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.
139Please respect copyright.ＰＥＮＡＮＡuTHNy5Uw17
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:139Please respect copyright.ＰＥＮＡＮＡi0BrnQS2Jj
---------------http response with etag----------------139Please respect copyright.ＰＥＮＡＮＡD7gvY2PJeU
HTTP/1.1 200 OK139Please respect copyright.ＰＥＮＡＮＡ5lchdOk1qb
Date: Mon, 25 Nov 2024 10:56:12 GMT139Please respect copyright.ＰＥＮＡＮＡjoOeAkCIOP
accept-ranges: bytes139Please respect copyright.ＰＥＮＡＮＡkwD4Ax58sX
ETag: "534231200_1732277116"139Please respect copyright.ＰＥＮＡＮＡpnoMOyDavX
Content-Length: 534231200139Please respect copyright.ＰＥＮＡＮＡxjxMdMkTi5
Content-Type: application/octet-stream139Please respect copyright.ＰＥＮＡＮＡF7p0wiNpYC
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)139Please respect copyright.ＰＥＮＡＮＡw4r96qyGns
-----------------------------------------------------
139Please respect copyright.ＰＥＮＡＮＡdcUE2vXQ0g
Section B. if-none-match
The browser send back the etag to ask server if it has been changed by header like this:139Please respect copyright.ＰＥＮＡＮＡkkRQdv1SS3
-----http request for a cached resource------139Please respect copyright.ＰＥＮＡＮＡhH8ROquRTq
GET /zen.iso HTTP/1.1139Please respect copyright.ＰＥＮＡＮＡ22Xr9xEK9Z
if-none-match: "534231200_1732277116"139Please respect copyright.ＰＥＮＡＮＡYTWt7TRFkn
---------------------------------------------
If the etag is unchanged, the server response like this:
1. for get and head requests:139Please respect copyright.ＰＥＮＡＮＡZE5MhLqOAk
-----------http response for unchanged resource------139Please respect copyright.ＰＥＮＡＮＡUcGzaOkAgt
HTTP/1.1 304 Not Modified
(http body canbe empty, the client will used its cache)139Please respect copyright.ＰＥＮＡＮＡ5xFXPMNM3G
----------------------------------------------------
2. for other methods requests:139Please respect copyright.ＰＥＮＡＮＡDhz2Ts1ApA
-----------http response for unchanged resource------139Please respect copyright.ＰＥＮＡＮＡm6nCfYlYw1
HTTP/1.1 412 Precondition Failed
(http body canbe empty, the client will used its cache)139Please respect copyright.ＰＥＮＡＮＡRmIw2iaIv2
----------------------------------------------------
139Please respect copyright.ＰＥＮＡＮＡQMchLPjnMf
If the etag changes, the server response 200 OK with the new file content like this:139Please respect copyright.ＰＥＮＡＮＡ0OQdzvDHrN
---------------http response with etag----------------139Please respect copyright.ＰＥＮＡＮＡqDXWse1tIg
HTTP/1.1 200 OK139Please respect copyright.ＰＥＮＡＮＡPYgt5mvmVC
Date: Mon, 25 Nov 2024 10:56:12 GMT139Please respect copyright.ＰＥＮＡＮＡ2TxCjgAtFn
accept-ranges: bytes139Please respect copyright.ＰＥＮＡＮＡi8UtLabpq5
ETag: "534231200_1732277116"139Please respect copyright.ＰＥＮＡＮＡMPRkMaBCCd
Content-Length: 534231200139Please respect copyright.ＰＥＮＡＮＡWyXlri6QtW
Content-Type: application/octet-stream139Please respect copyright.ＰＥＮＡＮＡGAD7n6pZ0L
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)139Please respect copyright.ＰＥＮＡＮＡhK7JMkewV7
-----------------------------------------------------
139Please respect copyright.ＰＥＮＡＮＡHDQ36JeTsK
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:139Please respect copyright.ＰＥＮＡＮＡ9bvvhJEnbO
------resume download request-------139Please respect copyright.ＰＥＮＡＮＡspbgj9X0WR
GET /zen.iso HTTP/1.1139Please respect copyright.ＰＥＮＡＮＡeP3q3zymeL
Range: bytes=287718144-139Please respect copyright.ＰＥＮＡＮＡjp1RGdljN1
If-Match: "534231200_1732277116"139Please respect copyright.ＰＥＮＡＮＡLrq1y8Lwz2
-----------------------------------139Please respect copyright.ＰＥＮＡＮＡExUUDvIEWU
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:139Please respect copyright.ＰＥＮＡＮＡbMyNWJfx6i
