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.
160Please respect copyright.ＰＥＮＡＮＡ3r6cRpzW7L
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:160Please respect copyright.ＰＥＮＡＮＡg33LtFzt3f
---------------http response with etag----------------160Please respect copyright.ＰＥＮＡＮＡehB0iFyaqG
HTTP/1.1 200 OK160Please respect copyright.ＰＥＮＡＮＡXAEUULFgNC
Date: Mon, 25 Nov 2024 10:56:12 GMT160Please respect copyright.ＰＥＮＡＮＡunN40wc8ww
accept-ranges: bytes160Please respect copyright.ＰＥＮＡＮＡPba4a8PCHR
ETag: "534231200_1732277116"160Please respect copyright.ＰＥＮＡＮＡxSI7H1ZAFt
Content-Length: 534231200160Please respect copyright.ＰＥＮＡＮＡu3hf8Df76m
Content-Type: application/octet-stream160Please respect copyright.ＰＥＮＡＮＡdmyEDGFelt
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)160Please respect copyright.ＰＥＮＡＮＡgykE3jPUPk
-----------------------------------------------------
160Please respect copyright.ＰＥＮＡＮＡ7jeuawHOak
Section B. if-none-match
The browser send back the etag to ask server if it has been changed by header like this:160Please respect copyright.ＰＥＮＡＮＡW8KwPicDcP
-----http request for a cached resource------160Please respect copyright.ＰＥＮＡＮＡwLeTFTb4Ow
GET /zen.iso HTTP/1.1160Please respect copyright.ＰＥＮＡＮＡpMXn7rwJhs
if-none-match: "534231200_1732277116"160Please respect copyright.ＰＥＮＡＮＡyacV0bWkMs
---------------------------------------------
If the etag is unchanged, the server response like this:
1. for get and head requests:160Please respect copyright.ＰＥＮＡＮＡfhnjrQm280
-----------http response for unchanged resource------160Please respect copyright.ＰＥＮＡＮＡUuphyUTBon
HTTP/1.1 304 Not Modified
(http body canbe empty, the client will used its cache)160Please respect copyright.ＰＥＮＡＮＡ87Q2wM8Wir
----------------------------------------------------
2. for other methods requests:160Please respect copyright.ＰＥＮＡＮＡlF98jb8G6d
-----------http response for unchanged resource------160Please respect copyright.ＰＥＮＡＮＡFrODX72Ypl
HTTP/1.1 412 Precondition Failed
(http body canbe empty, the client will used its cache)160Please respect copyright.ＰＥＮＡＮＡvlIJyKKNqV
----------------------------------------------------
160Please respect copyright.ＰＥＮＡＮＡOJVmQEjAXd
If the etag changes, the server response 200 OK with the new file content like this:160Please respect copyright.ＰＥＮＡＮＡwxaEDvYoYQ
---------------http response with etag----------------160Please respect copyright.ＰＥＮＡＮＡtqJvqjGr8X
HTTP/1.1 200 OK160Please respect copyright.ＰＥＮＡＮＡYWbIPr56A4
Date: Mon, 25 Nov 2024 10:56:12 GMT160Please respect copyright.ＰＥＮＡＮＡqQ3c4RgGeY
accept-ranges: bytes160Please respect copyright.ＰＥＮＡＮＡE5BwJzYunO
ETag: "534231200_1732277116"160Please respect copyright.ＰＥＮＡＮＡT3EnmRN3gF
Content-Length: 534231200160Please respect copyright.ＰＥＮＡＮＡKZBNa4xilM
Content-Type: application/octet-stream160Please respect copyright.ＰＥＮＡＮＡq49NGxXUUS
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)160Please respect copyright.ＰＥＮＡＮＡOcbILwTUok
-----------------------------------------------------
160Please respect copyright.ＰＥＮＡＮＡZECYQKK7wq
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:160Please respect copyright.ＰＥＮＡＮＡaP3o13nk4F
------resume download request-------160Please respect copyright.ＰＥＮＡＮＡJ6aD733G40
GET /zen.iso HTTP/1.1160Please respect copyright.ＰＥＮＡＮＡ3CcZ9qSwOR
Range: bytes=287718144-160Please respect copyright.ＰＥＮＡＮＡneIPZLaN9P
If-Match: "534231200_1732277116"160Please respect copyright.ＰＥＮＡＮＡD06kizBxPj
-----------------------------------160Please respect copyright.ＰＥＮＡＮＡCb94ldojmb
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:160Please respect copyright.ＰＥＮＡＮＡkQcVf5OaK6
