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 protection367ＰＥＮＡＮＡTqXXHSwJjz 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 protection367ＰＥＮＡＮＡqF39CB45rI 尼

371Please respect copyright.ＰＥＮＡＮＡvUydqkuvPS
Section A. ETag1234 copyright protection367ＰＥＮＡＮＡ5xIxomTui8 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 protection367ＰＥＮＡＮＡsFNrEzEeDG 尼

The server send the etag of a resource by header like this:371Please respect copyright.ＰＥＮＡＮＡQixOcLSRXn
---------------http response with etag----------------371Please respect copyright.ＰＥＮＡＮＡwKs29mEbO9
HTTP/1.1 200 OK371Please respect copyright.ＰＥＮＡＮＡtefdTSe90N
Date: Mon, 25 Nov 2024 10:56:12 GMT371Please respect copyright.ＰＥＮＡＮＡbHBPko2tnM
accept-ranges: bytes371Please respect copyright.ＰＥＮＡＮＡj3OHmF5XyT
ETag: "534231200_1732277116"371Please respect copyright.ＰＥＮＡＮＡx86N7XOydv
Content-Length: 534231200371Please respect copyright.ＰＥＮＡＮＡ2w4nomAlLv
Content-Type: application/octet-stream371Please respect copyright.ＰＥＮＡＮＡzTyaMPthWc
Content-Disposition: attachment; filename="zen.iso"1234 copyright protection367ＰＥＮＡＮＡ6n8bDLBSvi 尼

(http body for the resource)371Please respect copyright.ＰＥＮＡＮＡD55BnMg0va
-----------------------------------------------------1234 copyright protection367ＰＥＮＡＮＡEe2QobcMCE 尼

371Please respect copyright.ＰＥＮＡＮＡoMGV791bae
Section B. if-none-match1234 copyright protection367ＰＥＮＡＮＡkCJNtxY453 尼

The browser send back the etag to ask server if it has been changed by header like this:371Please respect copyright.ＰＥＮＡＮＡqZWIEPu4ZN
-----http request for a cached resource------371Please respect copyright.ＰＥＮＡＮＡVFrB4UtdQb
GET /zen.iso HTTP/1.1371Please respect copyright.ＰＥＮＡＮＡGhnXQ8tdyC
if-none-match: "534231200_1732277116"371Please respect copyright.ＰＥＮＡＮＡWzdX3opfHM
---------------------------------------------1234 copyright protection367ＰＥＮＡＮＡbdu1hJWxxo 尼

If the etag is unchanged, the server response like this:1234 copyright protection367ＰＥＮＡＮＡPvGZWPpd4G 尼

1. for get and head requests:371Please respect copyright.ＰＥＮＡＮＡGk9fpWpzRL
-----------http response for unchanged resource------371Please respect copyright.ＰＥＮＡＮＡ8o83Osmoxe
HTTP/1.1 304 Not Modified1234 copyright protection367ＰＥＮＡＮＡrYOnstnF2r 尼

(http body canbe empty, the client will used its cache)371Please respect copyright.ＰＥＮＡＮＡZNYtDCjXJa
----------------------------------------------------1234 copyright protection367ＰＥＮＡＮＡCqGviBMA0H 尼

2. for other methods requests:371Please respect copyright.ＰＥＮＡＮＡ3YbzwMbrOC
-----------http response for unchanged resource------371Please respect copyright.ＰＥＮＡＮＡsPxuCbB2ix
HTTP/1.1 412 Precondition Failed 1234 copyright protection367ＰＥＮＡＮＡ2hZy0ajyNd 尼

(http body canbe empty, the client will used its cache)371Please respect copyright.ＰＥＮＡＮＡqATpIJmjP6
----------------------------------------------------1234 copyright protection367ＰＥＮＡＮＡCNokTJCxVB 尼

