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.
64Please respect copyright.ＰＥＮＡＮＡIh3dul7l28
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:64Please respect copyright.ＰＥＮＡＮＡQCDydBpbgS
---------------http response with etag----------------64Please respect copyright.ＰＥＮＡＮＡgXFWwwPCgo
HTTP/1.1 200 OK64Please respect copyright.ＰＥＮＡＮＡdvM8d6ELka
Date: Mon, 25 Nov 2024 10:56:12 GMT64Please respect copyright.ＰＥＮＡＮＡ6j75uTzKB3
accept-ranges: bytes64Please respect copyright.ＰＥＮＡＮＡuUROkCjinj
ETag: "534231200_1732277116"64Please respect copyright.ＰＥＮＡＮＡD8fwz1PDwF
Content-Length: 53423120064Please respect copyright.ＰＥＮＡＮＡJh8ks8GXpS
Content-Type: application/octet-stream64Please respect copyright.ＰＥＮＡＮＡWGb0wO4HDC
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)64Please respect copyright.ＰＥＮＡＮＡWMSMfb42jm
-----------------------------------------------------
64Please respect copyright.ＰＥＮＡＮＡylqPCAlLdF
Section B. if-none-match
The browser send back the etag to ask server if it has been changed by header like this:64Please respect copyright.ＰＥＮＡＮＡAnxfar4inr
-----http request for a cached resource------64Please respect copyright.ＰＥＮＡＮＡhvWmwRu2L3
GET /zen.iso HTTP/1.164Please respect copyright.ＰＥＮＡＮＡfkxbheU53n
if-none-match: "534231200_1732277116"64Please respect copyright.ＰＥＮＡＮＡlq1gmhs9ul
---------------------------------------------
If the etag is unchanged, the server response like this:
1. for get and head requests:64Please respect copyright.ＰＥＮＡＮＡReSPrbNtiZ
-----------http response for unchanged resource------64Please respect copyright.ＰＥＮＡＮＡchV8wZANST
HTTP/1.1 304 Not Modified
(http body canbe empty, the client will used its cache)64Please respect copyright.ＰＥＮＡＮＡMhZUY5k9wx
----------------------------------------------------
2. for other methods requests:64Please respect copyright.ＰＥＮＡＮＡCKZB8HBceF
-----------http response for unchanged resource------64Please respect copyright.ＰＥＮＡＮＡuJs7xjuQiZ
HTTP/1.1 412 Precondition Failed
(http body canbe empty, the client will used its cache)64Please respect copyright.ＰＥＮＡＮＡPfeyLdh9SO
----------------------------------------------------
64Please respect copyright.ＰＥＮＡＮＡeaqBX458wV
If the etag changes, the server response 200 OK with the new file content like this:64Please respect copyright.ＰＥＮＡＮＡ0OdyH1Hho5
---------------http response with etag----------------64Please respect copyright.ＰＥＮＡＮＡekbdHltBeM
HTTP/1.1 200 OK64Please respect copyright.ＰＥＮＡＮＡU0Hq6UPTZD
Date: Mon, 25 Nov 2024 10:56:12 GMT64Please respect copyright.ＰＥＮＡＮＡm5mh23xZzd
accept-ranges: bytes64Please respect copyright.ＰＥＮＡＮＡngpL2pCluU
ETag: "534231200_1732277116"64Please respect copyright.ＰＥＮＡＮＡqEOEFOtAIT
Content-Length: 53423120064Please respect copyright.ＰＥＮＡＮＡbPH32JoPeS
Content-Type: application/octet-stream64Please respect copyright.ＰＥＮＡＮＡFLPKDywRIW
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)64Please respect copyright.ＰＥＮＡＮＡduaEWbpDZ1
-----------------------------------------------------
64Please respect copyright.ＰＥＮＡＮＡkHXbaZvcCz
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:64Please respect copyright.ＰＥＮＡＮＡeMoq8Z4nPl
------resume download request-------64Please respect copyright.ＰＥＮＡＮＡ1RY2dcyUST
GET /zen.iso HTTP/1.164Please respect copyright.ＰＥＮＡＮＡyL4URVcvAf
Range: bytes=287718144-64Please respect copyright.ＰＥＮＡＮＡXteSsTKecg
If-Match: "534231200_1732277116"64Please respect copyright.ＰＥＮＡＮＡ4D2INa9ZmH
-----------------------------------64Please respect copyright.ＰＥＮＡＮＡSrNUcmkmcH
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:64Please respect copyright.ＰＥＮＡＮＡ53pRogIbNX
