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.
94Please respect copyright.ＰＥＮＡＮＡu7OexP54o9
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:94Please respect copyright.ＰＥＮＡＮＡwA9xkI84LZ
---------------http response with etag----------------94Please respect copyright.ＰＥＮＡＮＡCV3ok4WKba
HTTP/1.1 200 OK94Please respect copyright.ＰＥＮＡＮＡswSmUt5Xo9
Date: Mon, 25 Nov 2024 10:56:12 GMT94Please respect copyright.ＰＥＮＡＮＡ6NCjDOxslF
accept-ranges: bytes94Please respect copyright.ＰＥＮＡＮＡmGIjWYNmlY
ETag: "534231200_1732277116"94Please respect copyright.ＰＥＮＡＮＡvZ61YnhoI3
Content-Length: 53423120094Please respect copyright.ＰＥＮＡＮＡWTgR0sWh7M
Content-Type: application/octet-stream94Please respect copyright.ＰＥＮＡＮＡwDgb7FRRjt
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)94Please respect copyright.ＰＥＮＡＮＡINVC7B0JVF
-----------------------------------------------------
94Please respect copyright.ＰＥＮＡＮＡg9IgSXj5QZ
Section B. if-none-match
The browser send back the etag to ask server if it has been changed by header like this:94Please respect copyright.ＰＥＮＡＮＡOoKjcBRzaI
-----http request for a cached resource------94Please respect copyright.ＰＥＮＡＮＡ09r6g50Gw4
GET /zen.iso HTTP/1.194Please respect copyright.ＰＥＮＡＮＡpcbgtGD6Dv
if-none-match: "534231200_1732277116"94Please respect copyright.ＰＥＮＡＮＡo9qFH7dmVJ
---------------------------------------------
If the etag is unchanged, the server response like this:
1. for get and head requests:94Please respect copyright.ＰＥＮＡＮＡFR44krPwdl
-----------http response for unchanged resource------94Please respect copyright.ＰＥＮＡＮＡzPNsVzD1ER
HTTP/1.1 304 Not Modified
(http body canbe empty, the client will used its cache)94Please respect copyright.ＰＥＮＡＮＡBlbAxJkM0E
----------------------------------------------------
2. for other methods requests:94Please respect copyright.ＰＥＮＡＮＡyF5jM0fLwo
-----------http response for unchanged resource------94Please respect copyright.ＰＥＮＡＮＡ7rfhH5MGr7
HTTP/1.1 412 Precondition Failed
(http body canbe empty, the client will used its cache)94Please respect copyright.ＰＥＮＡＮＡRewmWsWlvy
----------------------------------------------------
94Please respect copyright.ＰＥＮＡＮＡh0JEnHsuCQ
If the etag changes, the server response 200 OK with the new file content like this:94Please respect copyright.ＰＥＮＡＮＡohffJAaBIf
---------------http response with etag----------------94Please respect copyright.ＰＥＮＡＮＡF39RNXb0cN
HTTP/1.1 200 OK94Please respect copyright.ＰＥＮＡＮＡkAhOnEtKbM
Date: Mon, 25 Nov 2024 10:56:12 GMT94Please respect copyright.ＰＥＮＡＮＡWlDmkFGLLC
accept-ranges: bytes94Please respect copyright.ＰＥＮＡＮＡHx4azvDeyL
ETag: "534231200_1732277116"94Please respect copyright.ＰＥＮＡＮＡ3bKGIyhA0d
Content-Length: 53423120094Please respect copyright.ＰＥＮＡＮＡjbSovCh0gm
Content-Type: application/octet-stream94Please respect copyright.ＰＥＮＡＮＡk94qI8q2rn
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)94Please respect copyright.ＰＥＮＡＮＡmalunmp3W1
-----------------------------------------------------
94Please respect copyright.ＰＥＮＡＮＡO0Jh4wCmm1
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:94Please respect copyright.ＰＥＮＡＮＡoM0DvifPeD
------resume download request-------94Please respect copyright.ＰＥＮＡＮＡheVLIQeB6b
GET /zen.iso HTTP/1.194Please respect copyright.ＰＥＮＡＮＡSNHxEOrNMn
Range: bytes=287718144-94Please respect copyright.ＰＥＮＡＮＡax6jVuc6FX
If-Match: "534231200_1732277116"94Please respect copyright.ＰＥＮＡＮＡLGu90CIuIf
-----------------------------------94Please respect copyright.ＰＥＮＡＮＡFOXYi3qThn
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:94Please respect copyright.ＰＥＮＡＮＡJaPZayCDsf
