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.
87Please respect copyright.ＰＥＮＡＮＡsXithExvC3
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:87Please respect copyright.ＰＥＮＡＮＡyo1GJZBOHU
---------------http response with etag----------------87Please respect copyright.ＰＥＮＡＮＡTDT3i2NaGW
HTTP/1.1 200 OK87Please respect copyright.ＰＥＮＡＮＡUxNRjVdcoz
Date: Mon, 25 Nov 2024 10:56:12 GMT87Please respect copyright.ＰＥＮＡＮＡm6DOFrGtYo
accept-ranges: bytes87Please respect copyright.ＰＥＮＡＮＡnTd7ft4s6i
ETag: "534231200_1732277116"87Please respect copyright.ＰＥＮＡＮＡwYzQzUx6uJ
Content-Length: 53423120087Please respect copyright.ＰＥＮＡＮＡps2AE2B5D0
Content-Type: application/octet-stream87Please respect copyright.ＰＥＮＡＮＡGo7gmWerNE
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)87Please respect copyright.ＰＥＮＡＮＡ7CkRz3wnkI
-----------------------------------------------------
87Please respect copyright.ＰＥＮＡＮＡEl2S7biElX
Section B. if-none-match
The browser send back the etag to ask server if it has been changed by header like this:87Please respect copyright.ＰＥＮＡＮＡ6t35rRdiby
-----http request for a cached resource------87Please respect copyright.ＰＥＮＡＮＡLUb409OxJ3
GET /zen.iso HTTP/1.187Please respect copyright.ＰＥＮＡＮＡz2EhQWC5vb
if-none-match: "534231200_1732277116"87Please respect copyright.ＰＥＮＡＮＡNX4JMmvgAy
---------------------------------------------
If the etag is unchanged, the server response like this:
1. for get and head requests:87Please respect copyright.ＰＥＮＡＮＡdaYNQgVnla
-----------http response for unchanged resource------87Please respect copyright.ＰＥＮＡＮＡM4IMpnukC5
HTTP/1.1 304 Not Modified
(http body canbe empty, the client will used its cache)87Please respect copyright.ＰＥＮＡＮＡc9fmhkfUnc
----------------------------------------------------
2. for other methods requests:87Please respect copyright.ＰＥＮＡＮＡLxD7ZHutrN
-----------http response for unchanged resource------87Please respect copyright.ＰＥＮＡＮＡaoZ90kd173
HTTP/1.1 412 Precondition Failed
(http body canbe empty, the client will used its cache)87Please respect copyright.ＰＥＮＡＮＡU0W3gxseVC
----------------------------------------------------
87Please respect copyright.ＰＥＮＡＮＡGWvu2hHAWa
If the etag changes, the server response 200 OK with the new file content like this:87Please respect copyright.ＰＥＮＡＮＡmty8CYFlHY
---------------http response with etag----------------87Please respect copyright.ＰＥＮＡＮＡEXd0lhStaX
HTTP/1.1 200 OK87Please respect copyright.ＰＥＮＡＮＡ6SSVezbtpN
Date: Mon, 25 Nov 2024 10:56:12 GMT87Please respect copyright.ＰＥＮＡＮＡAA90VNStU6
accept-ranges: bytes87Please respect copyright.ＰＥＮＡＮＡnxYU2uT9tR
ETag: "534231200_1732277116"87Please respect copyright.ＰＥＮＡＮＡFrD9adct1X
Content-Length: 53423120087Please respect copyright.ＰＥＮＡＮＡ7jInDNOdFD
Content-Type: application/octet-stream87Please respect copyright.ＰＥＮＡＮＡs21Rzptr2H
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)87Please respect copyright.ＰＥＮＡＮＡZBSXzEmn1M
-----------------------------------------------------
87Please respect copyright.ＰＥＮＡＮＡQrc5pTXbLn
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:87Please respect copyright.ＰＥＮＡＮＡaqKUfbzBSo
------resume download request-------87Please respect copyright.ＰＥＮＡＮＡntIFhx3AT3
GET /zen.iso HTTP/1.187Please respect copyright.ＰＥＮＡＮＡqiJIdkm09s
Range: bytes=287718144-87Please respect copyright.ＰＥＮＡＮＡE6kFlKOfpj
If-Match: "534231200_1732277116"87Please respect copyright.ＰＥＮＡＮＡAi0Zic4dG3
-----------------------------------87Please respect copyright.ＰＥＮＡＮＡnZj6ZRP0Qv
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:87Please respect copyright.ＰＥＮＡＮＡdnRDBSIcbm
-------------range response-------------------87Please respect copyright.ＰＥＮＡＮＡNuKj5SDbHV
