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.
211Please respect copyright.ＰＥＮＡＮＡMZSc7xq6DE
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:211Please respect copyright.ＰＥＮＡＮＡNuF1hSQHAJ
---------------http response with etag----------------211Please respect copyright.ＰＥＮＡＮＡRgmL2N2jN7
HTTP/1.1 200 OK211Please respect copyright.ＰＥＮＡＮＡC0kkQKovHn
Date: Mon, 25 Nov 2024 10:56:12 GMT211Please respect copyright.ＰＥＮＡＮＡOn1mZkKi4G
accept-ranges: bytes211Please respect copyright.ＰＥＮＡＮＡwgQFmyuwHU
ETag: "534231200_1732277116"211Please respect copyright.ＰＥＮＡＮＡdqKpxcmYjw
Content-Length: 534231200211Please respect copyright.ＰＥＮＡＮＡF1nH0NyAxh
Content-Type: application/octet-stream211Please respect copyright.ＰＥＮＡＮＡemRIekznnt
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)211Please respect copyright.ＰＥＮＡＮＡ3AjaY5nhM9
-----------------------------------------------------
211Please respect copyright.ＰＥＮＡＮＡIV2RwB0ZbJ
Section B. if-none-match
The browser send back the etag to ask server if it has been changed by header like this:211Please respect copyright.ＰＥＮＡＮＡHhaFWCko36
-----http request for a cached resource------211Please respect copyright.ＰＥＮＡＮＡfsJpouOQjb
GET /zen.iso HTTP/1.1211Please respect copyright.ＰＥＮＡＮＡWzDNMY6ChO
if-none-match: "534231200_1732277116"211Please respect copyright.ＰＥＮＡＮＡnMHmUKSkXD
---------------------------------------------
If the etag is unchanged, the server response like this:
1. for get and head requests:211Please respect copyright.ＰＥＮＡＮＡQ2146yQsCQ
-----------http response for unchanged resource------211Please respect copyright.ＰＥＮＡＮＡz78XB5veXw
HTTP/1.1 304 Not Modified
(http body canbe empty, the client will used its cache)211Please respect copyright.ＰＥＮＡＮＡHWLqaKtb96
----------------------------------------------------
2. for other methods requests:211Please respect copyright.ＰＥＮＡＮＡPJC5PUhWI2
-----------http response for unchanged resource------211Please respect copyright.ＰＥＮＡＮＡiOYfZfyRgB
HTTP/1.1 412 Precondition Failed
(http body canbe empty, the client will used its cache)211Please respect copyright.ＰＥＮＡＮＡjGLypREWL4
----------------------------------------------------
211Please respect copyright.ＰＥＮＡＮＡonkaWegEVY
If the etag changes, the server response 200 OK with the new file content like this:211Please respect copyright.ＰＥＮＡＮＡwOlkRG09iv
---------------http response with etag----------------211Please respect copyright.ＰＥＮＡＮＡ94Qzll7540
HTTP/1.1 200 OK211Please respect copyright.ＰＥＮＡＮＡyRZzDVDFlr
Date: Mon, 25 Nov 2024 10:56:12 GMT211Please respect copyright.ＰＥＮＡＮＡACAnmuVXhz
accept-ranges: bytes211Please respect copyright.ＰＥＮＡＮＡQ4zDCTHdMR
ETag: "534231200_1732277116"211Please respect copyright.ＰＥＮＡＮＡqEPEsFr95v
Content-Length: 534231200211Please respect copyright.ＰＥＮＡＮＡQqpYacDwm5
Content-Type: application/octet-stream211Please respect copyright.ＰＥＮＡＮＡKSlJHa8uwA
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)211Please respect copyright.ＰＥＮＡＮＡdnwiG8bwsX
-----------------------------------------------------
211Please respect copyright.ＰＥＮＡＮＡl8ni7OXfDT
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:211Please respect copyright.ＰＥＮＡＮＡxbFVDBFAb7
------resume download request-------211Please respect copyright.ＰＥＮＡＮＡvk4OxBHKUh
GET /zen.iso HTTP/1.1211Please respect copyright.ＰＥＮＡＮＡniTjwPPYOW
Range: bytes=287718144-211Please respect copyright.ＰＥＮＡＮＡ9oecMlsxxp
If-Match: "534231200_1732277116"211Please respect copyright.ＰＥＮＡＮＡRFeE0aZtnG
-----------------------------------211Please respect copyright.ＰＥＮＡＮＡrh5QLTbgiS
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:211Please respect copyright.ＰＥＮＡＮＡdLWZftShXs
