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.
223Please respect copyright.ＰＥＮＡＮＡsFWTh6qT28
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:223Please respect copyright.ＰＥＮＡＮＡmoWXNm94cE
---------------http response with etag----------------223Please respect copyright.ＰＥＮＡＮＡswFlO8ewVW
HTTP/1.1 200 OK223Please respect copyright.ＰＥＮＡＮＡusqxahrQGw
Date: Mon, 25 Nov 2024 10:56:12 GMT223Please respect copyright.ＰＥＮＡＮＡikcGS1QYHV
accept-ranges: bytes223Please respect copyright.ＰＥＮＡＮＡxU893q7x48
ETag: "534231200_1732277116"223Please respect copyright.ＰＥＮＡＮＡGcBidW3CWC
Content-Length: 534231200223Please respect copyright.ＰＥＮＡＮＡqVpQ5uHNgH
Content-Type: application/octet-stream223Please respect copyright.ＰＥＮＡＮＡfgfwXFtcfn
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)223Please respect copyright.ＰＥＮＡＮＡGVgTch1wG0
-----------------------------------------------------
223Please respect copyright.ＰＥＮＡＮＡe0pQ072FmN
Section B. if-none-match
The browser send back the etag to ask server if it has been changed by header like this:223Please respect copyright.ＰＥＮＡＮＡfgeKDoQQEQ
-----http request for a cached resource------223Please respect copyright.ＰＥＮＡＮＡyEkOccD1L5
GET /zen.iso HTTP/1.1223Please respect copyright.ＰＥＮＡＮＡhwCTfVMtAK
if-none-match: "534231200_1732277116"223Please respect copyright.ＰＥＮＡＮＡwOIpBOZPRH
---------------------------------------------
If the etag is unchanged, the server response like this:
1. for get and head requests:223Please respect copyright.ＰＥＮＡＮＡrOV5g5YU5n
-----------http response for unchanged resource------223Please respect copyright.ＰＥＮＡＮＡaFhb38uRp4
HTTP/1.1 304 Not Modified
(http body canbe empty, the client will used its cache)223Please respect copyright.ＰＥＮＡＮＡfTv8zVdQvD
----------------------------------------------------
2. for other methods requests:223Please respect copyright.ＰＥＮＡＮＡ3ARVmkKJyr
-----------http response for unchanged resource------223Please respect copyright.ＰＥＮＡＮＡrWqfq15NOc
HTTP/1.1 412 Precondition Failed
(http body canbe empty, the client will used its cache)223Please respect copyright.ＰＥＮＡＮＡ65W8sMMdoP
----------------------------------------------------
223Please respect copyright.ＰＥＮＡＮＡ3z0ChLNNrs
If the etag changes, the server response 200 OK with the new file content like this:223Please respect copyright.ＰＥＮＡＮＡu1pugZjhat
---------------http response with etag----------------223Please respect copyright.ＰＥＮＡＮＡFNQ5kwO4it
HTTP/1.1 200 OK223Please respect copyright.ＰＥＮＡＮＡHpp7Ae5fe9
Date: Mon, 25 Nov 2024 10:56:12 GMT223Please respect copyright.ＰＥＮＡＮＡau42s7oQ1C
accept-ranges: bytes223Please respect copyright.ＰＥＮＡＮＡ4odhUBJc84
ETag: "534231200_1732277116"223Please respect copyright.ＰＥＮＡＮＡPi8ViiUgn3
Content-Length: 534231200223Please respect copyright.ＰＥＮＡＮＡVhsdAiRSnj
Content-Type: application/octet-stream223Please respect copyright.ＰＥＮＡＮＡRe05P7TN15
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)223Please respect copyright.ＰＥＮＡＮＡVWMQ8fTzdZ
-----------------------------------------------------
223Please respect copyright.ＰＥＮＡＮＡQ6uQbMQlDr
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:223Please respect copyright.ＰＥＮＡＮＡ9OlyVfmS9I
------resume download request-------223Please respect copyright.ＰＥＮＡＮＡY5b5Wnp1gF
GET /zen.iso HTTP/1.1223Please respect copyright.ＰＥＮＡＮＡjNzVaCSdK1
Range: bytes=287718144-223Please respect copyright.ＰＥＮＡＮＡCIZijyfg9H
If-Match: "534231200_1732277116"223Please respect copyright.ＰＥＮＡＮＡXaZwm35sjl
-----------------------------------223Please respect copyright.ＰＥＮＡＮＡ22IrOHE96H
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:223Please respect copyright.ＰＥＮＡＮＡyuTDNZSeyU