HTTP/1.1 206 Partial Content87Please respect copyright.ＰＥＮＡＮＡVjCaBaYOea
accept-ranges: bytes87Please respect copyright.ＰＥＮＡＮＡJXdWNCZTOA
content-range: bytes 287718144-534231199/53423120087Please respect copyright.ＰＥＮＡＮＡ5b1kqxv0wg
ETag: "534231200_1732277116"87Please respect copyright.ＰＥＮＡＮＡ52APVnGw5V
Content-Length: 24651305687Please respect copyright.ＰＥＮＡＮＡGP9jtqJr6d
Content-Type: application/octet-stream87Please respect copyright.ＰＥＮＡＮＡv7HlqMpRr0
Content-Disposition: attachment; filename="zen.iso"
(http body is the remain file content)87Please respect copyright.ＰＥＮＡＮＡerMRaWiAtc
-------------------------------------------
If the etag changes, response 412 like this:87Please respect copyright.ＰＥＮＡＮＡQ2kV6hb9Jg
-----------http response for changed resource------87Please respect copyright.ＰＥＮＡＮＡiEPnZQLTc8
HTTP/1.1 412 Precondition Failed
(http body canbe empty, the client will stop downloading the resumed file)87Please respect copyright.ＰＥＮＡＮＡsh5Bv2ueff
---------------------------------------------------
87Please respect copyright.ＰＥＮＡＮＡTqeOLxUzkP
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---87Please respect copyright.ＰＥＮＡＮＡgSbFEnr0x5
GET /zen.iso HTTP/1.187Please respect copyright.ＰＥＮＡＮＡx3iKAV0weK
Range: bytes=287718144-87Please respect copyright.ＰＥＮＡＮＡCqeZt0Oeea
If-Range: "534231200_1732277116"87Please respect copyright.ＰＥＮＡＮＡmq5cv1Gvp0
------------------------------------87Please respect copyright.ＰＥＮＡＮＡ4qDObvzKvf
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:87Please respect copyright.ＰＥＮＡＮＡJ4DlwhbkXS
-------------range response-------------------87Please respect copyright.ＰＥＮＡＮＡEXJHgFBmri
HTTP/1.1 206 Partial Content87Please respect copyright.ＰＥＮＡＮＡoetKUrucST
accept-ranges: bytes87Please respect copyright.ＰＥＮＡＮＡB5b8slxr3g
content-range: bytes 287718144-534231199/53423120087Please respect copyright.ＰＥＮＡＮＡPqDdT6QlMV
ETag: "534231200_1732277116"87Please respect copyright.ＰＥＮＡＮＡVNzi14SKkp
Content-Length: 24651305687Please respect copyright.ＰＥＮＡＮＡ9KXnX5Oz17
Content-Type: application/octet-stream87Please respect copyright.ＰＥＮＡＮＡuXrGlogyhm
Content-Disposition: attachment; filename="zen.iso"
(http body is the remain file content)87Please respect copyright.ＰＥＮＡＮＡzB8P0n29Em
-------------------------------------------
If the etag changes, response 200 from beginning like this:87Please respect copyright.ＰＥＮＡＮＡQdDOJZf8Ii
-----------http response from the beginning------87Please respect copyright.ＰＥＮＡＮＡzMBBmL80h9
HTTP/1.1 200 OK87Please respect copyright.ＰＥＮＡＮＡbbxo3NBm67
Date: Mon, 25 Nov 2024 10:56:12 GMT87Please respect copyright.ＰＥＮＡＮＡbgZoH5Lsg8
accept-ranges: bytes87Please respect copyright.ＰＥＮＡＮＡCA3juPHLyF
ETag: "534231200_1732277116"87Please respect copyright.ＰＥＮＡＮＡ9WuFGQ1Vp9
Content-Length: 53423120087Please respect copyright.ＰＥＮＡＮＡfL2W58No6f
Content-Type: application/octet-stream87Please respect copyright.ＰＥＮＡＮＡuANaJdzbCu
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)87Please respect copyright.ＰＥＮＡＮＡiCesHDcOgO
---------------------------------------------------
87Please respect copyright.ＰＥＮＡＮＡqCw5GXqI6G
Section X. Thanks
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/If-None-Match87Please respect copyright.ＰＥＮＡＮＡpH3B62KikO
https://blog.csdn.net/catoop/article/details/13417465387Please respect copyright.ＰＥＮＡＮＡB0yKkMa3oR
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]87Please respect copyright.ＰＥＮＡＮＡ604zrfoiSu
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]
如果您對C/C++ Programming, Linux, HTTP Protocol, Website Development, Vue, Git, VsCode感興趣，邀請您加入「Linux/C/C++ Website Development」微信群，請加我的微信（si_jinmin）以便拉您进群。
ns3.128.153.31da2
然後按「加至主畫面」