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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