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.
180Please respect copyright.ＰＥＮＡＮＡU9fk0yUs4e
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:180Please respect copyright.ＰＥＮＡＮＡnpkrqZ9Rs0
---------------http response with etag----------------180Please respect copyright.ＰＥＮＡＮＡsd5c7nWmQG
HTTP/1.1 200 OK180Please respect copyright.ＰＥＮＡＮＡJkDIBxqr33
Date: Mon, 25 Nov 2024 10:56:12 GMT180Please respect copyright.ＰＥＮＡＮＡnA53Zxp7T1
accept-ranges: bytes180Please respect copyright.ＰＥＮＡＮＡ8qSkIRWCr6
ETag: "534231200_1732277116"180Please respect copyright.ＰＥＮＡＮＡ5wqVeBjW6G
Content-Length: 534231200180Please respect copyright.ＰＥＮＡＮＡZo5OW08OcM
Content-Type: application/octet-stream180Please respect copyright.ＰＥＮＡＮＡqwzAv0j5qn
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)180Please respect copyright.ＰＥＮＡＮＡAd6Z2J2my1
-----------------------------------------------------
180Please respect copyright.ＰＥＮＡＮＡUadtQTkNQ3
Section B. if-none-match
The browser send back the etag to ask server if it has been changed by header like this:180Please respect copyright.ＰＥＮＡＮＡHfJTi72hxU
-----http request for a cached resource------180Please respect copyright.ＰＥＮＡＮＡlIqkanAq14
GET /zen.iso HTTP/1.1180Please respect copyright.ＰＥＮＡＮＡHPe8moiTCp
if-none-match: "534231200_1732277116"180Please respect copyright.ＰＥＮＡＮＡJQhO01r7ak
---------------------------------------------
If the etag is unchanged, the server response like this:
1. for get and head requests:180Please respect copyright.ＰＥＮＡＮＡJmrpfS7dCE
-----------http response for unchanged resource------180Please respect copyright.ＰＥＮＡＮＡT8xPZV0bao
HTTP/1.1 304 Not Modified
(http body canbe empty, the client will used its cache)180Please respect copyright.ＰＥＮＡＮＡICUWThENh8
----------------------------------------------------
2. for other methods requests:180Please respect copyright.ＰＥＮＡＮＡhO3GxiKBHp
-----------http response for unchanged resource------180Please respect copyright.ＰＥＮＡＮＡYHqIUDKZKS
HTTP/1.1 412 Precondition Failed
(http body canbe empty, the client will used its cache)180Please respect copyright.ＰＥＮＡＮＡoNJYDDogOW
----------------------------------------------------
180Please respect copyright.ＰＥＮＡＮＡY9Zj2K66zJ
If the etag changes, the server response 200 OK with the new file content like this:180Please respect copyright.ＰＥＮＡＮＡRgbMA4zIcI
---------------http response with etag----------------180Please respect copyright.ＰＥＮＡＮＡtfJCoVzlnP
HTTP/1.1 200 OK180Please respect copyright.ＰＥＮＡＮＡBaU02Wi175
Date: Mon, 25 Nov 2024 10:56:12 GMT180Please respect copyright.ＰＥＮＡＮＡu8reUeVbjj
accept-ranges: bytes180Please respect copyright.ＰＥＮＡＮＡU2jrYMCCqG
ETag: "534231200_1732277116"180Please respect copyright.ＰＥＮＡＮＡjo8otbg4RA
Content-Length: 534231200180Please respect copyright.ＰＥＮＡＮＡsxfRlHjCfj
Content-Type: application/octet-stream180Please respect copyright.ＰＥＮＡＮＡJlYGEszRnW
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)180Please respect copyright.ＰＥＮＡＮＡeAAvsRUfUz
-----------------------------------------------------
180Please respect copyright.ＰＥＮＡＮＡt9THRkdd1D
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:180Please respect copyright.ＰＥＮＡＮＡw8oNAVB3eO
------resume download request-------180Please respect copyright.ＰＥＮＡＮＡZQRjNB04vg
GET /zen.iso HTTP/1.1180Please respect copyright.ＰＥＮＡＮＡJqiHMJbiXS
Range: bytes=287718144-180Please respect copyright.ＰＥＮＡＮＡXFvaJFi0fc
If-Match: "534231200_1732277116"180Please respect copyright.ＰＥＮＡＮＡB6KFGOEbTL
-----------------------------------180Please respect copyright.ＰＥＮＡＮＡPSgzJyATKU
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:180Please respect copyright.ＰＥＮＡＮＡBRFMNVhAKR
