209Please respect copyright.ＰＥＮＡＮＡX6QSEyrqTf
content-range: bytes 287718144-534231199/534231200209Please respect copyright.ＰＥＮＡＮＡvfaBJQZt0G
ETag: "534231200_1732277116"209Please respect copyright.ＰＥＮＡＮＡGTF983ZNBs
Content-Length: 246513056209Please respect copyright.ＰＥＮＡＮＡfnwe4Eko8Z
Content-Type: application/octet-stream209Please respect copyright.ＰＥＮＡＮＡY7aQ8iFW2B
Content-Disposition: attachment; filename="zen.iso"
(http body is the remain file content)209Please respect copyright.ＰＥＮＡＮＡtlieija1mm
-------------------------------------------
If the etag changes, response 412 like this:209Please respect copyright.ＰＥＮＡＮＡN2i1jZ4zQ8
-----------http response for changed resource------209Please respect copyright.ＰＥＮＡＮＡh0bWEsPnsS
HTTP/1.1 412 Precondition Failed
(http body canbe empty, the client will stop downloading the resumed file)209Please respect copyright.ＰＥＮＡＮＡ3wdn7UgLu2
---------------------------------------------------
209Please respect copyright.ＰＥＮＡＮＡ9QDFwuvqH8
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---209Please respect copyright.ＰＥＮＡＮＡfGSgADpFJD
GET /zen.iso HTTP/1.1209Please respect copyright.ＰＥＮＡＮＡjJspkCcPwN
Range: bytes=287718144-209Please respect copyright.ＰＥＮＡＮＡ5o6r6IBMLS
If-Range: "534231200_1732277116"209Please respect copyright.ＰＥＮＡＮＡrqjsfVPzw4
------------------------------------209Please respect copyright.ＰＥＮＡＮＡ2FYoq1sfsb
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:209Please respect copyright.ＰＥＮＡＮＡT1an01Bzl8
-------------range response-------------------209Please respect copyright.ＰＥＮＡＮＡEu89t7AAZe
HTTP/1.1 206 Partial Content209Please respect copyright.ＰＥＮＡＮＡcXUwXG5IIF
accept-ranges: bytes209Please respect copyright.ＰＥＮＡＮＡVhmxwBHnZ5
content-range: bytes 287718144-534231199/534231200209Please respect copyright.ＰＥＮＡＮＡZ7XWPOjmLA
ETag: "534231200_1732277116"209Please respect copyright.ＰＥＮＡＮＡRckcAiqjMA
Content-Length: 246513056209Please respect copyright.ＰＥＮＡＮＡXrMPysE1n0
Content-Type: application/octet-stream209Please respect copyright.ＰＥＮＡＮＡfWX2r83Ki2
Content-Disposition: attachment; filename="zen.iso"
(http body is the remain file content)209Please respect copyright.ＰＥＮＡＮＡMXaB6kmSMq
-------------------------------------------
If the etag changes, response 200 from beginning like this:209Please respect copyright.ＰＥＮＡＮＡDiEsPAsMvk
-----------http response from the beginning------209Please respect copyright.ＰＥＮＡＮＡV8eJwHbUkW
HTTP/1.1 200 OK209Please respect copyright.ＰＥＮＡＮＡbyNMsTIYZP
Date: Mon, 25 Nov 2024 10:56:12 GMT209Please respect copyright.ＰＥＮＡＮＡPJtU86xaGu
accept-ranges: bytes209Please respect copyright.ＰＥＮＡＮＡyTQpteDbP7
ETag: "534231200_1732277116"209Please respect copyright.ＰＥＮＡＮＡcBpeyXYAoU
Content-Length: 534231200209Please respect copyright.ＰＥＮＡＮＡrhVKLLIuCE
Content-Type: application/octet-stream209Please respect copyright.ＰＥＮＡＮＡTOilr7y0W2
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)209Please respect copyright.ＰＥＮＡＮＡwxp5MqMgZ1
---------------------------------------------------
209Please respect copyright.ＰＥＮＡＮＡso0oZpvsnt
Section X. Thanks
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/If-None-Match209Please respect copyright.ＰＥＮＡＮＡd3uzISlt6b
https://blog.csdn.net/catoop/article/details/134174653209Please respect copyright.ＰＥＮＡＮＡ4WKUm6HgRv
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]209Please respect copyright.ＰＥＮＡＮＡVJDNDCO8Sx
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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