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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