371Please respect copyright.ＰＥＮＡＮＡC0RN6UyGjm
If the etag changes, the server response 200 OK with the new file content like this:371Please respect copyright.ＰＥＮＡＮＡF1RG5IvZmi
---------------http response with etag----------------371Please respect copyright.ＰＥＮＡＮＡhI2JlYtE9c
HTTP/1.1 200 OK371Please respect copyright.ＰＥＮＡＮＡs1gLtRNk1s
Date: Mon, 25 Nov 2024 10:56:12 GMT371Please respect copyright.ＰＥＮＡＮＡe0yeyaqukI
accept-ranges: bytes371Please respect copyright.ＰＥＮＡＮＡLvC5VTx8rg
ETag: "534231200_1732277116"371Please respect copyright.ＰＥＮＡＮＡcpfIM2Cdg1
Content-Length: 534231200371Please respect copyright.ＰＥＮＡＮＡpmcuvMZtTj
Content-Type: application/octet-stream371Please respect copyright.ＰＥＮＡＮＡBAfVMXccD4
Content-Disposition: attachment; filename="zen.iso"1234 copyright protection367ＰＥＮＡＮＡa3OrSpV0VS 尼

(http body for the resource)371Please respect copyright.ＰＥＮＡＮＡkGDshrX09a
-----------------------------------------------------1234 copyright protection367ＰＥＮＡＮＡdehuqJxsQT 尼

371Please respect copyright.ＰＥＮＡＮＡb7st9rm4Gp
Section C. if-match1234 copyright protection367ＰＥＮＡＮＡ1XSHW9F9EF 尼

When resuming a download in the browser download manager, the browser sends a range request with the if-match header like this:371Please respect copyright.ＰＥＮＡＮＡDmQK92kxBH
------resume download request-------371Please respect copyright.ＰＥＮＡＮＡCldm0unYxt
GET /zen.iso HTTP/1.1371Please respect copyright.ＰＥＮＡＮＡYSWTe4AjO9
Range: bytes=287718144-371Please respect copyright.ＰＥＮＡＮＡI1AKrVgspa
If-Match: "534231200_1732277116"371Please respect copyright.ＰＥＮＡＮＡ2dHucWZ2SP
-----------------------------------371Please respect copyright.ＰＥＮＡＮＡ7DShyb0YKX
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 protection367ＰＥＮＡＮＡn1s3jg9szR 尼

The server response for unchanged etag is like this:371Please respect copyright.ＰＥＮＡＮＡ1SaqHixxdf
-------------range response-------------------371Please respect copyright.ＰＥＮＡＮＡB3rNKCUjJf
HTTP/1.1 206 Partial Content371Please respect copyright.ＰＥＮＡＮＡ5GSvHIk3f4
accept-ranges: bytes371Please respect copyright.ＰＥＮＡＮＡXqRChaF0Yd
content-range: bytes 287718144-534231199/534231200371Please respect copyright.ＰＥＮＡＮＡlY9Y25i9bs
ETag: "534231200_1732277116"371Please respect copyright.ＰＥＮＡＮＡ7PdorybYDz
Content-Length: 246513056371Please respect copyright.ＰＥＮＡＮＡwldZ6QyCGP
Content-Type: application/octet-stream371Please respect copyright.ＰＥＮＡＮＡ8QU45dRCLA
Content-Disposition: attachment; filename="zen.iso"1234 copyright protection367ＰＥＮＡＮＡ6wAUeaKSbj 尼

(http body is the remain file content)371Please respect copyright.ＰＥＮＡＮＡoGbejqIDsH
-------------------------------------------1234 copyright protection367ＰＥＮＡＮＡc7fcbfmcUh 尼

If the etag changes, response 412 like this:371Please respect copyright.ＰＥＮＡＮＡ0l0oe5zVsu
-----------http response for changed resource------371Please respect copyright.ＰＥＮＡＮＡhfDfhUwgTb
HTTP/1.1 412 Precondition Failed 1234 copyright protection367ＰＥＮＡＮＡe7AC4m383R 尼

(http body canbe empty, the client will stop downloading the resumed file)371Please respect copyright.ＰＥＮＡＮＡR5FFD0elAy
---------------------------------------------------1234 copyright protection367ＰＥＮＡＮＡCXjGjXYZQy 尼

371Please respect copyright.ＰＥＮＡＮＡ67uthVrjM7
Section D. if-range1234 copyright protection367ＰＥＮＡＮＡ1eMzCbeqeF 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 protection367ＰＥＮＡＮＡTLB4yd7DHE 尼