-------------range response-------------------160Please respect copyright.ＰＥＮＡＮＡjuUsW4NoRi
HTTP/1.1 206 Partial Content160Please respect copyright.ＰＥＮＡＮＡNbDEcKjs40
accept-ranges: bytes160Please respect copyright.ＰＥＮＡＮＡASADlfAVsN
content-range: bytes 287718144-534231199/534231200160Please respect copyright.ＰＥＮＡＮＡbP8tqAXuIZ
ETag: "534231200_1732277116"160Please respect copyright.ＰＥＮＡＮＡmBur2dGOfM
Content-Length: 246513056160Please respect copyright.ＰＥＮＡＮＡs5vmIOjgYF
Content-Type: application/octet-stream160Please respect copyright.ＰＥＮＡＮＡMNowoKlFWW
Content-Disposition: attachment; filename="zen.iso"
(http body is the remain file content)160Please respect copyright.ＰＥＮＡＮＡ2oMcXuNpXS
-------------------------------------------
If the etag changes, response 412 like this:160Please respect copyright.ＰＥＮＡＮＡ1NJc4r26AO
-----------http response for changed resource------160Please respect copyright.ＰＥＮＡＮＡTyuRWmPVTd
HTTP/1.1 412 Precondition Failed
(http body canbe empty, the client will stop downloading the resumed file)160Please respect copyright.ＰＥＮＡＮＡfLWBTHd4TY
---------------------------------------------------
160Please respect copyright.ＰＥＮＡＮＡqWuNm2l793
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---160Please respect copyright.ＰＥＮＡＮＡUNRidO9ZqP
GET /zen.iso HTTP/1.1160Please respect copyright.ＰＥＮＡＮＡRNftrA1Z5k
Range: bytes=287718144-160Please respect copyright.ＰＥＮＡＮＡhdCJcf358z
If-Range: "534231200_1732277116"160Please respect copyright.ＰＥＮＡＮＡd2xc8lJLgw
------------------------------------160Please respect copyright.ＰＥＮＡＮＡ1IiJ7dYXV8
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:160Please respect copyright.ＰＥＮＡＮＡkgE1aVbtfO
-------------range response-------------------160Please respect copyright.ＰＥＮＡＮＡeQbk0dtFlT
HTTP/1.1 206 Partial Content160Please respect copyright.ＰＥＮＡＮＡ5RNWXHt7ho
accept-ranges: bytes160Please respect copyright.ＰＥＮＡＮＡhqPkH6ECRt
content-range: bytes 287718144-534231199/534231200160Please respect copyright.ＰＥＮＡＮＡFiBpd3Nvm6
ETag: "534231200_1732277116"160Please respect copyright.ＰＥＮＡＮＡb5AFvM5icq
Content-Length: 246513056160Please respect copyright.ＰＥＮＡＮＡPMUig5LzJX
Content-Type: application/octet-stream160Please respect copyright.ＰＥＮＡＮＡY0cSPKZ7Ft
Content-Disposition: attachment; filename="zen.iso"
(http body is the remain file content)160Please respect copyright.ＰＥＮＡＮＡ1iVQpmYK5v
-------------------------------------------
If the etag changes, response 200 from beginning like this:160Please respect copyright.ＰＥＮＡＮＡUwwtl6odoi
-----------http response from the beginning------160Please respect copyright.ＰＥＮＡＮＡ3U75HWeYwO
HTTP/1.1 200 OK160Please respect copyright.ＰＥＮＡＮＡk0ZtrYHTcJ
Date: Mon, 25 Nov 2024 10:56:12 GMT160Please respect copyright.ＰＥＮＡＮＡtsTKjlAPjV
accept-ranges: bytes160Please respect copyright.ＰＥＮＡＮＡrzFaQoLf2D
ETag: "534231200_1732277116"160Please respect copyright.ＰＥＮＡＮＡBNM5T5I0og
Content-Length: 534231200160Please respect copyright.ＰＥＮＡＮＡwwbPyqanfm
Content-Type: application/octet-stream160Please respect copyright.ＰＥＮＡＮＡKHlS2Gzm2G
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)160Please respect copyright.ＰＥＮＡＮＡsJVJjp00xx
---------------------------------------------------
160Please respect copyright.ＰＥＮＡＮＡ4SU221DqFW
Section X. Thanks
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/If-None-Match160Please respect copyright.ＰＥＮＡＮＡ7QAiSi953g
https://blog.csdn.net/catoop/article/details/134174653160Please respect copyright.ＰＥＮＡＮＡPS9dnJk5wi
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]160Please respect copyright.ＰＥＮＡＮＡPrQfWYvMV0
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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