-------------range response-------------------139Please respect copyright.ＰＥＮＡＮＡWyQsojUKQA
HTTP/1.1 206 Partial Content139Please respect copyright.ＰＥＮＡＮＡkmS5flraaL
accept-ranges: bytes139Please respect copyright.ＰＥＮＡＮＡMoPNirEaET
content-range: bytes 287718144-534231199/534231200139Please respect copyright.ＰＥＮＡＮＡjPjrZV9YwY
ETag: "534231200_1732277116"139Please respect copyright.ＰＥＮＡＮＡ2RNdMvoKKa
Content-Length: 246513056139Please respect copyright.ＰＥＮＡＮＡoRzIBFRjr0
Content-Type: application/octet-stream139Please respect copyright.ＰＥＮＡＮＡPun87LhzVW
Content-Disposition: attachment; filename="zen.iso"
(http body is the remain file content)139Please respect copyright.ＰＥＮＡＮＡbzN3g80Pdv
-------------------------------------------
If the etag changes, response 412 like this:139Please respect copyright.ＰＥＮＡＮＡXQl9qfOjQA
-----------http response for changed resource------139Please respect copyright.ＰＥＮＡＮＡbsGROe330E
HTTP/1.1 412 Precondition Failed
(http body canbe empty, the client will stop downloading the resumed file)139Please respect copyright.ＰＥＮＡＮＡ23LJTABOeb
---------------------------------------------------
139Please respect copyright.ＰＥＮＡＮＡMwoNm9fM1W
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---139Please respect copyright.ＰＥＮＡＮＡXBAFQDtstr
GET /zen.iso HTTP/1.1139Please respect copyright.ＰＥＮＡＮＡW5VP6HfEhb
Range: bytes=287718144-139Please respect copyright.ＰＥＮＡＮＡjQZXko2vp9
If-Range: "534231200_1732277116"139Please respect copyright.ＰＥＮＡＮＡY7sERNg9uy
------------------------------------139Please respect copyright.ＰＥＮＡＮＡ5W0E8e3b9G
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:139Please respect copyright.ＰＥＮＡＮＡTELUSPwnQB
-------------range response-------------------139Please respect copyright.ＰＥＮＡＮＡESuKDE8aFA
HTTP/1.1 206 Partial Content139Please respect copyright.ＰＥＮＡＮＡn2LZXEFRau
accept-ranges: bytes139Please respect copyright.ＰＥＮＡＮＡNNtcfNHsXF
content-range: bytes 287718144-534231199/534231200139Please respect copyright.ＰＥＮＡＮＡsYsk7GxNIh
ETag: "534231200_1732277116"139Please respect copyright.ＰＥＮＡＮＡcKYFagAMvO
Content-Length: 246513056139Please respect copyright.ＰＥＮＡＮＡy8VC3iLjkP
Content-Type: application/octet-stream139Please respect copyright.ＰＥＮＡＮＡyFoKyBjEjx
Content-Disposition: attachment; filename="zen.iso"
(http body is the remain file content)139Please respect copyright.ＰＥＮＡＮＡF5WezzBbwd
-------------------------------------------
If the etag changes, response 200 from beginning like this:139Please respect copyright.ＰＥＮＡＮＡrapSHuWGRE
-----------http response from the beginning------139Please respect copyright.ＰＥＮＡＮＡa81vZ1JC9Z
HTTP/1.1 200 OK139Please respect copyright.ＰＥＮＡＮＡHtnLSN0Ly0
Date: Mon, 25 Nov 2024 10:56:12 GMT139Please respect copyright.ＰＥＮＡＮＡY7FEXvTP1O
accept-ranges: bytes139Please respect copyright.ＰＥＮＡＮＡzaVIDNGB2a
ETag: "534231200_1732277116"139Please respect copyright.ＰＥＮＡＮＡacblT4iqlV
Content-Length: 534231200139Please respect copyright.ＰＥＮＡＮＡVG3iqMjLFI
Content-Type: application/octet-stream139Please respect copyright.ＰＥＮＡＮＡ1WU5FIvZp6
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)139Please respect copyright.ＰＥＮＡＮＡWZxHfIHTOk
---------------------------------------------------
139Please respect copyright.ＰＥＮＡＮＡJFlfvIdYNQ
Section X. Thanks
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/If-None-Match139Please respect copyright.ＰＥＮＡＮＡk4rPkUl176
https://blog.csdn.net/catoop/article/details/134174653139Please respect copyright.ＰＥＮＡＮＡ6VvolRE8Mr
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]139Please respect copyright.ＰＥＮＡＮＡfnaXVn47JS
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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