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。1234 copyright protection371ＰＥＮＡＮＡnXRKnYxSiV 尼

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.1234 copyright protection371ＰＥＮＡＮＡH8J2WdzRby 尼

375Please respect copyright.ＰＥＮＡＮＡUt0CVn5O3A
Section A. ETag1234 copyright protection371ＰＥＮＡＮＡNloTvLet4c 尼

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.1234 copyright protection371ＰＥＮＡＮＡvgK3L62nDj 尼

The server send the etag of a resource by header like this:375Please respect copyright.ＰＥＮＡＮＡwSKFgMUWj2
---------------http response with etag----------------375Please respect copyright.ＰＥＮＡＮＡY2HdzmJSjp
HTTP/1.1 200 OK375Please respect copyright.ＰＥＮＡＮＡ7SjNHZe3lW
Date: Mon, 25 Nov 2024 10:56:12 GMT375Please respect copyright.ＰＥＮＡＮＡs846H3IAXK
accept-ranges: bytes375Please respect copyright.ＰＥＮＡＮＡZZb7W7B2J3
ETag: "534231200_1732277116"375Please respect copyright.ＰＥＮＡＮＡeRkFf2XGG3
Content-Length: 534231200375Please respect copyright.ＰＥＮＡＮＡ3cUQvzKsKA
Content-Type: application/octet-stream375Please respect copyright.ＰＥＮＡＮＡ1XJH0uPtPa
Content-Disposition: attachment; filename="zen.iso"1234 copyright protection371ＰＥＮＡＮＡRykoztkMM0 尼

(http body for the resource)375Please respect copyright.ＰＥＮＡＮＡZusSPbSOcK
-----------------------------------------------------1234 copyright protection371ＰＥＮＡＮＡkE0ZcdZVXI 尼

375Please respect copyright.ＰＥＮＡＮＡPrH3KO1bAB
Section B. if-none-match1234 copyright protection371ＰＥＮＡＮＡTvGE3PbYMA 尼

The browser send back the etag to ask server if it has been changed by header like this:375Please respect copyright.ＰＥＮＡＮＡyjdYMl1xZt
-----http request for a cached resource------375Please respect copyright.ＰＥＮＡＮＡ8hg6PqlYLp
GET /zen.iso HTTP/1.1375Please respect copyright.ＰＥＮＡＮＡXjdik51Pxp
if-none-match: "534231200_1732277116"375Please respect copyright.ＰＥＮＡＮＡlAgaW0bcKV
---------------------------------------------1234 copyright protection371ＰＥＮＡＮＡHNdBbN6pWH 尼

If the etag is unchanged, the server response like this:1234 copyright protection371ＰＥＮＡＮＡxTCoDNR5vP 尼

1. for get and head requests:375Please respect copyright.ＰＥＮＡＮＡm3Jh2bprtI
-----------http response for unchanged resource------375Please respect copyright.ＰＥＮＡＮＡAeDTKCuhKk
HTTP/1.1 304 Not Modified1234 copyright protection371ＰＥＮＡＮＡrXfnUooMi2 尼

(http body canbe empty, the client will used its cache)375Please respect copyright.ＰＥＮＡＮＡxWwHpKeljj
----------------------------------------------------1234 copyright protection371ＰＥＮＡＮＡkXSs6lBuGb 尼

2. for other methods requests:375Please respect copyright.ＰＥＮＡＮＡ706uQ7oab6
-----------http response for unchanged resource------375Please respect copyright.ＰＥＮＡＮＡHLoEqgmXrd
HTTP/1.1 412 Precondition Failed 1234 copyright protection371ＰＥＮＡＮＡpp6wdLdiaz 尼

(http body canbe empty, the client will used its cache)375Please respect copyright.ＰＥＮＡＮＡx9bAfUH9ow
----------------------------------------------------1234 copyright protection371ＰＥＮＡＮＡ4qGBcgiUqA 尼