-------------range response-------------------94Please respect copyright.ＰＥＮＡＮＡ8jvpFMlx7Q
HTTP/1.1 206 Partial Content94Please respect copyright.ＰＥＮＡＮＡaQodM4P0qf
accept-ranges: bytes94Please respect copyright.ＰＥＮＡＮＡblusNR2ysS
content-range: bytes 287718144-534231199/53423120094Please respect copyright.ＰＥＮＡＮＡ8SPAhQGThf
ETag: "534231200_1732277116"94Please respect copyright.ＰＥＮＡＮＡoRN3uWqCsc
Content-Length: 24651305694Please respect copyright.ＰＥＮＡＮＡIhdyTKZFyK
Content-Type: application/octet-stream94Please respect copyright.ＰＥＮＡＮＡsYT0RUpaJn
Content-Disposition: attachment; filename="zen.iso"
(http body is the remain file content)94Please respect copyright.ＰＥＮＡＮＡIgHl2ID3Ne
-------------------------------------------
If the etag changes, response 412 like this:94Please respect copyright.ＰＥＮＡＮＡBTU3btw255
-----------http response for changed resource------94Please respect copyright.ＰＥＮＡＮＡwhsMBeACVc
HTTP/1.1 412 Precondition Failed
(http body canbe empty, the client will stop downloading the resumed file)94Please respect copyright.ＰＥＮＡＮＡegjha1XnCY
---------------------------------------------------
94Please respect copyright.ＰＥＮＡＮＡApfSuquxVM
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---94Please respect copyright.ＰＥＮＡＮＡh7gySjuGbz
GET /zen.iso HTTP/1.194Please respect copyright.ＰＥＮＡＮＡnp2XaY7r5j
Range: bytes=287718144-94Please respect copyright.ＰＥＮＡＮＡSumB7T0EBU
If-Range: "534231200_1732277116"94Please respect copyright.ＰＥＮＡＮＡKd67sLRtZh
------------------------------------94Please respect copyright.ＰＥＮＡＮＡYINoao1CYV
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:94Please respect copyright.ＰＥＮＡＮＡUyahK4415r
-------------range response-------------------94Please respect copyright.ＰＥＮＡＮＡJhlnVsMeS4
HTTP/1.1 206 Partial Content94Please respect copyright.ＰＥＮＡＮＡ7RqdpOvDEy
accept-ranges: bytes94Please respect copyright.ＰＥＮＡＮＡ2ieu48o5Te
content-range: bytes 287718144-534231199/53423120094Please respect copyright.ＰＥＮＡＮＡYzHmCQEjxr
ETag: "534231200_1732277116"94Please respect copyright.ＰＥＮＡＮＡShNxxLD4Cy
Content-Length: 24651305694Please respect copyright.ＰＥＮＡＮＡXeO4VkeLag
Content-Type: application/octet-stream94Please respect copyright.ＰＥＮＡＮＡDQev98CqqB
Content-Disposition: attachment; filename="zen.iso"
(http body is the remain file content)94Please respect copyright.ＰＥＮＡＮＡsQ7ypgd0LR
-------------------------------------------
If the etag changes, response 200 from beginning like this:94Please respect copyright.ＰＥＮＡＮＡYq0CN5AIpt
-----------http response from the beginning------94Please respect copyright.ＰＥＮＡＮＡDqmodCqXX5
HTTP/1.1 200 OK94Please respect copyright.ＰＥＮＡＮＡeBENET4lJt
Date: Mon, 25 Nov 2024 10:56:12 GMT94Please respect copyright.ＰＥＮＡＮＡbkaiPDvRxx
accept-ranges: bytes94Please respect copyright.ＰＥＮＡＮＡL26ttnYLiY
ETag: "534231200_1732277116"94Please respect copyright.ＰＥＮＡＮＡkgAXiX35Z7
Content-Length: 53423120094Please respect copyright.ＰＥＮＡＮＡe9tIz7QmrI
Content-Type: application/octet-stream94Please respect copyright.ＰＥＮＡＮＡCbp54dySSa
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)94Please respect copyright.ＰＥＮＡＮＡ09f19NXNsY
---------------------------------------------------
94Please respect copyright.ＰＥＮＡＮＡu3Yvw85rPZ
Section X. Thanks
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/If-None-Match94Please respect copyright.ＰＥＮＡＮＡtlxVEqaFGc
https://blog.csdn.net/catoop/article/details/13417465394Please respect copyright.ＰＥＮＡＮＡrMBYQbtON7
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]94Please respect copyright.ＰＥＮＡＮＡBaRa9jem4V
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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