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.
191Please respect copyright.ＰＥＮＡＮＡr8laC1AFtT
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:191Please respect copyright.ＰＥＮＡＮＡfVArjyIx6n
---------------http response with etag----------------191Please respect copyright.ＰＥＮＡＮＡyd3RfSsArI
HTTP/1.1 200 OK191Please respect copyright.ＰＥＮＡＮＡKvTbIMifSB
Date: Mon, 25 Nov 2024 10:56:12 GMT191Please respect copyright.ＰＥＮＡＮＡwCCMPrP4VW
accept-ranges: bytes191Please respect copyright.ＰＥＮＡＮＡ4agiKdsG42
ETag: "534231200_1732277116"191Please respect copyright.ＰＥＮＡＮＡeidau6sdhk
Content-Length: 534231200191Please respect copyright.ＰＥＮＡＮＡFDRWbm249s
Content-Type: application/octet-stream191Please respect copyright.ＰＥＮＡＮＡ7JDcFjqGpP
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)191Please respect copyright.ＰＥＮＡＮＡTTRcwDoxxD
-----------------------------------------------------
191Please respect copyright.ＰＥＮＡＮＡdq5LnZfx9n
Section B. if-none-match
The browser send back the etag to ask server if it has been changed by header like this:191Please respect copyright.ＰＥＮＡＮＡ2tMTiCkTyo
-----http request for a cached resource------191Please respect copyright.ＰＥＮＡＮＡIYNTyYXS1U
GET /zen.iso HTTP/1.1191Please respect copyright.ＰＥＮＡＮＡqKLwhOE36b
if-none-match: "534231200_1732277116"191Please respect copyright.ＰＥＮＡＮＡTN0CI2KjRr
---------------------------------------------
If the etag is unchanged, the server response like this:
1. for get and head requests:191Please respect copyright.ＰＥＮＡＮＡmwGViASBuO
-----------http response for unchanged resource------191Please respect copyright.ＰＥＮＡＮＡhuOewh5RWu
HTTP/1.1 304 Not Modified
(http body canbe empty, the client will used its cache)191Please respect copyright.ＰＥＮＡＮＡ3WZHhSk8hw
----------------------------------------------------
2. for other methods requests:191Please respect copyright.ＰＥＮＡＮＡVz7jk6OuI7
-----------http response for unchanged resource------191Please respect copyright.ＰＥＮＡＮＡPYx7ExnDVb
HTTP/1.1 412 Precondition Failed
(http body canbe empty, the client will used its cache)191Please respect copyright.ＰＥＮＡＮＡw77Ktdc5Zj
----------------------------------------------------
191Please respect copyright.ＰＥＮＡＮＡRarAYLl2Z8
If the etag changes, the server response 200 OK with the new file content like this:191Please respect copyright.ＰＥＮＡＮＡAP371efSZC
---------------http response with etag----------------191Please respect copyright.ＰＥＮＡＮＡBQVJV3weFU
HTTP/1.1 200 OK191Please respect copyright.ＰＥＮＡＮＡq1YoL3MdyQ
Date: Mon, 25 Nov 2024 10:56:12 GMT191Please respect copyright.ＰＥＮＡＮＡP64YmlatI5
accept-ranges: bytes191Please respect copyright.ＰＥＮＡＮＡnWV47nXlTc
ETag: "534231200_1732277116"191Please respect copyright.ＰＥＮＡＮＡ15InYhpOg5
Content-Length: 534231200191Please respect copyright.ＰＥＮＡＮＡBlvQSfjviK
Content-Type: application/octet-stream191Please respect copyright.ＰＥＮＡＮＡVOKqbZOURb
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)191Please respect copyright.ＰＥＮＡＮＡbkWUIQxyYb
-----------------------------------------------------
191Please respect copyright.ＰＥＮＡＮＡiccuIrDZH3
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:191Please respect copyright.ＰＥＮＡＮＡxodBEy9oTB
------resume download request-------191Please respect copyright.ＰＥＮＡＮＡpaXyuHyM8R
GET /zen.iso HTTP/1.1191Please respect copyright.ＰＥＮＡＮＡizicMZNQC0
Range: bytes=287718144-191Please respect copyright.ＰＥＮＡＮＡmRMhLvA2aE
If-Match: "534231200_1732277116"191Please respect copyright.ＰＥＮＡＮＡVIRWgwzApg
-----------------------------------191Please respect copyright.ＰＥＮＡＮＡVcZLDy0FCt
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:191Please respect copyright.ＰＥＮＡＮＡYddVAVXIeO
