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.
66Please respect copyright.ＰＥＮＡＮＡ5hqYf8o8TC
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:66Please respect copyright.ＰＥＮＡＮＡesSTidYQOc
---------------http response with etag----------------66Please respect copyright.ＰＥＮＡＮＡuVeG7zx300
HTTP/1.1 200 OK66Please respect copyright.ＰＥＮＡＮＡQRfPD13WVk
Date: Mon, 25 Nov 2024 10:56:12 GMT66Please respect copyright.ＰＥＮＡＮＡE4sR8k0gb3
accept-ranges: bytes66Please respect copyright.ＰＥＮＡＮＡGCeWPBejbL
ETag: "534231200_1732277116"66Please respect copyright.ＰＥＮＡＮＡgLfg5dI3Sf
Content-Length: 53423120066Please respect copyright.ＰＥＮＡＮＡx91aDSStSS
Content-Type: application/octet-stream66Please respect copyright.ＰＥＮＡＮＡOrWd02KK1l
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)66Please respect copyright.ＰＥＮＡＮＡFtZdcHCipm
-----------------------------------------------------
66Please respect copyright.ＰＥＮＡＮＡKbtZwRPq50
Section B. if-none-match
The browser send back the etag to ask server if it has been changed by header like this:66Please respect copyright.ＰＥＮＡＮＡAZMZOCxFN7
-----http request for a cached resource------66Please respect copyright.ＰＥＮＡＮＡcqCyAmf1MM
GET /zen.iso HTTP/1.166Please respect copyright.ＰＥＮＡＮＡoo7ixdfa2o
if-none-match: "534231200_1732277116"66Please respect copyright.ＰＥＮＡＮＡbJw4KPxjlK
---------------------------------------------
If the etag is unchanged, the server response like this:
1. for get and head requests:66Please respect copyright.ＰＥＮＡＮＡZLFkoAa4EH
-----------http response for unchanged resource------66Please respect copyright.ＰＥＮＡＮＡtClwlr3ZKv
HTTP/1.1 304 Not Modified
(http body canbe empty, the client will used its cache)66Please respect copyright.ＰＥＮＡＮＡjJXD2I0H1e
----------------------------------------------------
2. for other methods requests:66Please respect copyright.ＰＥＮＡＮＡRIU4v1IPH1
-----------http response for unchanged resource------66Please respect copyright.ＰＥＮＡＮＡT8UQbR4I4E
HTTP/1.1 412 Precondition Failed
(http body canbe empty, the client will used its cache)66Please respect copyright.ＰＥＮＡＮＡuiD5SYOv1T
----------------------------------------------------
66Please respect copyright.ＰＥＮＡＮＡoFvRLIaVrE
If the etag changes, the server response 200 OK with the new file content like this:66Please respect copyright.ＰＥＮＡＮＡ8ELzsIWjTF
---------------http response with etag----------------66Please respect copyright.ＰＥＮＡＮＡqpsn22QAaA
HTTP/1.1 200 OK66Please respect copyright.ＰＥＮＡＮＡeoV4sH98CI
Date: Mon, 25 Nov 2024 10:56:12 GMT66Please respect copyright.ＰＥＮＡＮＡD80hhsKf8c
accept-ranges: bytes66Please respect copyright.ＰＥＮＡＮＡF1nxYJxmW6
ETag: "534231200_1732277116"66Please respect copyright.ＰＥＮＡＮＡ6L2hz6CmMk
Content-Length: 53423120066Please respect copyright.ＰＥＮＡＮＡu8umUuh7jV
Content-Type: application/octet-stream66Please respect copyright.ＰＥＮＡＮＡHyg8qrQa1X
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)66Please respect copyright.ＰＥＮＡＮＡV44mxFyDir
-----------------------------------------------------
66Please respect copyright.ＰＥＮＡＮＡD9qiAbYABU
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:66Please respect copyright.ＰＥＮＡＮＡTOsRhP34u9
------resume download request-------66Please respect copyright.ＰＥＮＡＮＡPg8PbhkWPC
GET /zen.iso HTTP/1.166Please respect copyright.ＰＥＮＡＮＡw6nmRpBug2
Range: bytes=287718144-66Please respect copyright.ＰＥＮＡＮＡbFOnruQjbx
If-Match: "534231200_1732277116"66Please respect copyright.ＰＥＮＡＮＡFyqoiXOd0r
-----------------------------------66Please respect copyright.ＰＥＮＡＮＡVBwYcfszJP
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:66Please respect copyright.ＰＥＮＡＮＡWSbbGOrB9t