375Please respect copyright.ＰＥＮＡＮＡwzMkZN8OOL
If the etag changes, the server response 200 OK with the new file content like this:375Please respect copyright.ＰＥＮＡＮＡZab0AVMZ5f
---------------http response with etag----------------375Please respect copyright.ＰＥＮＡＮＡBfFSDnBfKp
HTTP/1.1 200 OK375Please respect copyright.ＰＥＮＡＮＡjYjoQpyNR6
Date: Mon, 25 Nov 2024 10:56:12 GMT375Please respect copyright.ＰＥＮＡＮＡLs3j8BtXwy
accept-ranges: bytes375Please respect copyright.ＰＥＮＡＮＡLrMuDvUjef
ETag: "534231200_1732277116"375Please respect copyright.ＰＥＮＡＮＡf4oKopEZ8z
Content-Length: 534231200375Please respect copyright.ＰＥＮＡＮＡS4X00jw4Ka
Content-Type: application/octet-stream375Please respect copyright.ＰＥＮＡＮＡVJRvR1MmBL
Content-Disposition: attachment; filename="zen.iso"1234 copyright protection371ＰＥＮＡＮＡvdvtrpWjo7 尼

(http body for the resource)375Please respect copyright.ＰＥＮＡＮＡViPbQK5wcP
-----------------------------------------------------1234 copyright protection371ＰＥＮＡＮＡOQAmw2imyc 尼

375Please respect copyright.ＰＥＮＡＮＡXS6wiACmaH
Section C. if-match1234 copyright protection371ＰＥＮＡＮＡMFoQ7681u4 尼

When resuming a download in the browser download manager, the browser sends a range request with the if-match header like this:375Please respect copyright.ＰＥＮＡＮＡttcCcMiuqy
------resume download request-------375Please respect copyright.ＰＥＮＡＮＡcWAGlyZUTm
GET /zen.iso HTTP/1.1375Please respect copyright.ＰＥＮＡＮＡFgBcsVXqX9
Range: bytes=287718144-375Please respect copyright.ＰＥＮＡＮＡOqMRSg9Dh0
If-Match: "534231200_1732277116"375Please respect copyright.ＰＥＮＡＮＡc4WloJjmuH
-----------------------------------375Please respect copyright.ＰＥＮＡＮＡzVoHn3Qhe1
It tells the server to check the etag in If-Match, if unchanged, resume to get the remain content; otherwise, return 412 Precondition Failed.1234 copyright protection371ＰＥＮＡＮＡiTCqo2Ioxc 尼

The server response for unchanged etag is like this:375Please respect copyright.ＰＥＮＡＮＡaHzTvjTKfz
-------------range response-------------------375Please respect copyright.ＰＥＮＡＮＡ5A9VtqBOxf
HTTP/1.1 206 Partial Content375Please respect copyright.ＰＥＮＡＮＡ4H3XIUS9u6
accept-ranges: bytes375Please respect copyright.ＰＥＮＡＮＡ5Z4gN9pwnd
content-range: bytes 287718144-534231199/534231200375Please respect copyright.ＰＥＮＡＮＡ6TA79T1ipW
ETag: "534231200_1732277116"375Please respect copyright.ＰＥＮＡＮＡfZo581Usv4
Content-Length: 246513056375Please respect copyright.ＰＥＮＡＮＡzxxWL1dXG6
Content-Type: application/octet-stream375Please respect copyright.ＰＥＮＡＮＡvIWcDRUPBr
Content-Disposition: attachment; filename="zen.iso"1234 copyright protection371ＰＥＮＡＮＡjAdNCrw5vD 尼

(http body is the remain file content)375Please respect copyright.ＰＥＮＡＮＡ37ho7QGONO
-------------------------------------------1234 copyright protection371ＰＥＮＡＮＡbWQhAEe7vH 尼

If the etag changes, response 412 like this:375Please respect copyright.ＰＥＮＡＮＡ74wKcWOQEf
-----------http response for changed resource------375Please respect copyright.ＰＥＮＡＮＡzhkyE0X8qy
HTTP/1.1 412 Precondition Failed 1234 copyright protection371ＰＥＮＡＮＡJWWM9ermvK 尼

(http body canbe empty, the client will stop downloading the resumed file)375Please respect copyright.ＰＥＮＡＮＡD1NHO3mtJi
---------------------------------------------------1234 copyright protection371ＰＥＮＡＮＡBuEVUMcY0c 尼

375Please respect copyright.ＰＥＮＡＮＡxy7POaMkYa
Section D. if-range1234 copyright protection371ＰＥＮＡＮＡ72xYaxbELt 尼

