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.
179Please respect copyright.PENANAAa60f9b5MC
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:179Please respect copyright.PENANANndk5sBRzN
---------------http response with etag----------------179Please respect copyright.PENANAriYAAF7eoo
HTTP/1.1 200 OK179Please respect copyright.PENANAIqkn1i7rGD
Date: Mon, 25 Nov 2024 10:56:12 GMT179Please respect copyright.PENANAt9goX0HPyY
accept-ranges: bytes179Please respect copyright.PENANAbPJottgQ7B
ETag: "534231200_1732277116"179Please respect copyright.PENANArvgMP1hCJP
Content-Length: 534231200179Please respect copyright.PENANAzE872pXv4H
Content-Type: application/octet-stream179Please respect copyright.PENANALWoWkdrvva
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)179Please respect copyright.PENANAGkivQPzcbY
-----------------------------------------------------
179Please respect copyright.PENANARH5uOPXDEd
Section B. if-none-match
The browser send back the etag to ask server if it has been changed by header like this:179Please respect copyright.PENANAQwaKb0e0K9
-----http request for a cached resource------179Please respect copyright.PENANAarcOr65DFU
GET /zen.iso HTTP/1.1179Please respect copyright.PENANAZMLYELFiZk
if-none-match: "534231200_1732277116"179Please respect copyright.PENANAXUXeYwO5sX
---------------------------------------------
If the etag is unchanged, the server response like this:
1. for get and head requests:179Please respect copyright.PENANA4Bim8DYGaB
-----------http response for unchanged resource------179Please respect copyright.PENANAMOpKVbGIRU
HTTP/1.1 304 Not Modified
(http body canbe empty, the client will used its cache)179Please respect copyright.PENANATSMlPCfSl5
----------------------------------------------------
2. for other methods requests:179Please respect copyright.PENANAMjeXPsUDCu
-----------http response for unchanged resource------179Please respect copyright.PENANAIagFIM68KI
HTTP/1.1 412 Precondition Failed
(http body canbe empty, the client will used its cache)179Please respect copyright.PENANAp37MIJJtra
----------------------------------------------------
179Please respect copyright.PENANAHznvbNn2At
If the etag changes, the server response 200 OK with the new file content like this:179Please respect copyright.PENANAJabvP3R4sY
---------------http response with etag----------------179Please respect copyright.PENANAceUO5OXx5d
HTTP/1.1 200 OK179Please respect copyright.PENANAFhthpfouT3
Date: Mon, 25 Nov 2024 10:56:12 GMT179Please respect copyright.PENANAhyzphiEEgE
accept-ranges: bytes179Please respect copyright.PENANAYRMFV1BqRg
ETag: "534231200_1732277116"179Please respect copyright.PENANAkCIC4lXnSM
Content-Length: 534231200179Please respect copyright.PENANADcoyZrD17s
Content-Type: application/octet-stream179Please respect copyright.PENANA00d8941pP7
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)179Please respect copyright.PENANAAxy5WJhagS
-----------------------------------------------------
179Please respect copyright.PENANAQqHdpRUYg1
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:179Please respect copyright.PENANAHDjWLuzYWO
------resume download request-------179Please respect copyright.PENANAcd3v0ZyMBL
GET /zen.iso HTTP/1.1179Please respect copyright.PENANArY7ZPDZcdw
Range: bytes=287718144-179Please respect copyright.PENANAizrUtTDcPc
If-Match: "534231200_1732277116"179Please respect copyright.PENANAIshUsXfUqq
-----------------------------------179Please respect copyright.PENANAMKITb47gcY
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:179Please respect copyright.PENANAIxfxYa0V1g
-------------range response-------------------179Please respect copyright.PENANANzTzpIGMzC
HTTP/1.1 206 Partial Content179Please respect copyright.PENANAcKF40xsyeW
accept-ranges: bytes179Please respect copyright.PENANAu8Ls8lkfbo
content-range: bytes 287718144-534231199/534231200179Please respect copyright.PENANA6pMFvYpJ7V
ETag: "534231200_1732277116"179Please respect copyright.PENANAFi46Jzcvax
Content-Length: 246513056179Please respect copyright.PENANALeAMEJLKl7
Content-Type: application/octet-stream179Please respect copyright.PENANAziJbVxQaIP
Content-Disposition: attachment; filename="zen.iso"
(http body is the remain file content)179Please respect copyright.PENANAE7ljpnqvc0
-------------------------------------------
If the etag changes, response 412 like this:179Please respect copyright.PENANAT8bgz7WXFS
-----------http response for changed resource------179Please respect copyright.PENANAKSBOGYdZLI
HTTP/1.1 412 Precondition Failed
(http body canbe empty, the client will stop downloading the resumed file)179Please respect copyright.PENANAP5Xw1XErlN
---------------------------------------------------
179Please respect copyright.PENANAGQnZpKUd8T
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---179Please respect copyright.PENANAQDNQaNrcSj
GET /zen.iso HTTP/1.1179Please respect copyright.PENANAUDdJ6rVn9z
Range: bytes=287718144-179Please respect copyright.PENANA5J9xBcrsGH
If-Range: "534231200_1732277116"179Please respect copyright.PENANAmalOjSL7sC
------------------------------------179Please respect copyright.PENANAQiyfMIcZj5
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:179Please respect copyright.PENANAVIHJTYxPJx
-------------range response-------------------179Please respect copyright.PENANAJB9W5O5Xjq
HTTP/1.1 206 Partial Content179Please respect copyright.PENANAY39xMHgYaB
accept-ranges: bytes179Please respect copyright.PENANA5AEUxRzqsE
content-range: bytes 287718144-534231199/534231200179Please respect copyright.PENANAuwrj0JlMIy
ETag: "534231200_1732277116"179Please respect copyright.PENANATo23S7rKND
Content-Length: 246513056179Please respect copyright.PENANAjc3vMNcGhl
Content-Type: application/octet-stream179Please respect copyright.PENANAf57qOIKpfF
Content-Disposition: attachment; filename="zen.iso"
(http body is the remain file content)179Please respect copyright.PENANAuApPrgBWSx
-------------------------------------------
If the etag changes, response 200 from beginning like this:179Please respect copyright.PENANARiwIxtnomV
-----------http response from the beginning------179Please respect copyright.PENANAxez8HELhVp
HTTP/1.1 200 OK179Please respect copyright.PENANAJAr3Wj3TZj
Date: Mon, 25 Nov 2024 10:56:12 GMT179Please respect copyright.PENANAqKFcTBCpWX
accept-ranges: bytes179Please respect copyright.PENANAPEecLuhl93
ETag: "534231200_1732277116"179Please respect copyright.PENANAg8eP2Bo8SJ
Content-Length: 534231200179Please respect copyright.PENANAR97u0hNCYz
Content-Type: application/octet-stream179Please respect copyright.PENANAnGXtpsHfir
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)179Please respect copyright.PENANAdqAEN1WEWO
---------------------------------------------------
179Please respect copyright.PENANAnFlzm0HETR
Section X. Thanks
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/If-None-Match179Please respect copyright.PENANAzh7hc6Tjs5
https://blog.csdn.net/catoop/article/details/134174653179Please respect copyright.PENANA2K8R9or8Pv
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]179Please respect copyright.PENANAF4HY71Lnjd
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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