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.
190Please respect copyright.ＰＥＮＡＮＡsLAA2iyWqf
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:190Please respect copyright.ＰＥＮＡＮＡIgI7rXDgDx
---------------http response with etag----------------190Please respect copyright.ＰＥＮＡＮＡ5XqlL6VUmm
HTTP/1.1 200 OK190Please respect copyright.ＰＥＮＡＮＡGMDiDfYFNP
Date: Mon, 25 Nov 2024 10:56:12 GMT190Please respect copyright.ＰＥＮＡＮＡOuHAdRLvRK
accept-ranges: bytes190Please respect copyright.ＰＥＮＡＮＡ6amAh18fk8
ETag: "534231200_1732277116"190Please respect copyright.ＰＥＮＡＮＡU6qcroEvaG
Content-Length: 534231200190Please respect copyright.ＰＥＮＡＮＡZxkpwvpTjg
Content-Type: application/octet-stream190Please respect copyright.ＰＥＮＡＮＡhlMtSSTWdn
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)190Please respect copyright.ＰＥＮＡＮＡkRoCH02lcf
-----------------------------------------------------
190Please respect copyright.ＰＥＮＡＮＡQCw4oy9s5L
Section B. if-none-match
The browser send back the etag to ask server if it has been changed by header like this:190Please respect copyright.ＰＥＮＡＮＡyZjI63PAgV
-----http request for a cached resource------190Please respect copyright.ＰＥＮＡＮＡsPa6lTuGwD
GET /zen.iso HTTP/1.1190Please respect copyright.ＰＥＮＡＮＡlEkySRmlSo
if-none-match: "534231200_1732277116"190Please respect copyright.ＰＥＮＡＮＡtgK6omrZep
---------------------------------------------
If the etag is unchanged, the server response like this:
1. for get and head requests:190Please respect copyright.ＰＥＮＡＮＡThXWiiyel8
-----------http response for unchanged resource------190Please respect copyright.ＰＥＮＡＮＡWxhEekUHZc
HTTP/1.1 304 Not Modified
(http body canbe empty, the client will used its cache)190Please respect copyright.ＰＥＮＡＮＡOXu6c8pUJl
----------------------------------------------------
2. for other methods requests:190Please respect copyright.ＰＥＮＡＮＡlEa0dwJkAb
-----------http response for unchanged resource------190Please respect copyright.ＰＥＮＡＮＡldsZ3Yguy6
HTTP/1.1 412 Precondition Failed
(http body canbe empty, the client will used its cache)190Please respect copyright.ＰＥＮＡＮＡV0zkoaDBAa
----------------------------------------------------
190Please respect copyright.ＰＥＮＡＮＡ65WMIjvp37
If the etag changes, the server response 200 OK with the new file content like this:190Please respect copyright.ＰＥＮＡＮＡFzTc07qR5h
---------------http response with etag----------------190Please respect copyright.ＰＥＮＡＮＡqD17sG3JMR
HTTP/1.1 200 OK190Please respect copyright.ＰＥＮＡＮＡZBahAr8WKq
Date: Mon, 25 Nov 2024 10:56:12 GMT190Please respect copyright.ＰＥＮＡＮＡyTb0Iubp16
accept-ranges: bytes190Please respect copyright.ＰＥＮＡＮＡk9cWSH5Ghj
ETag: "534231200_1732277116"190Please respect copyright.ＰＥＮＡＮＡNM5okyyP2D
Content-Length: 534231200190Please respect copyright.ＰＥＮＡＮＡquaQg4qyD8
Content-Type: application/octet-stream190Please respect copyright.ＰＥＮＡＮＡK4alFpTW4U
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)190Please respect copyright.ＰＥＮＡＮＡG5ygQbyoGm
-----------------------------------------------------
190Please respect copyright.ＰＥＮＡＮＡaW2C2QxKYm
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:190Please respect copyright.ＰＥＮＡＮＡVxTCYXdVZC
------resume download request-------190Please respect copyright.ＰＥＮＡＮＡNhucTikQlc
GET /zen.iso HTTP/1.1190Please respect copyright.ＰＥＮＡＮＡiJUXqvfy9h
Range: bytes=287718144-190Please respect copyright.ＰＥＮＡＮＡMVY79ccMFd
If-Match: "534231200_1732277116"190Please respect copyright.ＰＥＮＡＮＡ2ROtGA19PY
-----------------------------------190Please respect copyright.ＰＥＮＡＮＡoYKGQs0uaJ
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:190Please respect copyright.ＰＥＮＡＮＡCRVaOZcSMg
-------------range response-------------------190Please respect copyright.ＰＥＮＡＮＡ13jiAk1gog
