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.
178Please respect copyright.ＰＥＮＡＮＡCSjdLiM7s2
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:178Please respect copyright.ＰＥＮＡＮＡ90PluQg0yv
---------------http response with etag----------------178Please respect copyright.ＰＥＮＡＮＡJxyQH6rf1p
HTTP/1.1 200 OK178Please respect copyright.ＰＥＮＡＮＡIgvdyocMsC
Date: Mon, 25 Nov 2024 10:56:12 GMT178Please respect copyright.ＰＥＮＡＮＡfoMIWSYOTG
accept-ranges: bytes178Please respect copyright.ＰＥＮＡＮＡoEnH0Kak2S
ETag: "534231200_1732277116"178Please respect copyright.ＰＥＮＡＮＡoSSsDGUXP2
Content-Length: 534231200178Please respect copyright.ＰＥＮＡＮＡZ9Td6yWs8T
Content-Type: application/octet-stream178Please respect copyright.ＰＥＮＡＮＡrpPFyglw2o
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)178Please respect copyright.ＰＥＮＡＮＡ9voiZFOYik
-----------------------------------------------------
178Please respect copyright.ＰＥＮＡＮＡHaXS13VBvY
Section B. if-none-match
The browser send back the etag to ask server if it has been changed by header like this:178Please respect copyright.ＰＥＮＡＮＡtL2zUjiCfU
-----http request for a cached resource------178Please respect copyright.ＰＥＮＡＮＡLsTlYUrq9t
GET /zen.iso HTTP/1.1178Please respect copyright.ＰＥＮＡＮＡfHxGqbGcZ8
if-none-match: "534231200_1732277116"178Please respect copyright.ＰＥＮＡＮＡiuAHiZj0Dx
---------------------------------------------
If the etag is unchanged, the server response like this:
1. for get and head requests:178Please respect copyright.ＰＥＮＡＮＡUMW4DnlJLO
-----------http response for unchanged resource------178Please respect copyright.ＰＥＮＡＮＡtWgHKlaJrt
HTTP/1.1 304 Not Modified
(http body canbe empty, the client will used its cache)178Please respect copyright.ＰＥＮＡＮＡRtWYAZpfeU
----------------------------------------------------
2. for other methods requests:178Please respect copyright.ＰＥＮＡＮＡa4LtoHlkVU
-----------http response for unchanged resource------178Please respect copyright.ＰＥＮＡＮＡW8xAlU31ln
HTTP/1.1 412 Precondition Failed
(http body canbe empty, the client will used its cache)178Please respect copyright.ＰＥＮＡＮＡ7Hmj6fwn7d
----------------------------------------------------
178Please respect copyright.ＰＥＮＡＮＡ845k1bDJe6
If the etag changes, the server response 200 OK with the new file content like this:178Please respect copyright.ＰＥＮＡＮＡLcy51mgrNH
---------------http response with etag----------------178Please respect copyright.ＰＥＮＡＮＡaNxaOsq51H
HTTP/1.1 200 OK178Please respect copyright.ＰＥＮＡＮＡ4hMTp4rwtV
Date: Mon, 25 Nov 2024 10:56:12 GMT178Please respect copyright.ＰＥＮＡＮＡ0Kk4XHBjkx
accept-ranges: bytes178Please respect copyright.ＰＥＮＡＮＡETAwli88mY
ETag: "534231200_1732277116"178Please respect copyright.ＰＥＮＡＮＡRXXy4lS6UP
Content-Length: 534231200178Please respect copyright.ＰＥＮＡＮＡBrekUiDOXR
Content-Type: application/octet-stream178Please respect copyright.ＰＥＮＡＮＡIpApfYXf5X
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)178Please respect copyright.ＰＥＮＡＮＡdFtnby0x8n
-----------------------------------------------------
178Please respect copyright.ＰＥＮＡＮＡCbGPnuNs0v
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:178Please respect copyright.ＰＥＮＡＮＡx4FbuYPvn9
------resume download request-------178Please respect copyright.ＰＥＮＡＮＡx0qF6u4FkV
GET /zen.iso HTTP/1.1178Please respect copyright.ＰＥＮＡＮＡWIrAQXNMdx
Range: bytes=287718144-178Please respect copyright.ＰＥＮＡＮＡIVCHAnkAo1
If-Match: "534231200_1732277116"178Please respect copyright.ＰＥＮＡＮＡ749KeFv1OL
-----------------------------------178Please respect copyright.ＰＥＮＡＮＡlbOpn6ogVi
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:178Please respect copyright.ＰＥＮＡＮＡmiMXrOe9lN