-------------range response-------------------223Please respect copyright.ＰＥＮＡＮＡSLcpw9ugH7
HTTP/1.1 206 Partial Content223Please respect copyright.ＰＥＮＡＮＡGjzXfJRY78
accept-ranges: bytes223Please respect copyright.ＰＥＮＡＮＡogOIjqeJEG
content-range: bytes 287718144-534231199/534231200223Please respect copyright.ＰＥＮＡＮＡYeP8mUwcLx
ETag: "534231200_1732277116"223Please respect copyright.ＰＥＮＡＮＡ9kkQl09YX9
Content-Length: 246513056223Please respect copyright.ＰＥＮＡＮＡqlsdgoY0MF
Content-Type: application/octet-stream223Please respect copyright.ＰＥＮＡＮＡcNYTcU4tn8
Content-Disposition: attachment; filename="zen.iso"
(http body is the remain file content)223Please respect copyright.ＰＥＮＡＮＡE1yoBviYHD
-------------------------------------------
If the etag changes, response 412 like this:223Please respect copyright.ＰＥＮＡＮＡh8snsaYapk
-----------http response for changed resource------223Please respect copyright.ＰＥＮＡＮＡ6iUz89CmH9
HTTP/1.1 412 Precondition Failed
(http body canbe empty, the client will stop downloading the resumed file)223Please respect copyright.ＰＥＮＡＮＡLXhpj1tVn5
---------------------------------------------------
223Please respect copyright.ＰＥＮＡＮＡWlihoEb5NK
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---223Please respect copyright.ＰＥＮＡＮＡmellyGWFJw
GET /zen.iso HTTP/1.1223Please respect copyright.ＰＥＮＡＮＡBiLYTECt9K
Range: bytes=287718144-223Please respect copyright.ＰＥＮＡＮＡln7ySUSPPu
If-Range: "534231200_1732277116"223Please respect copyright.ＰＥＮＡＮＡ0HnHjEeMw1
------------------------------------223Please respect copyright.ＰＥＮＡＮＡwpiu7s63wi
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:223Please respect copyright.ＰＥＮＡＮＡRhkMXosVSO
-------------range response-------------------223Please respect copyright.ＰＥＮＡＮＡzIvn9IjPck
HTTP/1.1 206 Partial Content223Please respect copyright.ＰＥＮＡＮＡgEhUZKv2oK
accept-ranges: bytes223Please respect copyright.ＰＥＮＡＮＡbHfrVay10U
content-range: bytes 287718144-534231199/534231200223Please respect copyright.ＰＥＮＡＮＡPMsRh7ExTq
ETag: "534231200_1732277116"223Please respect copyright.ＰＥＮＡＮＡXhcd5c1u3s
Content-Length: 246513056223Please respect copyright.ＰＥＮＡＮＡWKSbE24qwK
Content-Type: application/octet-stream223Please respect copyright.ＰＥＮＡＮＡTJTEE9xNmS
Content-Disposition: attachment; filename="zen.iso"
(http body is the remain file content)223Please respect copyright.ＰＥＮＡＮＡV5TbRhh8UD
-------------------------------------------
If the etag changes, response 200 from beginning like this:223Please respect copyright.ＰＥＮＡＮＡMzW1jVB3tG
-----------http response from the beginning------223Please respect copyright.ＰＥＮＡＮＡORFE5jv5q8
HTTP/1.1 200 OK223Please respect copyright.ＰＥＮＡＮＡlbY3g1fFmC
Date: Mon, 25 Nov 2024 10:56:12 GMT223Please respect copyright.ＰＥＮＡＮＡvVsVajfa3Z
accept-ranges: bytes223Please respect copyright.ＰＥＮＡＮＡacExUQH7sb
ETag: "534231200_1732277116"223Please respect copyright.ＰＥＮＡＮＡlDH1lIUK1U
Content-Length: 534231200223Please respect copyright.ＰＥＮＡＮＡfXG3CXsVeH
Content-Type: application/octet-stream223Please respect copyright.ＰＥＮＡＮＡW5Us3xjMRE
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)223Please respect copyright.ＰＥＮＡＮＡGBOiFsAxsX
---------------------------------------------------
223Please respect copyright.ＰＥＮＡＮＡQ0KzgVSI7t
Section X. Thanks
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/If-None-Match223Please respect copyright.ＰＥＮＡＮＡPJXSyNi20Q
https://blog.csdn.net/catoop/article/details/134174653223Please respect copyright.ＰＥＮＡＮＡiWFTA01zSO
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]223Please respect copyright.ＰＥＮＡＮＡ31AWkbnIkj
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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