-------------range response-------------------66Please respect copyright.ＰＥＮＡＮＡnJt76L562t
HTTP/1.1 206 Partial Content66Please respect copyright.ＰＥＮＡＮＡ9G6H4AE63w
accept-ranges: bytes66Please respect copyright.ＰＥＮＡＮＡsST6RwzuTf
content-range: bytes 287718144-534231199/53423120066Please respect copyright.ＰＥＮＡＮＡy9SbtLrnzE
ETag: "534231200_1732277116"66Please respect copyright.ＰＥＮＡＮＡiwmXacKKga
Content-Length: 24651305666Please respect copyright.ＰＥＮＡＮＡQZrZlN77EL
Content-Type: application/octet-stream66Please respect copyright.ＰＥＮＡＮＡas8CXrQNxa
Content-Disposition: attachment; filename="zen.iso"
(http body is the remain file content)66Please respect copyright.ＰＥＮＡＮＡAPK3FD4BqS
-------------------------------------------
If the etag changes, response 412 like this:66Please respect copyright.ＰＥＮＡＮＡpiyPicA0Ip
-----------http response for changed resource------66Please respect copyright.ＰＥＮＡＮＡQJGtz1iCB8
HTTP/1.1 412 Precondition Failed
(http body canbe empty, the client will stop downloading the resumed file)66Please respect copyright.ＰＥＮＡＮＡ1vGw5wgK8v
---------------------------------------------------
66Please respect copyright.ＰＥＮＡＮＡsyNckYZU31
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---66Please respect copyright.ＰＥＮＡＮＡDkRIpa7pd2
GET /zen.iso HTTP/1.166Please respect copyright.ＰＥＮＡＮＡYSdaeLsEsp
Range: bytes=287718144-66Please respect copyright.ＰＥＮＡＮＡuIXwh9CzTJ
If-Range: "534231200_1732277116"66Please respect copyright.ＰＥＮＡＮＡ3OtdTMFJiQ
------------------------------------66Please respect copyright.ＰＥＮＡＮＡdDepbTSUrM
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:66Please respect copyright.ＰＥＮＡＮＡh4KReEALKo
-------------range response-------------------66Please respect copyright.ＰＥＮＡＮＡarEAPzTXik
HTTP/1.1 206 Partial Content66Please respect copyright.ＰＥＮＡＮＡ60S5r9NIa1
accept-ranges: bytes66Please respect copyright.ＰＥＮＡＮＡM8Vf5hqlER
content-range: bytes 287718144-534231199/53423120066Please respect copyright.ＰＥＮＡＮＡBXBaMglfqp
ETag: "534231200_1732277116"66Please respect copyright.ＰＥＮＡＮＡrvGTrnLWEz
Content-Length: 24651305666Please respect copyright.ＰＥＮＡＮＡbmncfzMvoK
Content-Type: application/octet-stream66Please respect copyright.ＰＥＮＡＮＡhBrtjGcb8k
Content-Disposition: attachment; filename="zen.iso"
(http body is the remain file content)66Please respect copyright.ＰＥＮＡＮＡ6IcQpaWRhW
-------------------------------------------
If the etag changes, response 200 from beginning like this:66Please respect copyright.ＰＥＮＡＮＡLd3XYdkDup
-----------http response from the beginning------66Please respect copyright.ＰＥＮＡＮＡDmrRHJtBIh
HTTP/1.1 200 OK66Please respect copyright.ＰＥＮＡＮＡMRFnShCjZ9
Date: Mon, 25 Nov 2024 10:56:12 GMT66Please respect copyright.ＰＥＮＡＮＡ2IYynhtzoL
accept-ranges: bytes66Please respect copyright.ＰＥＮＡＮＡosDqbXBGr7
ETag: "534231200_1732277116"66Please respect copyright.ＰＥＮＡＮＡV8lOd9dCcU
Content-Length: 53423120066Please respect copyright.ＰＥＮＡＮＡnmwa5SGDWJ
Content-Type: application/octet-stream66Please respect copyright.ＰＥＮＡＮＡoQzL3HxyJq
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)66Please respect copyright.ＰＥＮＡＮＡFiZ2t2yjqe
---------------------------------------------------
66Please respect copyright.ＰＥＮＡＮＡoL3pabvZs6
Section X. Thanks
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/If-None-Match66Please respect copyright.ＰＥＮＡＮＡZHs7vRELTe
https://blog.csdn.net/catoop/article/details/13417465366Please respect copyright.ＰＥＮＡＮＡrouLVEX9NF
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]66Please respect copyright.ＰＥＮＡＮＡs0zGXgGHAG
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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