-------------range response-------------------178Please respect copyright.ＰＥＮＡＮＡ0dMCRVFGSg
HTTP/1.1 206 Partial Content178Please respect copyright.ＰＥＮＡＮＡhbrYJizUZH
accept-ranges: bytes178Please respect copyright.ＰＥＮＡＮＡS8GygRMlqm
content-range: bytes 287718144-534231199/534231200178Please respect copyright.ＰＥＮＡＮＡReccU31bII
ETag: "534231200_1732277116"178Please respect copyright.ＰＥＮＡＮＡlQ1OsoAy9i
Content-Length: 246513056178Please respect copyright.ＰＥＮＡＮＡl5iSyNbZAo
Content-Type: application/octet-stream178Please respect copyright.ＰＥＮＡＮＡgCVMwMWb5b
Content-Disposition: attachment; filename="zen.iso"
(http body is the remain file content)178Please respect copyright.ＰＥＮＡＮＡ1341xIhPua
-------------------------------------------
If the etag changes, response 412 like this:178Please respect copyright.ＰＥＮＡＮＡU6ahwHI54x
-----------http response for changed resource------178Please respect copyright.ＰＥＮＡＮＡm39ltwlTld
HTTP/1.1 412 Precondition Failed
(http body canbe empty, the client will stop downloading the resumed file)178Please respect copyright.ＰＥＮＡＮＡdROU4fF2DV
---------------------------------------------------
178Please respect copyright.ＰＥＮＡＮＡ13fSCADZp9
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---178Please respect copyright.ＰＥＮＡＮＡo4DP8dJvyp
GET /zen.iso HTTP/1.1178Please respect copyright.ＰＥＮＡＮＡ2L1oXwi4gj
Range: bytes=287718144-178Please respect copyright.ＰＥＮＡＮＡGjZ5xQkuAg
If-Range: "534231200_1732277116"178Please respect copyright.ＰＥＮＡＮＡeZr3Th5QGL
------------------------------------178Please respect copyright.ＰＥＮＡＮＡVNe4ht6a0M
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:178Please respect copyright.ＰＥＮＡＮＡE9BCvDQ9g8
-------------range response-------------------178Please respect copyright.ＰＥＮＡＮＡhd32TekpF3
HTTP/1.1 206 Partial Content178Please respect copyright.ＰＥＮＡＮＡanDJsFLNCS
accept-ranges: bytes178Please respect copyright.ＰＥＮＡＮＡIodCuzLbEl
content-range: bytes 287718144-534231199/534231200178Please respect copyright.ＰＥＮＡＮＡtMgL0APgxd
ETag: "534231200_1732277116"178Please respect copyright.ＰＥＮＡＮＡtk6vxcfxw9
Content-Length: 246513056178Please respect copyright.ＰＥＮＡＮＡZXNCylDaXE
Content-Type: application/octet-stream178Please respect copyright.ＰＥＮＡＮＡFOuCMp7lI6
Content-Disposition: attachment; filename="zen.iso"
(http body is the remain file content)178Please respect copyright.ＰＥＮＡＮＡaLapfhhOmB
-------------------------------------------
If the etag changes, response 200 from beginning like this:178Please respect copyright.ＰＥＮＡＮＡcpOvQhVRit
-----------http response from the beginning------178Please respect copyright.ＰＥＮＡＮＡ76qEtOg3mu
HTTP/1.1 200 OK178Please respect copyright.ＰＥＮＡＮＡvX27WdHsTY
Date: Mon, 25 Nov 2024 10:56:12 GMT178Please respect copyright.ＰＥＮＡＮＡnWLHX2GuG5
accept-ranges: bytes178Please respect copyright.ＰＥＮＡＮＡlaGlt6kzYa
ETag: "534231200_1732277116"178Please respect copyright.ＰＥＮＡＮＡYx3Z43h1cK
Content-Length: 534231200178Please respect copyright.ＰＥＮＡＮＡnidH0flaPk
Content-Type: application/octet-stream178Please respect copyright.ＰＥＮＡＮＡqvKiy8Pztb
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)178Please respect copyright.ＰＥＮＡＮＡBOHakZkRrn
---------------------------------------------------
178Please respect copyright.ＰＥＮＡＮＡrUoYxqmkTd
Section X. Thanks
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/If-None-Match178Please respect copyright.ＰＥＮＡＮＡdNyLvGDsj9
https://blog.csdn.net/catoop/article/details/134174653178Please respect copyright.ＰＥＮＡＮＡxvnQmhImLP
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]178Please respect copyright.ＰＥＮＡＮＡ5Zxvb6PDbK
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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