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.
179Please respect copyright.ＰＥＮＡＮＡIdFB09rxN0
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:179Please respect copyright.ＰＥＮＡＮＡwKuYOoAy8D
---------------http response with etag----------------179Please respect copyright.ＰＥＮＡＮＡwuUS57uAuA
HTTP/1.1 200 OK179Please respect copyright.ＰＥＮＡＮＡdkKE3GDWtT
Date: Mon, 25 Nov 2024 10:56:12 GMT179Please respect copyright.ＰＥＮＡＮＡYXETOxOBTH
accept-ranges: bytes179Please respect copyright.ＰＥＮＡＮＡsWxFsDy2lj
ETag: "534231200_1732277116"179Please respect copyright.ＰＥＮＡＮＡL9bIhwKHFi
Content-Length: 534231200179Please respect copyright.ＰＥＮＡＮＡsLIdUJ2lUD
Content-Type: application/octet-stream179Please respect copyright.ＰＥＮＡＮＡf9GogNphq8
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)179Please respect copyright.ＰＥＮＡＮＡOYZVcqPzvq
-----------------------------------------------------
179Please respect copyright.ＰＥＮＡＮＡ2vRcWGQXRg
Section B. if-none-match
The browser send back the etag to ask server if it has been changed by header like this:179Please respect copyright.ＰＥＮＡＮＡZUh2IWh4Vt
-----http request for a cached resource------179Please respect copyright.ＰＥＮＡＮＡAL7Gj0RAbt
GET /zen.iso HTTP/1.1179Please respect copyright.ＰＥＮＡＮＡ0ZWKKQ8dW6
if-none-match: "534231200_1732277116"179Please respect copyright.ＰＥＮＡＮＡCrXtBvF5mB
---------------------------------------------
If the etag is unchanged, the server response like this:
1. for get and head requests:179Please respect copyright.ＰＥＮＡＮＡiw4sMrAyJa
-----------http response for unchanged resource------179Please respect copyright.ＰＥＮＡＮＡvO3vJ8KjkK
HTTP/1.1 304 Not Modified
(http body canbe empty, the client will used its cache)179Please respect copyright.ＰＥＮＡＮＡZBcSBjbr1U
----------------------------------------------------
2. for other methods requests:179Please respect copyright.ＰＥＮＡＮＡZ34RBahvjq
-----------http response for unchanged resource------179Please respect copyright.ＰＥＮＡＮＡNVF11KcuIT
HTTP/1.1 412 Precondition Failed
(http body canbe empty, the client will used its cache)179Please respect copyright.ＰＥＮＡＮＡXl7mRmpmSa
----------------------------------------------------
179Please respect copyright.ＰＥＮＡＮＡBHUmKQs2lf
If the etag changes, the server response 200 OK with the new file content like this:179Please respect copyright.ＰＥＮＡＮＡPd1336K5Cs
---------------http response with etag----------------179Please respect copyright.ＰＥＮＡＮＡZ2D9nbub1y
HTTP/1.1 200 OK179Please respect copyright.ＰＥＮＡＮＡlTEfdnFcm2
Date: Mon, 25 Nov 2024 10:56:12 GMT179Please respect copyright.ＰＥＮＡＮＡyUohKMZKiM
accept-ranges: bytes179Please respect copyright.ＰＥＮＡＮＡ5Id2jOoxTy
ETag: "534231200_1732277116"179Please respect copyright.ＰＥＮＡＮＡhxJOTAC3fh
Content-Length: 534231200179Please respect copyright.ＰＥＮＡＮＡNYENAxdLTw
Content-Type: application/octet-stream179Please respect copyright.ＰＥＮＡＮＡwPCxTpG4Qk
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)179Please respect copyright.ＰＥＮＡＮＡHRqdPxltam
-----------------------------------------------------
179Please respect copyright.ＰＥＮＡＮＡHo84yxwIbR
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:179Please respect copyright.ＰＥＮＡＮＡtOEQmbg8IT
------resume download request-------179Please respect copyright.ＰＥＮＡＮＡvZt8c52dSL
GET /zen.iso HTTP/1.1179Please respect copyright.ＰＥＮＡＮＡICqtxMHkb1
Range: bytes=287718144-179Please respect copyright.ＰＥＮＡＮＡdIrDkvkh1K
If-Match: "534231200_1732277116"179Please respect copyright.ＰＥＮＡＮＡna3pNm0hKR
-----------------------------------179Please respect copyright.ＰＥＮＡＮＡj6g6BrcPsD
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:179Please respect copyright.ＰＥＮＡＮＡHZXswVee8r
