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.
184Please respect copyright.PENANAsM75f2vIea
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:184Please respect copyright.PENANAJgxfnOl1OY
---------------http response with etag----------------184Please respect copyright.PENANAZc0YUAR9ez
HTTP/1.1 200 OK184Please respect copyright.PENANAG5Yf1Vm9r1
Date: Mon, 25 Nov 2024 10:56:12 GMT184Please respect copyright.PENANACb0zNAmwwq
accept-ranges: bytes184Please respect copyright.PENANAtuhLQmvEUl
ETag: "534231200_1732277116"184Please respect copyright.PENANAq7jfkWNdYi
Content-Length: 534231200184Please respect copyright.PENANAGXoKyPVm7s
Content-Type: application/octet-stream184Please respect copyright.PENANAxpvGKHEw2e
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)184Please respect copyright.PENANAkwKMVWb5t3
-----------------------------------------------------
184Please respect copyright.PENANAif5ULSTfGk
Section B. if-none-match
The browser send back the etag to ask server if it has been changed by header like this:184Please respect copyright.PENANAYlPaOt57y7
-----http request for a cached resource------184Please respect copyright.PENANAWiyHt9aFBF
GET /zen.iso HTTP/1.1184Please respect copyright.PENANArpIu4p6yWR
if-none-match: "534231200_1732277116"184Please respect copyright.PENANAnNZZ8USqxS
---------------------------------------------
If the etag is unchanged, the server response like this:
1. for get and head requests:184Please respect copyright.PENANAU6bwykP73S
-----------http response for unchanged resource------184Please respect copyright.PENANAs3xXinqf1C
HTTP/1.1 304 Not Modified
(http body canbe empty, the client will used its cache)184Please respect copyright.PENANA1iC46n8zZk
----------------------------------------------------
2. for other methods requests:184Please respect copyright.PENANASl7TTD1H01
-----------http response for unchanged resource------184Please respect copyright.PENANASAtXYS5Ewt
HTTP/1.1 412 Precondition Failed
(http body canbe empty, the client will used its cache)184Please respect copyright.PENANAODDNdpkWsV
----------------------------------------------------
184Please respect copyright.PENANANJaTP0PlA7
If the etag changes, the server response 200 OK with the new file content like this:184Please respect copyright.PENANAUHTOQNh20R
---------------http response with etag----------------184Please respect copyright.PENANAH1llNQBVic
HTTP/1.1 200 OK184Please respect copyright.PENANAIZEN5kgqhn
Date: Mon, 25 Nov 2024 10:56:12 GMT184Please respect copyright.PENANA1YOVOsPVHA
accept-ranges: bytes184Please respect copyright.PENANALeGUlLZ6eH
ETag: "534231200_1732277116"184Please respect copyright.PENANAeKY7tbdGrz
Content-Length: 534231200184Please respect copyright.PENANA1RQTugQuWm
Content-Type: application/octet-stream184Please respect copyright.PENANApilRBpP9yO
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)184Please respect copyright.PENANAqd7ldygxDg
-----------------------------------------------------
184Please respect copyright.PENANAOkBBIZm5q6
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:184Please respect copyright.PENANALkoDeQLvlU
------resume download request-------184Please respect copyright.PENANAp4fXXpM9Tv
GET /zen.iso HTTP/1.1184Please respect copyright.PENANAeA9jKQrDpn
Range: bytes=287718144-184Please respect copyright.PENANAEPgaGNJ4rG
If-Match: "534231200_1732277116"184Please respect copyright.PENANAUrfK3WDfIq
-----------------------------------184Please respect copyright.PENANAifNTve5vkL
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:184Please respect copyright.PENANAItfi2O9NTE
-------------range response-------------------184Please respect copyright.PENANAz8nqWQgrrF
HTTP/1.1 206 Partial Content184Please respect copyright.PENANAiYtzaidRtY
accept-ranges: bytes184Please respect copyright.PENANAiXheSfmP43
content-range: bytes 287718144-534231199/534231200184Please respect copyright.PENANAlMsi2ptdLV
ETag: "534231200_1732277116"184Please respect copyright.PENANAi5f0Qo4l5B
Content-Length: 246513056184Please respect copyright.PENANAkPRf8QF690
Content-Type: application/octet-stream184Please respect copyright.PENANAPNOYnUZdiB
Content-Disposition: attachment; filename="zen.iso"
(http body is the remain file content)184Please respect copyright.PENANAT6xQs70U5A
-------------------------------------------
If the etag changes, response 412 like this:184Please respect copyright.PENANAQX2EjOkYiK
-----------http response for changed resource------184Please respect copyright.PENANAPvS8m2KIsu
HTTP/1.1 412 Precondition Failed
(http body canbe empty, the client will stop downloading the resumed file)184Please respect copyright.PENANADpT0ePavF2
---------------------------------------------------
184Please respect copyright.PENANAUIIpGfuqi3
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---184Please respect copyright.PENANA7C5bTQtoWO
GET /zen.iso HTTP/1.1184Please respect copyright.PENANA2rMFMBpdJ0
Range: bytes=287718144-184Please respect copyright.PENANAZRBFFlzOp4
If-Range: "534231200_1732277116"184Please respect copyright.PENANAMhUACRW2Ha
------------------------------------184Please respect copyright.PENANASVULwnfGzx
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:184Please respect copyright.PENANA0k2vnZ9lHE
-------------range response-------------------184Please respect copyright.PENANA2qkU1sSNMY
HTTP/1.1 206 Partial Content184Please respect copyright.PENANA0a3WTu1Gl2
accept-ranges: bytes184Please respect copyright.PENANAvYcruz8HpM
content-range: bytes 287718144-534231199/534231200184Please respect copyright.PENANAkVVwnYPFer
ETag: "534231200_1732277116"184Please respect copyright.PENANAW86w3Q6P8R
Content-Length: 246513056184Please respect copyright.PENANApEbQkeW9Hi
Content-Type: application/octet-stream184Please respect copyright.PENANAlTh2cGfC1K
Content-Disposition: attachment; filename="zen.iso"
(http body is the remain file content)184Please respect copyright.PENANA1qIJ6Q2gGp
-------------------------------------------
If the etag changes, response 200 from beginning like this:184Please respect copyright.PENANAOWyGs4LyeJ
-----------http response from the beginning------184Please respect copyright.PENANAUI3vXBcpUa
HTTP/1.1 200 OK184Please respect copyright.PENANA4FSmKdXLn5
Date: Mon, 25 Nov 2024 10:56:12 GMT184Please respect copyright.PENANAqHrWJIE0lE
accept-ranges: bytes184Please respect copyright.PENANAZQTlpXZwQo
ETag: "534231200_1732277116"184Please respect copyright.PENANAbZhwh4XJNF
Content-Length: 534231200184Please respect copyright.PENANAdRhbxO9NZG
Content-Type: application/octet-stream184Please respect copyright.PENANA9XdTOImr0h
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)184Please respect copyright.PENANAWxj16nsKf9
---------------------------------------------------
184Please respect copyright.PENANAXiRxhIKlXX
Section X. Thanks
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/If-None-Match184Please respect copyright.PENANAcH4dWaS1Mu
https://blog.csdn.net/catoop/article/details/134174653184Please respect copyright.PENANALfg5FHY8d2
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]184Please respect copyright.PENANAfhbhpB0sju
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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