-------------range response-------------------64Please respect copyright.ＰＥＮＡＮＡEgHEHNlJ3P
HTTP/1.1 206 Partial Content64Please respect copyright.ＰＥＮＡＮＡXYi4MXSXYX
accept-ranges: bytes64Please respect copyright.ＰＥＮＡＮＡ87GXEv3317
content-range: bytes 287718144-534231199/53423120064Please respect copyright.ＰＥＮＡＮＡqY1GBXwblG
ETag: "534231200_1732277116"64Please respect copyright.ＰＥＮＡＮＡxCR4hmG9Ah
Content-Length: 24651305664Please respect copyright.ＰＥＮＡＮＡLmTn7uQDZM
Content-Type: application/octet-stream64Please respect copyright.ＰＥＮＡＮＡzN0AvRjlKC
Content-Disposition: attachment; filename="zen.iso"
(http body is the remain file content)64Please respect copyright.ＰＥＮＡＮＡP2uDxbrA1b
-------------------------------------------
If the etag changes, response 412 like this:64Please respect copyright.ＰＥＮＡＮＡrf9Q7VyJ01
-----------http response for changed resource------64Please respect copyright.ＰＥＮＡＮＡ8GLpu3iJnQ
HTTP/1.1 412 Precondition Failed
(http body canbe empty, the client will stop downloading the resumed file)64Please respect copyright.ＰＥＮＡＮＡbVErczyixH
---------------------------------------------------
64Please respect copyright.ＰＥＮＡＮＡDmq1UW0ex5
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---64Please respect copyright.ＰＥＮＡＮＡvp1T0afahA
GET /zen.iso HTTP/1.164Please respect copyright.ＰＥＮＡＮＡCgLWxVShAV
Range: bytes=287718144-64Please respect copyright.ＰＥＮＡＮＡWegK6LQ18w
If-Range: "534231200_1732277116"64Please respect copyright.ＰＥＮＡＮＡn6z8SskrEu
------------------------------------64Please respect copyright.ＰＥＮＡＮＡOtAXrkd77g
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:64Please respect copyright.ＰＥＮＡＮＡ0NkxoDg2yt
-------------range response-------------------64Please respect copyright.ＰＥＮＡＮＡGlmQv6EJSP
HTTP/1.1 206 Partial Content64Please respect copyright.ＰＥＮＡＮＡXPkwbAkpCF
accept-ranges: bytes64Please respect copyright.ＰＥＮＡＮＡKGworphxra
content-range: bytes 287718144-534231199/53423120064Please respect copyright.ＰＥＮＡＮＡ3o9rXSxfen
ETag: "534231200_1732277116"64Please respect copyright.ＰＥＮＡＮＡW3ZLOe7by6
Content-Length: 24651305664Please respect copyright.ＰＥＮＡＮＡsx2R9G7cZE
Content-Type: application/octet-stream64Please respect copyright.ＰＥＮＡＮＡlvhmRUYuHE
Content-Disposition: attachment; filename="zen.iso"
(http body is the remain file content)64Please respect copyright.ＰＥＮＡＮＡcBMoatqqja
-------------------------------------------
If the etag changes, response 200 from beginning like this:64Please respect copyright.ＰＥＮＡＮＡjW4vAbZTVw
-----------http response from the beginning------64Please respect copyright.ＰＥＮＡＮＡzSAxufIrni
HTTP/1.1 200 OK64Please respect copyright.ＰＥＮＡＮＡkg4JhdtN29
Date: Mon, 25 Nov 2024 10:56:12 GMT64Please respect copyright.ＰＥＮＡＮＡIlt5AAxkFr
accept-ranges: bytes64Please respect copyright.ＰＥＮＡＮＡDFG5i6qg1G
ETag: "534231200_1732277116"64Please respect copyright.ＰＥＮＡＮＡ0hp1FYE1ZF
Content-Length: 53423120064Please respect copyright.ＰＥＮＡＮＡAX3j09vroH
Content-Type: application/octet-stream64Please respect copyright.ＰＥＮＡＮＡhq7EiklzT0
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)64Please respect copyright.ＰＥＮＡＮＡ2e5982Dz0O
---------------------------------------------------
64Please respect copyright.ＰＥＮＡＮＡbAfc20GhaD
Section X. Thanks
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/If-None-Match64Please respect copyright.ＰＥＮＡＮＡYEViIyQIUa
https://blog.csdn.net/catoop/article/details/13417465364Please respect copyright.ＰＥＮＡＮＡXiDAPiRRqV
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.com64Please respect copyright.ＰＥＮＡＮＡcjMIFfz2pp
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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