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.
137Please respect copyright.ＰＥＮＡＮＡ6B0Apbm8Rc
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:137Please respect copyright.ＰＥＮＡＮＡt0Si1aqvsQ
---------------http response with etag----------------137Please respect copyright.ＰＥＮＡＮＡAoWWehDnhn
HTTP/1.1 200 OK137Please respect copyright.ＰＥＮＡＮＡi7vrSd2L0L
Date: Mon, 25 Nov 2024 10:56:12 GMT137Please respect copyright.ＰＥＮＡＮＡ3XgKzrpTn6
accept-ranges: bytes137Please respect copyright.ＰＥＮＡＮＡQ5lwQmt51c
ETag: "534231200_1732277116"137Please respect copyright.ＰＥＮＡＮＡA3pCgMVj2f
Content-Length: 534231200137Please respect copyright.ＰＥＮＡＮＡk2QplNMpNJ
Content-Type: application/octet-stream137Please respect copyright.ＰＥＮＡＮＡXMmnzSd0jk
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)137Please respect copyright.ＰＥＮＡＮＡn27V5R8yMa
-----------------------------------------------------
137Please respect copyright.ＰＥＮＡＮＡi5dbzUV5qT
Section B. if-none-match
The browser send back the etag to ask server if it has been changed by header like this:137Please respect copyright.ＰＥＮＡＮＡJ9dFT9a29N
-----http request for a cached resource------137Please respect copyright.ＰＥＮＡＮＡGPePivd9rM
GET /zen.iso HTTP/1.1137Please respect copyright.ＰＥＮＡＮＡabQ5xDLhRU
if-none-match: "534231200_1732277116"137Please respect copyright.ＰＥＮＡＮＡwuR4BsEYa7
---------------------------------------------
If the etag is unchanged, the server response like this:
1. for get and head requests:137Please respect copyright.ＰＥＮＡＮＡq2SEBUlzsl
-----------http response for unchanged resource------137Please respect copyright.ＰＥＮＡＮＡzwk3mTDAnm
HTTP/1.1 304 Not Modified
(http body canbe empty, the client will used its cache)137Please respect copyright.ＰＥＮＡＮＡfA3GseoE6X
----------------------------------------------------
2. for other methods requests:137Please respect copyright.ＰＥＮＡＮＡ0BQWeqAdy5
-----------http response for unchanged resource------137Please respect copyright.ＰＥＮＡＮＡxn9UeX4WOV
HTTP/1.1 412 Precondition Failed
(http body canbe empty, the client will used its cache)137Please respect copyright.ＰＥＮＡＮＡ7EwTxbaHJL
----------------------------------------------------
137Please respect copyright.ＰＥＮＡＮＡT8buVwkphG
If the etag changes, the server response 200 OK with the new file content like this:137Please respect copyright.ＰＥＮＡＮＡxVEMf8uKQs
---------------http response with etag----------------137Please respect copyright.ＰＥＮＡＮＡwFdUVJDiK4
HTTP/1.1 200 OK137Please respect copyright.ＰＥＮＡＮＡjTrzQgmpAT
Date: Mon, 25 Nov 2024 10:56:12 GMT137Please respect copyright.ＰＥＮＡＮＡjlDVxsgkxw
accept-ranges: bytes137Please respect copyright.ＰＥＮＡＮＡQmw7Inqh2l
ETag: "534231200_1732277116"137Please respect copyright.ＰＥＮＡＮＡ20IPVfuamf
Content-Length: 534231200137Please respect copyright.ＰＥＮＡＮＡXvIYsPvGBB
Content-Type: application/octet-stream137Please respect copyright.ＰＥＮＡＮＡqRQMeLbLbJ
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)137Please respect copyright.ＰＥＮＡＮＡKqsTHmRrYm
-----------------------------------------------------
137Please respect copyright.ＰＥＮＡＮＡ03KuiF9lkB
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:137Please respect copyright.ＰＥＮＡＮＡBDoeYl2o6A
------resume download request-------137Please respect copyright.ＰＥＮＡＮＡjOQZhwq6I7
GET /zen.iso HTTP/1.1137Please respect copyright.ＰＥＮＡＮＡttz0Sh6AID
Range: bytes=287718144-137Please respect copyright.ＰＥＮＡＮＡiH0xsy09pv
If-Match: "534231200_1732277116"137Please respect copyright.ＰＥＮＡＮＡ8cBzTTcaEG
-----------------------------------137Please respect copyright.ＰＥＮＡＮＡFQccczePUI
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:137Please respect copyright.ＰＥＮＡＮＡikDcTUR1JH