-------------range response-------------------179Please respect copyright.ＰＥＮＡＮＡpx7q75b2F2
HTTP/1.1 206 Partial Content179Please respect copyright.ＰＥＮＡＮＡ5WBYuEfrm9
accept-ranges: bytes179Please respect copyright.ＰＥＮＡＮＡ0oYM69wkZi
content-range: bytes 287718144-534231199/534231200179Please respect copyright.ＰＥＮＡＮＡaPksNNqrVC
ETag: "534231200_1732277116"179Please respect copyright.ＰＥＮＡＮＡr4DnoWPIG9
Content-Length: 246513056179Please respect copyright.ＰＥＮＡＮＡvxg6eLhrWo
Content-Type: application/octet-stream179Please respect copyright.ＰＥＮＡＮＡZCzwR6Xy4n
Content-Disposition: attachment; filename="zen.iso"
(http body is the remain file content)179Please respect copyright.ＰＥＮＡＮＡL6ePIwJnik
-------------------------------------------
If the etag changes, response 412 like this:179Please respect copyright.ＰＥＮＡＮＡfAS9BWbHzn
-----------http response for changed resource------179Please respect copyright.ＰＥＮＡＮＡ49BczeVzQn
HTTP/1.1 412 Precondition Failed
(http body canbe empty, the client will stop downloading the resumed file)179Please respect copyright.ＰＥＮＡＮＡbpl164g8Yx
---------------------------------------------------
179Please respect copyright.ＰＥＮＡＮＡerNuQ61U4H
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---179Please respect copyright.ＰＥＮＡＮＡtN3C4eJbk2
GET /zen.iso HTTP/1.1179Please respect copyright.ＰＥＮＡＮＡVHK04NUCvF
Range: bytes=287718144-179Please respect copyright.ＰＥＮＡＮＡF1B91EHds8
If-Range: "534231200_1732277116"179Please respect copyright.ＰＥＮＡＮＡDEH1KsJ9bo
------------------------------------179Please respect copyright.ＰＥＮＡＮＡC18eOsKz5o
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:179Please respect copyright.ＰＥＮＡＮＡEEKFcUGj4m
-------------range response-------------------179Please respect copyright.ＰＥＮＡＮＡjf1xlnbsbS
HTTP/1.1 206 Partial Content179Please respect copyright.ＰＥＮＡＮＡi3TVke59rv
accept-ranges: bytes179Please respect copyright.ＰＥＮＡＮＡDjVvIFNW5c
content-range: bytes 287718144-534231199/534231200179Please respect copyright.ＰＥＮＡＮＡyl32k5vMdd
ETag: "534231200_1732277116"179Please respect copyright.ＰＥＮＡＮＡvELFx5lhmz
Content-Length: 246513056179Please respect copyright.ＰＥＮＡＮＡKXNIn2QFZV
Content-Type: application/octet-stream179Please respect copyright.ＰＥＮＡＮＡzoRTZOFpFc
Content-Disposition: attachment; filename="zen.iso"
(http body is the remain file content)179Please respect copyright.ＰＥＮＡＮＡo26FndSxc5
-------------------------------------------
If the etag changes, response 200 from beginning like this:179Please respect copyright.ＰＥＮＡＮＡVNH6LwXUB1
-----------http response from the beginning------179Please respect copyright.ＰＥＮＡＮＡicByRXnGkH
HTTP/1.1 200 OK179Please respect copyright.ＰＥＮＡＮＡ3gpxaKdkK1
Date: Mon, 25 Nov 2024 10:56:12 GMT179Please respect copyright.ＰＥＮＡＮＡjEXJuSV1ct
accept-ranges: bytes179Please respect copyright.ＰＥＮＡＮＡwT6p2GXAHG
ETag: "534231200_1732277116"179Please respect copyright.ＰＥＮＡＮＡJexfnNNvSK
Content-Length: 534231200179Please respect copyright.ＰＥＮＡＮＡhmBuwk78kv
Content-Type: application/octet-stream179Please respect copyright.ＰＥＮＡＮＡ0TVJygAk9g
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)179Please respect copyright.ＰＥＮＡＮＡsSfl23g9oc
---------------------------------------------------
179Please respect copyright.ＰＥＮＡＮＡ0rDSKwldhr
Section X. Thanks
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/If-None-Match179Please respect copyright.ＰＥＮＡＮＡc4fw7x9cx3
https://blog.csdn.net/catoop/article/details/134174653179Please respect copyright.ＰＥＮＡＮＡvMa1kbhKi7
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]179Please respect copyright.ＰＥＮＡＮＡwvRryLx5Ib
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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