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.
157Please respect copyright.ＰＥＮＡＮＡelDy2EgvT1
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:157Please respect copyright.ＰＥＮＡＮＡf7fR5ywyE6
---------------http response with etag----------------157Please respect copyright.ＰＥＮＡＮＡi8SmEaJy0O
HTTP/1.1 200 OK157Please respect copyright.ＰＥＮＡＮＡ8Cd9QGbETt
Date: Mon, 25 Nov 2024 10:56:12 GMT157Please respect copyright.ＰＥＮＡＮＡvXxfSUdRtV
accept-ranges: bytes157Please respect copyright.ＰＥＮＡＮＡXSRMQXgBKC
ETag: "534231200_1732277116"157Please respect copyright.ＰＥＮＡＮＡVuMoTRvRn1
Content-Length: 534231200157Please respect copyright.ＰＥＮＡＮＡOuU9rmj4D5
Content-Type: application/octet-stream157Please respect copyright.ＰＥＮＡＮＡ2HzZroqIhv
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)157Please respect copyright.ＰＥＮＡＮＡoRQuEURlVV
-----------------------------------------------------
157Please respect copyright.ＰＥＮＡＮＡIjJkdZDFOv
Section B. if-none-match
The browser send back the etag to ask server if it has been changed by header like this:157Please respect copyright.ＰＥＮＡＮＡiOzcThZ0PH
-----http request for a cached resource------157Please respect copyright.ＰＥＮＡＮＡNxwoVkPkML
GET /zen.iso HTTP/1.1157Please respect copyright.ＰＥＮＡＮＡ5s4ZZ23dxR
if-none-match: "534231200_1732277116"157Please respect copyright.ＰＥＮＡＮＡU30stlcZDh
---------------------------------------------
If the etag is unchanged, the server response like this:
1. for get and head requests:157Please respect copyright.ＰＥＮＡＮＡGYrLQ67BS4
-----------http response for unchanged resource------157Please respect copyright.ＰＥＮＡＮＡkSYh1ZmD7g
HTTP/1.1 304 Not Modified
(http body canbe empty, the client will used its cache)157Please respect copyright.ＰＥＮＡＮＡAbomunXLhp
----------------------------------------------------
2. for other methods requests:157Please respect copyright.ＰＥＮＡＮＡQdWnBffJTt
-----------http response for unchanged resource------157Please respect copyright.ＰＥＮＡＮＡpPsQEk5SX9
HTTP/1.1 412 Precondition Failed
(http body canbe empty, the client will used its cache)157Please respect copyright.ＰＥＮＡＮＡEsSN3yAvmR
----------------------------------------------------
157Please respect copyright.ＰＥＮＡＮＡuB250tbZs6
If the etag changes, the server response 200 OK with the new file content like this:157Please respect copyright.ＰＥＮＡＮＡMJVpVEvfWb
---------------http response with etag----------------157Please respect copyright.ＰＥＮＡＮＡ97o94Ydo78
HTTP/1.1 200 OK157Please respect copyright.ＰＥＮＡＮＡGhBpuyt9Y2
Date: Mon, 25 Nov 2024 10:56:12 GMT157Please respect copyright.ＰＥＮＡＮＡ1hr62QnZTb
accept-ranges: bytes157Please respect copyright.ＰＥＮＡＮＡlXLbiQXHyS
ETag: "534231200_1732277116"157Please respect copyright.ＰＥＮＡＮＡsSTHeLKpZB
Content-Length: 534231200157Please respect copyright.ＰＥＮＡＮＡTXtyeSxeh8
Content-Type: application/octet-stream157Please respect copyright.ＰＥＮＡＮＡpQmGpQoRyA
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)157Please respect copyright.ＰＥＮＡＮＡAPHC5CwxVc
-----------------------------------------------------
157Please respect copyright.ＰＥＮＡＮＡ3u4vQ2iHqS
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:157Please respect copyright.ＰＥＮＡＮＡKee52AVydK
------resume download request-------157Please respect copyright.ＰＥＮＡＮＡjVZLvCbFmi
GET /zen.iso HTTP/1.1157Please respect copyright.ＰＥＮＡＮＡdjm6wzQLXB
Range: bytes=287718144-157Please respect copyright.ＰＥＮＡＮＡRBZJl9YFmS
If-Match: "534231200_1732277116"157Please respect copyright.ＰＥＮＡＮＡvJU5jZ5ZHX
-----------------------------------157Please respect copyright.ＰＥＮＡＮＡVWfH5QQq51
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:157Please respect copyright.ＰＥＮＡＮＡf8dZ7cbdVH