-------------range response-------------------137Please respect copyright.ＰＥＮＡＮＡUWOQQt0Siv
HTTP/1.1 206 Partial Content137Please respect copyright.ＰＥＮＡＮＡ4eiynpW18f
accept-ranges: bytes137Please respect copyright.ＰＥＮＡＮＡvY1PTEKkYt
content-range: bytes 287718144-534231199/534231200137Please respect copyright.ＰＥＮＡＮＡfGkhVyzHJ8
ETag: "534231200_1732277116"137Please respect copyright.ＰＥＮＡＮＡPxQWupLmnW
Content-Length: 246513056137Please respect copyright.ＰＥＮＡＮＡrJiviw6WET
Content-Type: application/octet-stream137Please respect copyright.ＰＥＮＡＮＡ9tqCTdXT2m
Content-Disposition: attachment; filename="zen.iso"
(http body is the remain file content)137Please respect copyright.ＰＥＮＡＮＡZqF59oEJbn
-------------------------------------------
If the etag changes, response 412 like this:137Please respect copyright.ＰＥＮＡＮＡ8RNwCAtZG2
-----------http response for changed resource------137Please respect copyright.ＰＥＮＡＮＡ1vXeSiMNMI
HTTP/1.1 412 Precondition Failed
(http body canbe empty, the client will stop downloading the resumed file)137Please respect copyright.ＰＥＮＡＮＡBXFiluOrWV
---------------------------------------------------
137Please respect copyright.ＰＥＮＡＮＡfmSSBaHtbH
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---137Please respect copyright.ＰＥＮＡＮＡAiWBIIRlX2
GET /zen.iso HTTP/1.1137Please respect copyright.ＰＥＮＡＮＡFEIUm7XjMg
Range: bytes=287718144-137Please respect copyright.ＰＥＮＡＮＡrZS1yuQYNI
If-Range: "534231200_1732277116"137Please respect copyright.ＰＥＮＡＮＡd1mBFo0x0o
------------------------------------137Please respect copyright.ＰＥＮＡＮＡMWFiMMj5eJ
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:137Please respect copyright.ＰＥＮＡＮＡFh9RK4R7O4
-------------range response-------------------137Please respect copyright.ＰＥＮＡＮＡmkigheV9DE
HTTP/1.1 206 Partial Content137Please respect copyright.ＰＥＮＡＮＡoCDq5TvTEf
accept-ranges: bytes137Please respect copyright.ＰＥＮＡＮＡwraJEczRJd
content-range: bytes 287718144-534231199/534231200137Please respect copyright.ＰＥＮＡＮＡMDXmvXIaT3
ETag: "534231200_1732277116"137Please respect copyright.ＰＥＮＡＮＡtlJ8YbPjv3
Content-Length: 246513056137Please respect copyright.ＰＥＮＡＮＡhRbDV5qg1r
Content-Type: application/octet-stream137Please respect copyright.ＰＥＮＡＮＡHFZC0HO1np
Content-Disposition: attachment; filename="zen.iso"
(http body is the remain file content)137Please respect copyright.ＰＥＮＡＮＡV4DhAuSKbI
-------------------------------------------
If the etag changes, response 200 from beginning like this:137Please respect copyright.ＰＥＮＡＮＡlcVe1DJqB2
-----------http response from the beginning------137Please respect copyright.ＰＥＮＡＮＡ5rGw67Seuv
HTTP/1.1 200 OK137Please respect copyright.ＰＥＮＡＮＡROPE6tb3RZ
Date: Mon, 25 Nov 2024 10:56:12 GMT137Please respect copyright.ＰＥＮＡＮＡ3fRCPB4uAE
accept-ranges: bytes137Please respect copyright.ＰＥＮＡＮＡzeteV4YqA4
ETag: "534231200_1732277116"137Please respect copyright.ＰＥＮＡＮＡpmpyqlb2Kq
Content-Length: 534231200137Please respect copyright.ＰＥＮＡＮＡRiqZuq823o
Content-Type: application/octet-stream137Please respect copyright.ＰＥＮＡＮＡCtB1cxpRAd
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)137Please respect copyright.ＰＥＮＡＮＡKwdQBhAA3w
---------------------------------------------------
137Please respect copyright.ＰＥＮＡＮＡkz09H2Ao0P
Section X. Thanks
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/If-None-Match137Please respect copyright.ＰＥＮＡＮＡAdRE5oRUvR
https://blog.csdn.net/catoop/article/details/134174653137Please respect copyright.ＰＥＮＡＮＡK72w8phXCH
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]137Please respect copyright.ＰＥＮＡＮＡRoG6Eld9P4
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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and then Add to home screen.