---resume download request by chrome---371Please respect copyright.ＰＥＮＡＮＡmIp5h5mepk
GET /zen.iso HTTP/1.1371Please respect copyright.ＰＥＮＡＮＡdBiVyKgvMI
Range: bytes=287718144-371Please respect copyright.ＰＥＮＡＮＡwRxRhdHjU4
If-Range: "534231200_1732277116"371Please respect copyright.ＰＥＮＡＮＡe1Tm84M6UU
------------------------------------371Please respect copyright.ＰＥＮＡＮＡy0IcGPSYma
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 protection367ＰＥＮＡＮＡnmyABtSLOD 尼

The server response for unchanged etag is like this:371Please respect copyright.ＰＥＮＡＮＡQ1GSbZqKh5
-------------range response-------------------371Please respect copyright.ＰＥＮＡＮＡ43IGcVeITM
HTTP/1.1 206 Partial Content371Please respect copyright.ＰＥＮＡＮＡ699LgGYe6a
accept-ranges: bytes371Please respect copyright.ＰＥＮＡＮＡALurK1RzVQ
content-range: bytes 287718144-534231199/534231200371Please respect copyright.ＰＥＮＡＮＡt8HDZcgbfy
ETag: "534231200_1732277116"371Please respect copyright.ＰＥＮＡＮＡgzyOHEAFu6
Content-Length: 246513056371Please respect copyright.ＰＥＮＡＮＡUfKS3yENzL
Content-Type: application/octet-stream371Please respect copyright.ＰＥＮＡＮＡI062ByraTf
Content-Disposition: attachment; filename="zen.iso"1234 copyright protection367ＰＥＮＡＮＡUvvkbj6GGL 尼

(http body is the remain file content)371Please respect copyright.ＰＥＮＡＮＡNvv6o0uFZq
-------------------------------------------1234 copyright protection367ＰＥＮＡＮＡZydAeviPeF 尼

If the etag changes, response 200 from beginning like this:371Please respect copyright.ＰＥＮＡＮＡDNyv9HsZ5J
-----------http response from the beginning------371Please respect copyright.ＰＥＮＡＮＡMeGZmHMlGZ
HTTP/1.1 200 OK371Please respect copyright.ＰＥＮＡＮＡNy5pX2qSj7
Date: Mon, 25 Nov 2024 10:56:12 GMT371Please respect copyright.ＰＥＮＡＮＡXyAXfL8pBp
accept-ranges: bytes371Please respect copyright.ＰＥＮＡＮＡpk24u344Fp
ETag: "534231200_1732277116"371Please respect copyright.ＰＥＮＡＮＡiCXWQVgqmH
Content-Length: 534231200371Please respect copyright.ＰＥＮＡＮＡJhjDY6KnlV
Content-Type: application/octet-stream371Please respect copyright.ＰＥＮＡＮＡH9KXa1thUC
Content-Disposition: attachment; filename="zen.iso"1234 copyright protection367ＰＥＮＡＮＡu3JSG9VpgI 尼

(http body for the resource)371Please respect copyright.ＰＥＮＡＮＡZpK03PK1he
---------------------------------------------------1234 copyright protection367ＰＥＮＡＮＡyPbn5TH8ny 尼

371Please respect copyright.ＰＥＮＡＮＡKgD6H3Hujv
Section X. Thanks1234 copyright protection367ＰＥＮＡＮＡGh8kf2Y38k 尼

https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/If-None-Match371Please respect copyright.ＰＥＮＡＮＡLBpZa0pox4
https://blog.csdn.net/catoop/article/details/134174653371Please respect copyright.ＰＥＮＡＮＡ5P3Gg2zd4x
https://blog.csdn.net/phker/article/details/507226191234 copyright protection367ＰＥＮＡＮＡF0sw6KTGaB 尼

Section Y. Contacts Me1234 copyright protection367ＰＥＮＡＮＡfp6IjQ4Jkj 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]371Please respect copyright.ＰＥＮＡＮＡD8vT2xIbRI
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 protection367ＰＥＮＡＮＡmHHOJIoCvC 尼

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