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.1234 copyright protection371ＰＥＮＡＮＡYvjsZv1Rb3 尼

---resume download request by chrome---375Please respect copyright.ＰＥＮＡＮＡ3hEST0OzjQ
GET /zen.iso HTTP/1.1375Please respect copyright.ＰＥＮＡＮＡsy9OInoTys
Range: bytes=287718144-375Please respect copyright.ＰＥＮＡＮＡzrIO2tAkGC
If-Range: "534231200_1732277116"375Please respect copyright.ＰＥＮＡＮＡWSE0l2I5cz
------------------------------------375Please respect copyright.ＰＥＮＡＮＡ0ibfrcfHNQ
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.1234 copyright protection371ＰＥＮＡＮＡkTwMtRUYBa 尼

The server response for unchanged etag is like this:375Please respect copyright.ＰＥＮＡＮＡLfswHmVHv2
-------------range response-------------------375Please respect copyright.ＰＥＮＡＮＡtQ22IWPVqR
HTTP/1.1 206 Partial Content375Please respect copyright.ＰＥＮＡＮＡ5HQzaSNgjM
accept-ranges: bytes375Please respect copyright.ＰＥＮＡＮＡsJJCZLw4c9
content-range: bytes 287718144-534231199/534231200375Please respect copyright.ＰＥＮＡＮＡOkRBwVbfjL
ETag: "534231200_1732277116"375Please respect copyright.ＰＥＮＡＮＡ7k2IWqQiu6
Content-Length: 246513056375Please respect copyright.ＰＥＮＡＮＡddMNxzqxKj
Content-Type: application/octet-stream375Please respect copyright.ＰＥＮＡＮＡkJq9YVEAoK
Content-Disposition: attachment; filename="zen.iso"1234 copyright protection371ＰＥＮＡＮＡx73MxlkqhL 尼

(http body is the remain file content)375Please respect copyright.ＰＥＮＡＮＡLv3R6sHL5x
-------------------------------------------1234 copyright protection371ＰＥＮＡＮＡmo95rn1EBY 尼

If the etag changes, response 200 from beginning like this:375Please respect copyright.ＰＥＮＡＮＡIobuOO8S1a
-----------http response from the beginning------375Please respect copyright.ＰＥＮＡＮＡxU6lnFvysf
HTTP/1.1 200 OK375Please respect copyright.ＰＥＮＡＮＡnIGpceGmZw
Date: Mon, 25 Nov 2024 10:56:12 GMT375Please respect copyright.ＰＥＮＡＮＡFHCh2LPmPk
accept-ranges: bytes375Please respect copyright.ＰＥＮＡＮＡnwYFc0plu3
ETag: "534231200_1732277116"375Please respect copyright.ＰＥＮＡＮＡs86n2u8Qa6
Content-Length: 534231200375Please respect copyright.ＰＥＮＡＮＡJVNLVOia5H
Content-Type: application/octet-stream375Please respect copyright.ＰＥＮＡＮＡCuEVYswMs4
Content-Disposition: attachment; filename="zen.iso"1234 copyright protection371ＰＥＮＡＮＡKmvTdI85Ay 尼

(http body for the resource)375Please respect copyright.ＰＥＮＡＮＡHFD6mRtqBF
---------------------------------------------------1234 copyright protection371ＰＥＮＡＮＡIIGhjjOBCs 尼

375Please respect copyright.ＰＥＮＡＮＡqfW1q0hipN
Section X. Thanks1234 copyright protection371ＰＥＮＡＮＡS5SE8oh2BH 尼

https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/If-None-Match375Please respect copyright.ＰＥＮＡＮＡnnNmMvCfvL
https://blog.csdn.net/catoop/article/details/134174653375Please respect copyright.ＰＥＮＡＮＡcKqqacieeC
https://blog.csdn.net/phker/article/details/507226191234 copyright protection371ＰＥＮＡＮＡNbWVXNMgXu 尼

Section Y. Contacts Me1234 copyright protection371ＰＥＮＡＮＡB1tou9ew8e 尼

If you found any errors or have any suggestions for this article, please let me know, my wechat: si_jinmin, my email: [email protected]375Please respect copyright.ＰＥＮＡＮＡrqx1aHDGxZ
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]1234 copyright protection371ＰＥＮＡＮＡP03gWJ77j8 尼

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