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.
136Please respect copyright.ＰＥＮＡＮＡ4NUhAKTOuJ
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:136Please respect copyright.ＰＥＮＡＮＡ0dK7ELvn2H
---------------http response with etag----------------136Please respect copyright.ＰＥＮＡＮＡmu5h7R3tQ0
HTTP/1.1 200 OK136Please respect copyright.ＰＥＮＡＮＡFBx7IVm89h
Date: Mon, 25 Nov 2024 10:56:12 GMT136Please respect copyright.ＰＥＮＡＮＡSz4hscVjVr
accept-ranges: bytes136Please respect copyright.ＰＥＮＡＮＡuu2WG70Xnb
ETag: "534231200_1732277116"136Please respect copyright.ＰＥＮＡＮＡ3dAW7aq1lg
Content-Length: 534231200136Please respect copyright.ＰＥＮＡＮＡ86R8eBMuaC
Content-Type: application/octet-stream136Please respect copyright.ＰＥＮＡＮＡHns4FiJAUK
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)136Please respect copyright.ＰＥＮＡＮＡ3Q2hYimMqf
-----------------------------------------------------
136Please respect copyright.ＰＥＮＡＮＡ6SRB6gyniO
Section B. if-none-match
The browser send back the etag to ask server if it has been changed by header like this:136Please respect copyright.ＰＥＮＡＮＡu7Bn319aCS
-----http request for a cached resource------136Please respect copyright.ＰＥＮＡＮＡ5dpNTEENGi
GET /zen.iso HTTP/1.1136Please respect copyright.ＰＥＮＡＮＡJAv1Ay0uIP
if-none-match: "534231200_1732277116"136Please respect copyright.ＰＥＮＡＮＡbIq8GntRPz
---------------------------------------------
If the etag is unchanged, the server response like this:
1. for get and head requests:136Please respect copyright.ＰＥＮＡＮＡpg7r2cX3GM
-----------http response for unchanged resource------136Please respect copyright.ＰＥＮＡＮＡyiBhc6T74E
HTTP/1.1 304 Not Modified
(http body canbe empty, the client will used its cache)136Please respect copyright.ＰＥＮＡＮＡXlgJTx1Fts
----------------------------------------------------
2. for other methods requests:136Please respect copyright.ＰＥＮＡＮＡSEbluOmugn
-----------http response for unchanged resource------136Please respect copyright.ＰＥＮＡＮＡTskew6lKZ7
HTTP/1.1 412 Precondition Failed
(http body canbe empty, the client will used its cache)136Please respect copyright.ＰＥＮＡＮＡaGrjujS4Ws
----------------------------------------------------
136Please respect copyright.ＰＥＮＡＮＡKIBFJ3q6QX
If the etag changes, the server response 200 OK with the new file content like this:136Please respect copyright.ＰＥＮＡＮＡXCwl6z4e7X
---------------http response with etag----------------136Please respect copyright.ＰＥＮＡＮＡQLwlTUC5zl
HTTP/1.1 200 OK136Please respect copyright.ＰＥＮＡＮＡLcVeqNLKfO
Date: Mon, 25 Nov 2024 10:56:12 GMT136Please respect copyright.ＰＥＮＡＮＡxO1z9zGkk5
accept-ranges: bytes136Please respect copyright.ＰＥＮＡＮＡMruGTiGxlB
ETag: "534231200_1732277116"136Please respect copyright.ＰＥＮＡＮＡOomle6JzTj
Content-Length: 534231200136Please respect copyright.ＰＥＮＡＮＡXvkRMvWtE5
Content-Type: application/octet-stream136Please respect copyright.ＰＥＮＡＮＡS1DvWzmedl
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)136Please respect copyright.ＰＥＮＡＮＡ16wT5vNMa9
-----------------------------------------------------
136Please respect copyright.ＰＥＮＡＮＡhu3KjfFJok
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:136Please respect copyright.ＰＥＮＡＮＡ0MG4b87ckt
------resume download request-------136Please respect copyright.ＰＥＮＡＮＡhckKhecmFk
GET /zen.iso HTTP/1.1136Please respect copyright.ＰＥＮＡＮＡvdPz8ObxDR
Range: bytes=287718144-136Please respect copyright.ＰＥＮＡＮＡCwGqtXZMMo
If-Match: "534231200_1732277116"136Please respect copyright.ＰＥＮＡＮＡuSLiCz2KHq
-----------------------------------136Please respect copyright.ＰＥＮＡＮＡrHkxN8mtbi
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:136Please respect copyright.ＰＥＮＡＮＡ3R2dFZ5UBW