-------------range response-------------------191Please respect copyright.ＰＥＮＡＮＡICDP3bpbAf
HTTP/1.1 206 Partial Content191Please respect copyright.ＰＥＮＡＮＡ64aNLtXsMm
accept-ranges: bytes191Please respect copyright.ＰＥＮＡＮＡC7ULUU2LVP
content-range: bytes 287718144-534231199/534231200191Please respect copyright.ＰＥＮＡＮＡNdLhooMYLP
ETag: "534231200_1732277116"191Please respect copyright.ＰＥＮＡＮＡTwWtiQiJnG
Content-Length: 246513056191Please respect copyright.ＰＥＮＡＮＡuHvZYVUQIS
Content-Type: application/octet-stream191Please respect copyright.ＰＥＮＡＮＡUNlZIRzKST
Content-Disposition: attachment; filename="zen.iso"
(http body is the remain file content)191Please respect copyright.ＰＥＮＡＮＡbmNGbiAIiM
-------------------------------------------
If the etag changes, response 412 like this:191Please respect copyright.ＰＥＮＡＮＡFdwRT8XCuf
-----------http response for changed resource------191Please respect copyright.ＰＥＮＡＮＡkAbRxCIdla
HTTP/1.1 412 Precondition Failed
(http body canbe empty, the client will stop downloading the resumed file)191Please respect copyright.ＰＥＮＡＮＡLY133tUefM
---------------------------------------------------
191Please respect copyright.ＰＥＮＡＮＡXkBgPCtFDD
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---191Please respect copyright.ＰＥＮＡＮＡksGQDJl9Fg
GET /zen.iso HTTP/1.1191Please respect copyright.ＰＥＮＡＮＡ13SuFem75T
Range: bytes=287718144-191Please respect copyright.ＰＥＮＡＮＡ0PFPW0QFMK
If-Range: "534231200_1732277116"191Please respect copyright.ＰＥＮＡＮＡjwAGzFQ6Q6
------------------------------------191Please respect copyright.ＰＥＮＡＮＡoTiJeV4Pgu
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:191Please respect copyright.ＰＥＮＡＮＡGvii7DHabp
-------------range response-------------------191Please respect copyright.ＰＥＮＡＮＡ5jsdz9Q5Sn
HTTP/1.1 206 Partial Content191Please respect copyright.ＰＥＮＡＮＡLEaqFfP8rW
accept-ranges: bytes191Please respect copyright.ＰＥＮＡＮＡbMYYc7kMs8
content-range: bytes 287718144-534231199/534231200191Please respect copyright.ＰＥＮＡＮＡ4OKfnTD8o4
ETag: "534231200_1732277116"191Please respect copyright.ＰＥＮＡＮＡXVXGZaGGZu
Content-Length: 246513056191Please respect copyright.ＰＥＮＡＮＡNyDfw6X6Ac
Content-Type: application/octet-stream191Please respect copyright.ＰＥＮＡＮＡDQIRqSqgXy
Content-Disposition: attachment; filename="zen.iso"
(http body is the remain file content)191Please respect copyright.ＰＥＮＡＮＡRpr2Bf580K
-------------------------------------------
If the etag changes, response 200 from beginning like this:191Please respect copyright.ＰＥＮＡＮＡVSojsIZtz9
-----------http response from the beginning------191Please respect copyright.ＰＥＮＡＮＡ8nGnp3Hi02
HTTP/1.1 200 OK191Please respect copyright.ＰＥＮＡＮＡu0G3W3TcJt
Date: Mon, 25 Nov 2024 10:56:12 GMT191Please respect copyright.ＰＥＮＡＮＡFCyxQJlnRN
accept-ranges: bytes191Please respect copyright.ＰＥＮＡＮＡG8G92wmcnD
ETag: "534231200_1732277116"191Please respect copyright.ＰＥＮＡＮＡhNAk0RwxuO
Content-Length: 534231200191Please respect copyright.ＰＥＮＡＮＡcru4wkxCys
Content-Type: application/octet-stream191Please respect copyright.ＰＥＮＡＮＡI3peoic9sO
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)191Please respect copyright.ＰＥＮＡＮＡt3vo1wzWkC
---------------------------------------------------
191Please respect copyright.ＰＥＮＡＮＡJ6EBC9lY8T
Section X. Thanks
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/If-None-Match191Please respect copyright.ＰＥＮＡＮＡqtDSTOHlU8
https://blog.csdn.net/catoop/article/details/134174653191Please respect copyright.ＰＥＮＡＮＡUUzwtHhClx
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]191Please respect copyright.ＰＥＮＡＮＡ3r7AMvfH08
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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