-------------range response-------------------211Please respect copyright.ＰＥＮＡＮＡg5Gg75N6Ae
HTTP/1.1 206 Partial Content211Please respect copyright.ＰＥＮＡＮＡsUMl9RedvO
accept-ranges: bytes211Please respect copyright.ＰＥＮＡＮＡA6Od7ulEcT
content-range: bytes 287718144-534231199/534231200211Please respect copyright.ＰＥＮＡＮＡHSrxaUhNLn
ETag: "534231200_1732277116"211Please respect copyright.ＰＥＮＡＮＡw7c3ZGaYKG
Content-Length: 246513056211Please respect copyright.ＰＥＮＡＮＡjVDlHHFkzZ
Content-Type: application/octet-stream211Please respect copyright.ＰＥＮＡＮＡKBwBTE8WQw
Content-Disposition: attachment; filename="zen.iso"
(http body is the remain file content)211Please respect copyright.ＰＥＮＡＮＡmjfIeGuAUe
-------------------------------------------
If the etag changes, response 412 like this:211Please respect copyright.ＰＥＮＡＮＡPUrOh1mNMS
-----------http response for changed resource------211Please respect copyright.ＰＥＮＡＮＡYsWWoBiVx3
HTTP/1.1 412 Precondition Failed
(http body canbe empty, the client will stop downloading the resumed file)211Please respect copyright.ＰＥＮＡＮＡXiwqxsr1Ze
---------------------------------------------------
211Please respect copyright.ＰＥＮＡＮＡqx85Kz5otl
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---211Please respect copyright.ＰＥＮＡＮＡhH2ZSKrdJC
GET /zen.iso HTTP/1.1211Please respect copyright.ＰＥＮＡＮＡpMolhTMfNl
Range: bytes=287718144-211Please respect copyright.ＰＥＮＡＮＡkWG0uyfJsJ
If-Range: "534231200_1732277116"211Please respect copyright.ＰＥＮＡＮＡBudNzPUQgW
------------------------------------211Please respect copyright.ＰＥＮＡＮＡZBf2AKu2K6
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:211Please respect copyright.ＰＥＮＡＮＡ5bUtag8fzU
-------------range response-------------------211Please respect copyright.ＰＥＮＡＮＡMuetDAmb0y
HTTP/1.1 206 Partial Content211Please respect copyright.ＰＥＮＡＮＡX2aEhESHDs
accept-ranges: bytes211Please respect copyright.ＰＥＮＡＮＡJgBhhm4wGE
content-range: bytes 287718144-534231199/534231200211Please respect copyright.ＰＥＮＡＮＡGO6pOfWUvv
ETag: "534231200_1732277116"211Please respect copyright.ＰＥＮＡＮＡMv691kHOsa
Content-Length: 246513056211Please respect copyright.ＰＥＮＡＮＡnSEW4PUhPn
Content-Type: application/octet-stream211Please respect copyright.ＰＥＮＡＮＡ1dKx6gdxIF
Content-Disposition: attachment; filename="zen.iso"
(http body is the remain file content)211Please respect copyright.ＰＥＮＡＮＡMkPQ5cMt1N
-------------------------------------------
If the etag changes, response 200 from beginning like this:211Please respect copyright.ＰＥＮＡＮＡconzCtoByG
-----------http response from the beginning------211Please respect copyright.ＰＥＮＡＮＡ3kkraFJlCN
HTTP/1.1 200 OK211Please respect copyright.ＰＥＮＡＮＡcCdZbYF4hD
Date: Mon, 25 Nov 2024 10:56:12 GMT211Please respect copyright.ＰＥＮＡＮＡlxxBDI2e5R
accept-ranges: bytes211Please respect copyright.ＰＥＮＡＮＡQUX47es7qM
ETag: "534231200_1732277116"211Please respect copyright.ＰＥＮＡＮＡx8gIsoAVvD
Content-Length: 534231200211Please respect copyright.ＰＥＮＡＮＡpZSnlbVS2o
Content-Type: application/octet-stream211Please respect copyright.ＰＥＮＡＮＡlm7Hv0Is7c
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)211Please respect copyright.ＰＥＮＡＮＡHBNxcoCeXA
---------------------------------------------------
211Please respect copyright.ＰＥＮＡＮＡmptvn9j4Ls
Section X. Thanks
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/If-None-Match211Please respect copyright.ＰＥＮＡＮＡKTvM63pQ8e
https://blog.csdn.net/catoop/article/details/134174653211Please respect copyright.ＰＥＮＡＮＡEImuSTKPbe
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]211Please respect copyright.ＰＥＮＡＮＡ32pwfrQkV7
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。
ns216.73.216.238da2
