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.
87Please respect copyright.ＰＥＮＡＮＡHnkykepGK1
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:87Please respect copyright.ＰＥＮＡＮＡ5pnlzx48pj
---------------http response with etag----------------87Please respect copyright.ＰＥＮＡＮＡhbIuelMIcg
HTTP/1.1 200 OK87Please respect copyright.ＰＥＮＡＮＡpcNQiL5cK5
Date: Mon, 25 Nov 2024 10:56:12 GMT87Please respect copyright.ＰＥＮＡＮＡMB1NRT5HMT
accept-ranges: bytes87Please respect copyright.ＰＥＮＡＮＡlpVC9ccAQN
ETag: "534231200_1732277116"87Please respect copyright.ＰＥＮＡＮＡeKTGAhkLec
Content-Length: 53423120087Please respect copyright.ＰＥＮＡＮＡbfUYg0JyYl
Content-Type: application/octet-stream87Please respect copyright.ＰＥＮＡＮＡzQpvxU08S5
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)87Please respect copyright.ＰＥＮＡＮＡtX0fJVjxWT
-----------------------------------------------------
87Please respect copyright.ＰＥＮＡＮＡQ7GV1Nu54X
Section B. if-none-match
The browser send back the etag to ask server if it has been changed by header like this:87Please respect copyright.ＰＥＮＡＮＡ4PW5dMqeCY
-----http request for a cached resource------87Please respect copyright.ＰＥＮＡＮＡEyHbECaVIi
GET /zen.iso HTTP/1.187Please respect copyright.ＰＥＮＡＮＡxl8k0ATphI
if-none-match: "534231200_1732277116"87Please respect copyright.ＰＥＮＡＮＡBSKqOG0qIF
---------------------------------------------
If the etag is unchanged, the server response like this:
1. for get and head requests:87Please respect copyright.ＰＥＮＡＮＡxXcQFv0fnu
-----------http response for unchanged resource------87Please respect copyright.ＰＥＮＡＮＡcRN7VrbTW1
HTTP/1.1 304 Not Modified
(http body canbe empty, the client will used its cache)87Please respect copyright.ＰＥＮＡＮＡVck0QtrE3I
----------------------------------------------------
2. for other methods requests:87Please respect copyright.ＰＥＮＡＮＡGWTlI2ua9M
-----------http response for unchanged resource------87Please respect copyright.ＰＥＮＡＮＡgCNBejTgZF
HTTP/1.1 412 Precondition Failed
(http body canbe empty, the client will used its cache)87Please respect copyright.ＰＥＮＡＮＡrMzvNeeOp3
----------------------------------------------------
87Please respect copyright.ＰＥＮＡＮＡPGZ9lsKBCS
If the etag changes, the server response 200 OK with the new file content like this:87Please respect copyright.ＰＥＮＡＮＡrZ7cu4fTOc
---------------http response with etag----------------87Please respect copyright.ＰＥＮＡＮＡLzscaR0eTK
HTTP/1.1 200 OK87Please respect copyright.ＰＥＮＡＮＡLhM4Mmvbuw
Date: Mon, 25 Nov 2024 10:56:12 GMT87Please respect copyright.ＰