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 protection349ＰＥＮＡＮＡzVkJVS4SH2 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 protection349ＰＥＮＡＮＡeQibpNbZXC 尼

353Please respect copyright.ＰＥＮＡＮＡPkwZJd1s72
Section A. ETag1234 copyright protection349ＰＥＮＡＮＡbCPDPfe6BN 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 protection349ＰＥＮＡＮＡ091HfjWxea 尼

The server send the etag of a resource by header like this:353Please respect copyright.ＰＥＮＡＮＡmpTePV4934
---------------http response with etag----------------353Please respect copyright.ＰＥＮＡＮＡ5WtkddS7Ws
HTTP/1.1 200 OK353Please respect copyright.ＰＥＮＡＮＡQmCSoYTSfN
Date: Mon, 25 Nov 2024 10:56:12 GMT353Please respect copyright.ＰＥＮＡＮＡ7kSIinC1rg
accept-ranges: bytes353Please respect copyright.ＰＥＮＡＮＡ6t88kASIkR
ETag: "534231200_1732277116"353Please respect copyright.ＰＥＮＡＮＡMmYXsuX5cn
Content-Length: 534231200353Please respect copyright.ＰＥＮＡＮＡ7SuDPZpazT
Content-Type: application/octet-stream353Please respect copyright.ＰＥＮＡＮＡZGAU6oIvyE
Content-Disposition: attachment; filename="zen.iso"1234 copyright protection349ＰＥＮＡＮＡ85yObP6UhK 尼

(http body for the resource)353Please respect copyright.ＰＥＮＡＮＡIhmCsCJ3n7
-----------------------------------------------------1234 copyright protection349ＰＥＮＡＮＡMLj85WAADv 尼

353Please respect copyright.ＰＥＮＡＮＡC8cR1ho1zC
Section B. if-none-match1234 copyright protection349ＰＥＮＡＮＡEgjkiyZysR 尼

The browser send back the etag to ask server if it has been changed by header like this:353Please respect copyright.ＰＥＮＡＮＡubCSA8lWU9
-----http request for a cached resource------353Please respect copyright.ＰＥＮＡＮＡDeUFBt4iby
GET /zen.iso HTTP/1.1353Please respect copyright.ＰＥＮＡＮＡkcaKZ0w59A
if-none-match: "534231200_1732277116"353Please respect copyright.ＰＥＮＡＮＡ214T7CU2Pz
---------------------------------------------1234 copyright protection349ＰＥＮＡＮＡ0bGkUlax82 尼

If the etag is unchanged, the server response like this:1234 copyright protection349ＰＥＮＡＮＡPpcslH2yYx 尼

1. for get and head requests:353Please respect copyright.ＰＥＮＡＮＡJ8SywfXuwg
-----------http response for unchanged resource------353Please respect copyright.ＰＥＮＡＮＡyT5NWy0n0y
HTTP/1.1 304 Not Modified1234 copyright protection349ＰＥＮＡＮＡYWLplRahYc 尼

(http body canbe empty, the client will used its cache)353Please respect copyright.ＰＥＮＡＮＡAZGWWEUqTJ
----------------------------------------------------1234 copyright protection349ＰＥＮＡＮＡ62HAH3Ovzn 尼

2. for other methods requests:353Please respect copyright.ＰＥＮＡＮＡxo42Q7QEGO
-----------http response for unchanged resource------353Please respect copyright.ＰＥＮＡＮＡGBhbdAU2JW
HTTP/1.1 412 Precondition Failed 1234 copyright protection349ＰＥＮＡＮＡLdhqcDRz2V 尼

(http body canbe empty, the client will used its cache)353Please respect copyright.ＰＥＮＡＮＡfx5w9RyjAB
----------------------------------------------------1234 copyright protection349ＰＥＮＡＮＡRCwkNtuzwr 尼

353Please respect copyright.ＰＥＮＡＮＡSDHbAhHEyv
If the etag changes, the server response 200 OK with the new file content like this:353Please respect copyright.ＰＥＮＡＮＡcjJxPWB6Uj
---------------http response with etag----------------353Please respect copyright.ＰＥＮＡＮＡxs5ld0olfB
HTTP/1.1 200 OK353Please respect copyright.ＰＥＮＡＮＡT90WuaQy1f
Date: Mon, 25 Nov 2024 10:56:12 GMT353Please respect copyright.ＰＥＮＡＮＡaGO9eMGAV0
accept-ranges: bytes353Please respect copyright.ＰＥＮＡＮＡULT6N3C9Qq
ETag: "534231200_1732277116"353Please respect copyright.ＰＥＮＡＮＡmqiopgZhti
Content-Length: 534231200353Please respect copyright.ＰＥＮＡＮＡW0XiO3HGjR
Content-Type: application/octet-stream353Please respect copyright.ＰＥＮＡＮＡOn9Wkp0ZSD
Content-Disposition: attachment; filename="zen.iso"1234 copyright protection349ＰＥＮＡＮＡW2INqEHQx6 尼

(http body for the resource)353Please respect copyright.ＰＥＮＡＮＡ53LUJLpb9f
-----------------------------------------------------1234 copyright protection349ＰＥＮＡＮＡrE76QshJgO 尼

353Please respect copyright.ＰＥＮＡＮＡbRqYl5oWpF
Section C. if-match1234 copyright protection349ＰＥＮＡＮＡ6ZOz9NIfWp 尼

When resuming a download in the browser download manager, the browser sends a range request with the if-match header like this:353Please respect copyright.ＰＥＮＡＮＡOKBCa46WOT
------resume download request-------353Please respect copyright.ＰＥＮＡＮＡQ46WGxPez4
GET /zen.iso HTTP/1.1353Please respect copyright.ＰＥＮＡＮＡX67SEj2vCd
Range: bytes=287718144-353Please respect copyright.ＰＥＮＡＮＡdOigdgnuiz
If-Match: "534231200_1732277116"353Please respect copyright.ＰＥＮＡＮＡUlIw7mr3BZ
-----------------------------------353Please respect copyright.ＰＥＮＡＮＡyqi7D9cMA1
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 protection349ＰＥＮＡＮＡukk266ap3Y 尼

The server response for unchanged etag is like this:353Please respect copyright.ＰＥＮＡＮＡ01Evn83U23
-------------range response-------------------353Please respect copyright.ＰＥＮＡＮＡSdCHKLAynE
HTTP/1.1 206 Partial Content353Please respect copyright.ＰＥＮＡＮＡXuTQlihDNO
accept-ranges: bytes353Please respect copyright.ＰＥＮＡＮＡL77TgpKs6B
content-range: bytes 287718144-534231199/534231200353Please respect copyright.ＰＥＮＡＮＡH2ewSFlylt
ETag: "534231200_1732277116"353Please respect copyright.ＰＥＮＡＮＡZgJpkx8dh7
Content-Length: 246513056353Please respect copyright.ＰＥＮＡＮＡj8yJUdo1lk
Content-Type: application/octet-stream353Please respect copyright.ＰＥＮＡＮＡBO2rlko8tZ
Content-Disposition: attachment; filename="zen.iso"1234 copyright protection349ＰＥＮＡＮＡgTzhZn9rx3 尼

(http body is the remain file content)353Please respect copyright.ＰＥＮＡＮＡPnTf4unyY0
-------------------------------------------1234 copyright protection349ＰＥＮＡＮＡqsIhSQbkNl 尼

If the etag changes, response 412 like this:353Please respect copyright.ＰＥＮＡＮＡLWAhLcSqei
-----------http response for changed resource------353Please respect copyright.ＰＥＮＡＮＡdtNgyIT4tC
HTTP/1.1 412 Precondition Failed 1234 copyright protection349ＰＥＮＡＮＡHYvMZS97rj 尼

(http body canbe empty, the client will stop downloading the resumed file)353Please respect copyright.ＰＥＮＡＮＡ8lLHpfCGCv
---------------------------------------------------1234 copyright protection349ＰＥＮＡＮＡAtKS6VgfeW 尼

353Please respect copyright.ＰＥＮＡＮＡuuDoN27TT5
Section D. if-range1234 copyright protection349ＰＥＮＡＮＡdMDPMXfaNf 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 protection349ＰＥＮＡＮＡbTQ3MY4VjM 尼

