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.
154Please respect copyright.ＰＥＮＡＮＡPMzTcxym6J
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:154Please respect copyright.ＰＥＮＡＮＡM140uSNXS0
---------------http response with etag----------------154Please respect copyright.ＰＥＮＡＮＡD007kFNuIc
HTTP/1.1 200 OK154Please respect copyright.ＰＥＮＡＮＡe1Fme4tOI0
Date: Mon, 25 Nov 2024 10:56:12 GMT154Please respect copyright.ＰＥＮＡＮＡK3RRoY6wKv
accept-ranges: bytes154Please respect copyright.ＰＥＮＡＮＡRvAHNmMSdn
ETag: "534231200_1732277116"154Please respect copyright.ＰＥＮＡＮＡKzFQfyOAV8
Content-Length: 534231200154Please respect copyright.ＰＥＮＡＮＡaAEHQ7EcGj
Content-Type: application/octet-stream154Please respect copyright.ＰＥＮＡＮＡ8jaAQfp627
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)154Please respect copyright.ＰＥＮＡＮＡKTPFmXetQp
-----------------------------------------------------
154Please respect copyright.ＰＥＮＡＮＡiAseAb6CUz
Section B. if-none-match
The browser send back the etag to ask server if it has been changed by header like this:154Please respect copyright.ＰＥＮＡＮＡuedW9V3ucq
-----http request for a cached resource------154Please respect copyright.ＰＥＮＡＮＡwHYXNK3Ahc
GET /zen.iso HTTP/1.1154Please respect copyright.ＰＥＮＡＮＡs9zB4ZmLZA
if-none-match: "534231200_1732277116"154Please respect copyright.ＰＥＮＡＮＡSEnP61qZ1b
---------------------------------------------
If the etag is unchanged, the server response like this:
1. for get and head requests:154Please respect copyright.ＰＥＮＡＮＡY6FNY2NdXN
-----------http response for unchanged resource------154Please respect copyright.ＰＥＮＡＮＡtGcA9YbAzk
HTTP/1.1 304 Not Modified
(http body canbe empty, the client will used its cache)154Please respect copyright.ＰＥＮＡＮＡ8lc7EP4kj3
----------------------------------------------------
2. for other methods requests:154Please respect copyright.ＰＥＮＡＮＡ7pV3QMb2ct
-----------http response for unchanged resource------154Please respect copyright.ＰＥＮＡＮＡKRhyDiCHUO
HTTP/1.1 412 Precondition Failed
(http body canbe empty, the client will used its cache)154Please respect copyright.ＰＥＮＡＮＡgPTid8cFU3
----------------------------------------------------
154Please respect copyright.ＰＥＮＡＮＡBvcbMigLlP
If the etag changes, the server response 200 OK with the new file content like this:154Please respect copyright.ＰＥＮＡＮＡxUacYqYzQH
---------------http response with etag----------------154Please respect copyright.ＰＥＮＡＮＡuruG6r70ig
HTTP/1.1 200 OK154Please respect copyright.ＰＥＮＡＮＡSeyR9pSfCh
Date: Mon, 25 Nov 2024 10:56:12 GMT154Please respect copyright.ＰＥＮＡＮＡoUkaEmIQeq
accept-ranges: bytes154Please respect copyright.ＰＥＮＡＮＡJ4BQoGF1Ks
ETag: "534231200_1732277116"154Please respect copyright.ＰＥＮＡＮＡ5wZh1L007i
Content-Length: 534231200154Please respect copyright.ＰＥＮＡＮＡP6xbMHv3Is
Content-Type: application/octet-stream154Please respect copyright.ＰＥＮＡＮＡTdJrfooTuq
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)154Please respect copyright.ＰＥＮＡＮＡX0FYbhZqSp
-----------------------------------------------------
154Please respect copyright.ＰＥＮＡＮＡNLeSYGTBOV
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:154Please respect copyright.ＰＥＮＡＮＡhiBzSHZxOg
------resume download request-------154Please respect copyright.ＰＥＮＡＮＡEdr2CNS5fE
GET /zen.iso HTTP/1.1154Please respect copyright.ＰＥＮＡＮＡAVL3jweEOe
Range: bytes=287718144-154Please respect copyright.ＰＥＮＡＮＡ2mipX14Xqh
If-Match: "534231200_1732277116"154Please respect copyright.ＰＥＮＡＮＡZQ9vYw9kwQ
-----------------------------------154Please respect copyright.ＰＥＮＡＮＡ3xIZ5hi9Wn
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:154Please respect copyright.ＰＥＮＡＮＡDqdwGieuSp