-------------range response-------------------180Please respect copyright.ＰＥＮＡＮＡnixL4q3yDF
HTTP/1.1 206 Partial Content180Please respect copyright.ＰＥＮＡＮＡXUTrwpTyt1
accept-ranges: bytes180Please respect copyright.ＰＥＮＡＮＡr8n5tNWroJ
content-range: bytes 287718144-534231199/534231200180Please respect copyright.ＰＥＮＡＮＡDhhn741BUh
ETag: "534231200_1732277116"180Please respect copyright.ＰＥＮＡＮＡa8lJPD02Kt
Content-Length: 246513056180Please respect copyright.ＰＥＮＡＮＡsPN8pf76S7
Content-Type: application/octet-stream180Please respect copyright.ＰＥＮＡＮＡ5BPn5Jqy8X
Content-Disposition: attachment; filename="zen.iso"
(http body is the remain file content)180Please respect copyright.ＰＥＮＡＮＡiFbl5rE1FA
-------------------------------------------
If the etag changes, response 412 like this:180Please respect copyright.ＰＥＮＡＮＡV5iNUv0xrk
-----------http response for changed resource------180Please respect copyright.ＰＥＮＡＮＡeavofdYP3u
HTTP/1.1 412 Precondition Failed
(http body canbe empty, the client will stop downloading the resumed file)180Please respect copyright.ＰＥＮＡＮＡD7m0JNwPPD
---------------------------------------------------
180Please respect copyright.ＰＥＮＡＮＡOpoAZ3DIJe
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---180Please respect copyright.ＰＥＮＡＮＡKh5uNOlFpf
GET /zen.iso HTTP/1.1180Please respect copyright.ＰＥＮＡＮＡH58nOd4bN3
Range: bytes=287718144-180Please respect copyright.ＰＥＮＡＮＡfO1hprNSs4
If-Range: "534231200_1732277116"180Please respect copyright.ＰＥＮＡＮＡn2NYKhnZ6l
------------------------------------180Please respect copyright.ＰＥＮＡＮＡleIESbZ3EJ
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:180Please respect copyright.ＰＥＮＡＮＡummyRTc8Cs
-------------range response-------------------180Please respect copyright.ＰＥＮＡＮＡovTRn5zicX
HTTP/1.1 206 Partial Content180Please respect copyright.ＰＥＮＡＮＡLJmsqJlGeg
accept-ranges: bytes180Please respect copyright.ＰＥＮＡＮＡUvNpEG6t8E
content-range: bytes 287718144-534231199/534231200180Please respect copyright.ＰＥＮＡＮＡM3aFCLfx1q
ETag: "534231200_1732277116"180Please respect copyright.ＰＥＮＡＮＡ65XqvMExaG
Content-Length: 246513056180Please respect copyright.ＰＥＮＡＮＡPXHE6IYx6T
Content-Type: application/octet-stream180Please respect copyright.ＰＥＮＡＮＡt8Q2WNtkoo
Content-Disposition: attachment; filename="zen.iso"
(http body is the remain file content)180Please respect copyright.ＰＥＮＡＮＡwCWiQPYNd5
-------------------------------------------
If the etag changes, response 200 from beginning like this:180Please respect copyright.ＰＥＮＡＮＡJTxiwtSv8F
-----------http response from the beginning------180Please respect copyright.ＰＥＮＡＮＡa2QRCkFvk1
HTTP/1.1 200 OK180Please respect copyright.ＰＥＮＡＮＡH2laLTQv5C
Date: Mon, 25 Nov 2024 10:56:12 GMT180Please respect copyright.ＰＥＮＡＮＡMMPp7caZUy
accept-ranges: bytes180Please respect copyright.ＰＥＮＡＮＡQ3qyYjmsct
ETag: "534231200_1732277116"180Please respect copyright.ＰＥＮＡＮＡ0uWF9n3ZIU
Content-Length: 534231200180Please respect copyright.ＰＥＮＡＮＡIy4ODHE4cU
Content-Type: application/octet-stream180Please respect copyright.ＰＥＮＡＮＡ8cnHDNYTKr
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)180Please respect copyright.ＰＥＮＡＮＡB6yZoI0Zjg
---------------------------------------------------
180Please respect copyright.ＰＥＮＡＮＡPAOhNiSuRr
Section X. Thanks
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/If-None-Match180Please respect copyright.ＰＥＮＡＮＡ3N9zyPtYEx
https://blog.csdn.net/catoop/article/details/134174653180Please respect copyright.ＰＥＮＡＮＡdYh1XMdoi7
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]180Please respect copyright.ＰＥＮＡＮＡbGszCnFAQD
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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