HTTP/1.1 206 Partial Content190Please respect copyright.ＰＥＮＡＮＡOItMNAKcOG
accept-ranges: bytes190Please respect copyright.ＰＥＮＡＮＡLaCwB5CTeW
content-range: bytes 287718144-534231199/534231200190Please respect copyright.ＰＥＮＡＮＡZuI94dpNbI
ETag: "534231200_1732277116"190Please respect copyright.ＰＥＮＡＮＡKyuEYo33K9
Content-Length: 246513056190Please respect copyright.ＰＥＮＡＮＡKwL9GNEZlh
Content-Type: application/octet-stream190Please respect copyright.ＰＥＮＡＮＡvv1vrDPUw6
Content-Disposition: attachment; filename="zen.iso"
(http body is the remain file content)190Please respect copyright.ＰＥＮＡＮＡvluhRO2RD8
-------------------------------------------
If the etag changes, response 412 like this:190Please respect copyright.ＰＥＮＡＮＡy8LcmhPEVI
-----------http response for changed resource------190Please respect copyright.ＰＥＮＡＮＡajML6Uphif
HTTP/1.1 412 Precondition Failed
(http body canbe empty, the client will stop downloading the resumed file)190Please respect copyright.ＰＥＮＡＮＡ1oJ897xQOp
---------------------------------------------------
190Please respect copyright.ＰＥＮＡＮＡrcWYrdmSa7
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---190Please respect copyright.ＰＥＮＡＮＡqn2yHr2W7N
GET /zen.iso HTTP/1.1190Please respect copyright.ＰＥＮＡＮＡLIet19lN2p
Range: bytes=287718144-190Please respect copyright.ＰＥＮＡＮＡ9JeaUEPMzz
If-Range: "534231200_1732277116"190Please respect copyright.ＰＥＮＡＮＡvXSl7N9Dqm
------------------------------------190Please respect copyright.ＰＥＮＡＮＡ3LYH9uKRpx
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:190Please respect copyright.ＰＥＮＡＮＡu8N2WiVcuo
-------------range response-------------------190Please respect copyright.ＰＥＮＡＮＡaCpYMhyyQP
HTTP/1.1 206 Partial Content190Please respect copyright.ＰＥＮＡＮＡCFccdHZ91v
accept-ranges: bytes190Please respect copyright.ＰＥＮＡＮＡAnrVb30JUQ
content-range: bytes 287718144-534231199/534231200190Please respect copyright.ＰＥＮＡＮＡ66gc5Q72AW
ETag: "534231200_1732277116"190Please respect copyright.ＰＥＮＡＮＡknUtUz54Fy
Content-Length: 246513056190Please respect copyright.ＰＥＮＡＮＡ1qvHBYiAsR
Content-Type: application/octet-stream190Please respect copyright.ＰＥＮＡＮＡ3sL2wglSEq
Content-Disposition: attachment; filename="zen.iso"
(http body is the remain file content)190Please respect copyright.ＰＥＮＡＮＡJ1WVM2PCm5
-------------------------------------------
If the etag changes, response 200 from beginning like this:190Please respect copyright.ＰＥＮＡＮＡhCutIvtJAw
-----------http response from the beginning------190Please respect copyright.ＰＥＮＡＮＡF5Ek7cnD9X
HTTP/1.1 200 OK190Please respect copyright.ＰＥＮＡＮＡcsKKTKhmWo
Date: Mon, 25 Nov 2024 10:56:12 GMT190Please respect copyright.ＰＥＮＡＮＡhZB38eY5Sj
accept-ranges: bytes190Please respect copyright.ＰＥＮＡＮＡ15o0LWlvDL
ETag: "534231200_1732277116"190Please respect copyright.ＰＥＮＡＮＡzRGFQMgO0a
Content-Length: 534231200190Please respect copyright.ＰＥＮＡＮＡTq0jqofZjJ
Content-Type: application/octet-stream190Please respect copyright.ＰＥＮＡＮＡilqTQzptcf
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)190Please respect copyright.ＰＥＮＡＮＡQiFc4ZlzDG
---------------------------------------------------
190Please respect copyright.ＰＥＮＡＮＡo50hb5njQn
Section X. Thanks
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/If-None-Match190Please respect copyright.ＰＥＮＡＮＡmAqyWY3zpX
https://blog.csdn.net/catoop/article/details/134174653190Please respect copyright.ＰＥＮＡＮＡAshs35UFDi
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]190Please respect copyright.ＰＥＮＡＮＡdC536Xddkn
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]
如果您對C/C++ Programming, Linux, HTTP Protocol, Website Development, Vue, Git, VsCode感興趣，邀請您加入「Linux/C/C++ Website Development」微信群，請加我的微信（si_jinmin）以便拉您进群。
ns216.73.216.113da2
然後按「加至主畫面」