---resume download request by chrome---353Please respect copyright.ＰＥＮＡＮＡ5aSr6b2az5
GET /zen.iso HTTP/1.1353Please respect copyright.ＰＥＮＡＮＡuZ7Jc4HF4d
Range: bytes=287718144-353Please respect copyright.ＰＥＮＡＮＡV2cNKwTbCo
If-Range: "534231200_1732277116"353Please respect copyright.ＰＥＮＡＮＡVTnBQtWv9C
------------------------------------353Please respect copyright.ＰＥＮＡＮＡZ6yasbkpRz
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 protection349ＰＥＮＡＮＡq7XWGoQfvI 尼

The server response for unchanged etag is like this:353Please respect copyright.ＰＥＮＡＮＡoiIv1erD5P
-------------range response-------------------353Please respect copyright.ＰＥＮＡＮＡRDwnz6qwLV
HTTP/1.1 206 Partial Content353Please respect copyright.ＰＥＮＡＮＡ3zSuR6hR4s
accept-ranges: bytes353Please respect copyright.ＰＥＮＡＮＡEfYicy0gcu
content-range: bytes 287718144-534231199/534231200353Please respect copyright.ＰＥＮＡＮＡ0OCYmZHaQB
ETag: "534231200_1732277116"353Please respect copyright.ＰＥＮＡＮＡPSCnRmWPTL
Content-Length: 246513056353Please respect copyright.ＰＥＮＡＮＡZtN6nnfZ4o
Content-Type: application/octet-stream353Please respect copyright.ＰＥＮＡＮＡi1CSQ7TyPy
Content-Disposition: attachment; filename="zen.iso"1234 copyright protection349ＰＥＮＡＮＡhVHtVHXKUP 尼

(http body is the remain file content)353Please respect copyright.ＰＥＮＡＮＡFqo62nJz2Z
-------------------------------------------1234 copyright protection349ＰＥＮＡＮＡW2Iv16U0aQ 尼

If the etag changes, response 200 from beginning like this:353Please respect copyright.ＰＥＮＡＮＡadyZD4tzJp
-----------http response from the beginning------353Please respect copyright.ＰＥＮＡＮＡGlrdQ8UJe4
HTTP/1.1 200 OK353Please respect copyright.ＰＥＮＡＮＡWCyr30IhCs
Date: Mon, 25 Nov 2024 10:56:12 GMT353Please respect copyright.ＰＥＮＡＮＡNVQt9E1KZO
accept-ranges: bytes353Please respect copyright.ＰＥＮＡＮＡNxVgZhfFNi
ETag: "534231200_1732277116"353Please respect copyright.ＰＥＮＡＮＡoi6eMDCAjA
Content-Length: 534231200353Please respect copyright.ＰＥＮＡＮＡxeanF7W7s6
Content-Type: application/octet-stream353Please respect copyright.ＰＥＮＡＮＡVvM2U8bTft
Content-Disposition: attachment; filename="zen.iso"1234 copyright protection349ＰＥＮＡＮＡiFOEzav56b 尼

(http body for the resource)353Please respect copyright.ＰＥＮＡＮＡntUS7cbTg1
---------------------------------------------------1234 copyright protection349ＰＥＮＡＮＡ0z2z36zM2c 尼

353Please respect copyright.ＰＥＮＡＮＡCzbENpvnMX
Section X. Thanks1234 copyright protection349ＰＥＮＡＮＡbWkT6lKeUQ 尼

https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/If-None-Match353Please respect copyright.ＰＥＮＡＮＡYeft79kWcK
https://blog.csdn.net/catoop/article/details/134174653353Please respect copyright.ＰＥＮＡＮＡqtEF1O7RnA
https://blog.csdn.net/phker/article/details/507226191234 copyright protection349ＰＥＮＡＮＡ0rUYrjKdV4 尼

Section Y. Contacts Me1234 copyright protection349ＰＥＮＡＮＡp1MRokyuNM 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]353Please respect copyright.ＰＥＮＡＮＡKpD9WrCkuo
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 protection349ＰＥＮＡＮＡgRb3vKE71r 尼

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