-------------range response-------------------157Please respect copyright.ＰＥＮＡＮＡ7jgL1nFriy
HTTP/1.1 206 Partial Content157Please respect copyright.ＰＥＮＡＮＡ0j089mgl7j
accept-ranges: bytes157Please respect copyright.ＰＥＮＡＮＡ9tvwztDECa
content-range: bytes 287718144-534231199/534231200157Please respect copyright.ＰＥＮＡＮＡlScfgchzQQ
ETag: "534231200_1732277116"157Please respect copyright.ＰＥＮＡＮＡQE4kuYzRT5
Content-Length: 246513056157Please respect copyright.ＰＥＮＡＮＡ0mV1rHfrSS
Content-Type: application/octet-stream157Please respect copyright.ＰＥＮＡＮＡN7sukpUBDR
Content-Disposition: attachment; filename="zen.iso"
(http body is the remain file content)157Please respect copyright.ＰＥＮＡＮＡIXOzApRiIQ
-------------------------------------------
If the etag changes, response 412 like this:157Please respect copyright.ＰＥＮＡＮＡuuqJ0k1g9F
-----------http response for changed resource------157Please respect copyright.ＰＥＮＡＮＡhF3leAS8EY
HTTP/1.1 412 Precondition Failed
(http body canbe empty, the client will stop downloading the resumed file)157Please respect copyright.ＰＥＮＡＮＡ5wJCscCKRF
---------------------------------------------------
157Please respect copyright.ＰＥＮＡＮＡitnTO2DZR8
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---157Please respect copyright.ＰＥＮＡＮＡux7Zdoqv5Z
GET /zen.iso HTTP/1.1157Please respect copyright.ＰＥＮＡＮＡY8hN5DO5Gg
Range: bytes=287718144-157Please respect copyright.ＰＥＮＡＮＡna8uQo5OJk
If-Range: "534231200_1732277116"157Please respect copyright.ＰＥＮＡＮＡtTR4b2go0O
------------------------------------157Please respect copyright.ＰＥＮＡＮＡPyoH6iP286
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:157Please respect copyright.ＰＥＮＡＮＡXwqiw8zXwe
-------------range response-------------------157Please respect copyright.ＰＥＮＡＮＡtopumUE9YA
HTTP/1.1 206 Partial Content157Please respect copyright.ＰＥＮＡＮＡ1WV9jxjPbt
accept-ranges: bytes157Please respect copyright.ＰＥＮＡＮＡsk3kYt2oJr
content-range: bytes 287718144-534231199/534231200157Please respect copyright.ＰＥＮＡＮＡhHBJyZ2eyJ
ETag: "534231200_1732277116"157Please respect copyright.ＰＥＮＡＮＡeEHac9t3K8
Content-Length: 246513056157Please respect copyright.ＰＥＮＡＮＡmtnK1gDwnw
Content-Type: application/octet-stream157Please respect copyright.ＰＥＮＡＮＡKMUt276DsM
Content-Disposition: attachment; filename="zen.iso"
(http body is the remain file content)157Please respect copyright.ＰＥＮＡＮＡfhlogBzCvZ
-------------------------------------------
If the etag changes, response 200 from beginning like this:157Please respect copyright.ＰＥＮＡＮＡwstM1JKNIS
-----------http response from the beginning------157Please respect copyright.ＰＥＮＡＮＡ8kVPXVr4P0
HTTP/1.1 200 OK157Please respect copyright.ＰＥＮＡＮＡ9jagHeb4Tf
Date: Mon, 25 Nov 2024 10:56:12 GMT157Please respect copyright.ＰＥＮＡＮＡhwOdI7OEDm
accept-ranges: bytes157Please respect copyright.ＰＥＮＡＮＡR4bFmpM43S
ETag: "534231200_1732277116"157Please respect copyright.ＰＥＮＡＮＡLhmQqS7dxS
Content-Length: 534231200157Please respect copyright.ＰＥＮＡＮＡtckzpTy6Jy
Content-Type: application/octet-stream157Please respect copyright.ＰＥＮＡＮＡyT95ovkCIO
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)157Please respect copyright.ＰＥＮＡＮＡLtIHb213HN
---------------------------------------------------
157Please respect copyright.ＰＥＮＡＮＡJShGYZBMoN
Section X. Thanks
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/If-None-Match157Please respect copyright.ＰＥＮＡＮＡz0qwLXhAiC
https://blog.csdn.net/catoop/article/details/134174653157Please respect copyright.ＰＥＮＡＮＡ5G17T2zIpe
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]157Please respect copyright.ＰＥＮＡＮＡ2EpNbNbpzp
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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