-------------range response-------------------154Please respect copyright.ＰＥＮＡＮＡ2T8sjKXzOb
HTTP/1.1 206 Partial Content154Please respect copyright.ＰＥＮＡＮＡ14m7ZpNCHW
accept-ranges: bytes154Please respect copyright.ＰＥＮＡＮＡDag1OIik2A
content-range: bytes 287718144-534231199/534231200154Please respect copyright.ＰＥＮＡＮＡg878lK3qRs
ETag: "534231200_1732277116"154Please respect copyright.ＰＥＮＡＮＡo76d1277eW
Content-Length: 246513056154Please respect copyright.ＰＥＮＡＮＡ1G6FiuOg5h
Content-Type: application/octet-stream154Please respect copyright.ＰＥＮＡＮＡPgXPFyHqqy
Content-Disposition: attachment; filename="zen.iso"
(http body is the remain file content)154Please respect copyright.ＰＥＮＡＮＡ56HO8YkQfI
-------------------------------------------
If the etag changes, response 412 like this:154Please respect copyright.ＰＥＮＡＮＡGGui5kMKbE
-----------http response for changed resource------154Please respect copyright.ＰＥＮＡＮＡcpEdfkerQS
HTTP/1.1 412 Precondition Failed
(http body canbe empty, the client will stop downloading the resumed file)154Please respect copyright.ＰＥＮＡＮＡ4y4RWYwm32
---------------------------------------------------
154Please respect copyright.ＰＥＮＡＮＡB7vhponXRl
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---154Please respect copyright.ＰＥＮＡＮＡ3LedHdUR5G
GET /zen.iso HTTP/1.1154Please respect copyright.ＰＥＮＡＮＡhymbbbFKJk
Range: bytes=287718144-154Please respect copyright.ＰＥＮＡＮＡWjY3Dya4Nv
If-Range: "534231200_1732277116"154Please respect copyright.ＰＥＮＡＮＡbUtkMvKhFK
------------------------------------154Please respect copyright.ＰＥＮＡＮＡiSneHMXFea
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:154Please respect copyright.ＰＥＮＡＮＡcyDggPcf41
-------------range response-------------------154Please respect copyright.ＰＥＮＡＮＡyT1B0814PV
HTTP/1.1 206 Partial Content154Please respect copyright.ＰＥＮＡＮＡr2sLnSFwDj
accept-ranges: bytes154Please respect copyright.ＰＥＮＡＮＡBggpxuIfsS
content-range: bytes 287718144-534231199/534231200154Please respect copyright.ＰＥＮＡＮＡoTEbMHbhm8
ETag: "534231200_1732277116"154Please respect copyright.ＰＥＮＡＮＡLvt8Hef6UV
Content-Length: 246513056154Please respect copyright.ＰＥＮＡＮＡAEUS6EukU0
Content-Type: application/octet-stream154Please respect copyright.ＰＥＮＡＮＡOkUEcJBNaD
Content-Disposition: attachment; filename="zen.iso"
(http body is the remain file content)154Please respect copyright.ＰＥＮＡＮＡr4UCpEMzMD
-------------------------------------------
If the etag changes, response 200 from beginning like this:154Please respect copyright.ＰＥＮＡＮＡ9EhhxMVD1P
-----------http response from the beginning------154Please respect copyright.ＰＥＮＡＮＡzRy7x7LG3n
HTTP/1.1 200 OK154Please respect copyright.ＰＥＮＡＮＡ93kSLaDeX2
Date: Mon, 25 Nov 2024 10:56:12 GMT154Please respect copyright.ＰＥＮＡＮＡcGLGqDzjk6
accept-ranges: bytes154Please respect copyright.ＰＥＮＡＮＡovE93nAQOv
ETag: "534231200_1732277116"154Please respect copyright.ＰＥＮＡＮＡHxXNGcqwpF
Content-Length: 534231200154Please respect copyright.ＰＥＮＡＮＡc7fKp2yDLd
Content-Type: application/octet-stream154Please respect copyright.ＰＥＮＡＮＡ01bEi93yf9
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)154Please respect copyright.ＰＥＮＡＮＡaVUANA0Gqg
---------------------------------------------------
154Please respect copyright.ＰＥＮＡＮＡpYOXw3iLnN
Section X. Thanks
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/If-None-Match154Please respect copyright.ＰＥＮＡＮＡkz36ada4vi
https://blog.csdn.net/catoop/article/details/134174653154Please respect copyright.ＰＥＮＡＮＡIFcVsGIMYJ
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]154Please respect copyright.ＰＥＮＡＮＡGZeqfDeSlZ
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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