-------------range response-------------------136Please respect copyright.ＰＥＮＡＮＡRdNFIeFTiH
HTTP/1.1 206 Partial Content136Please respect copyright.ＰＥＮＡＮＡ3WxvPk43ny
accept-ranges: bytes136Please respect copyright.ＰＥＮＡＮＡDUstiIlKxz
content-range: bytes 287718144-534231199/534231200136Please respect copyright.ＰＥＮＡＮＡk1GznuBdX9
ETag: "534231200_1732277116"136Please respect copyright.ＰＥＮＡＮＡ2WyUKiKd5l
Content-Length: 246513056136Please respect copyright.ＰＥＮＡＮＡxW1tzqXei3
Content-Type: application/octet-stream136Please respect copyright.ＰＥＮＡＮＡwDhC3vSztk
Content-Disposition: attachment; filename="zen.iso"
(http body is the remain file content)136Please respect copyright.ＰＥＮＡＮＡfUYOtqu6Rv
-------------------------------------------
If the etag changes, response 412 like this:136Please respect copyright.ＰＥＮＡＮＡ7NbqbAejck
-----------http response for changed resource------136Please respect copyright.ＰＥＮＡＮＡHvIGjuOgy0
HTTP/1.1 412 Precondition Failed
(http body canbe empty, the client will stop downloading the resumed file)136Please respect copyright.ＰＥＮＡＮＡ3augMhO1Zh
---------------------------------------------------
136Please respect copyright.ＰＥＮＡＮＡTLko4YElTK
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---136Please respect copyright.ＰＥＮＡＮＡ8WKr0LwO3j
GET /zen.iso HTTP/1.1136Please respect copyright.ＰＥＮＡＮＡ93913Hs78l
Range: bytes=287718144-136Please respect copyright.ＰＥＮＡＮＡJj5wbxwDJu
If-Range: "534231200_1732277116"136Please respect copyright.ＰＥＮＡＮＡ0OBr08Bxuc
------------------------------------136Please respect copyright.ＰＥＮＡＮＡJeij1jydOT
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:136Please respect copyright.ＰＥＮＡＮＡ7WUBm22Q88
-------------range response-------------------136Please respect copyright.ＰＥＮＡＮＡI3BLpPVFJy
HTTP/1.1 206 Partial Content136Please respect copyright.ＰＥＮＡＮＡ91wXoby8qf
accept-ranges: bytes136Please respect copyright.ＰＥＮＡＮＡxi1k2bLyGG
content-range: bytes 287718144-534231199/534231200136Please respect copyright.ＰＥＮＡＮＡ4wbqCmzWkT
ETag: "534231200_1732277116"136Please respect copyright.ＰＥＮＡＮＡeRWKC5hn9K
Content-Length: 246513056136Please respect copyright.ＰＥＮＡＮＡERqIg42UmR
Content-Type: application/octet-stream136Please respect copyright.ＰＥＮＡＮＡJHAA7h2ui3
Content-Disposition: attachment; filename="zen.iso"
(http body is the remain file content)136Please respect copyright.ＰＥＮＡＮＡfuoZGFjW5A
-------------------------------------------
If the etag changes, response 200 from beginning like this:136Please respect copyright.ＰＥＮＡＮＡozxLmZuMgW
-----------http response from the beginning------136Please respect copyright.ＰＥＮＡＮＡJbPS1up8L6
HTTP/1.1 200 OK136Please respect copyright.ＰＥＮＡＮＡwNqFDNRPwN
Date: Mon, 25 Nov 2024 10:56:12 GMT136Please respect copyright.ＰＥＮＡＮＡpE1t4mR572
accept-ranges: bytes136Please respect copyright.ＰＥＮＡＮＡDgQDdUNu8R
ETag: "534231200_1732277116"136Please respect copyright.ＰＥＮＡＮＡZ8gFDcDkal
Content-Length: 534231200136Please respect copyright.ＰＥＮＡＮＡktXzIUoXRp
Content-Type: application/octet-stream136Please respect copyright.ＰＥＮＡＮＡCmCs50NZrs
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)136Please respect copyright.ＰＥＮＡＮＡ0T89cecupt
---------------------------------------------------
136Please respect copyright.ＰＥＮＡＮＡL7hnA0xU0I
Section X. Thanks
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/If-None-Match136Please respect copyright.ＰＥＮＡＮＡae2JNYU7dk
https://blog.csdn.net/catoop/article/details/134174653136Please respect copyright.ＰＥＮＡＮＡSGfZlaLtdW
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]136Please respect copyright.ＰＥＮＡＮＡXjDRFxnR1T
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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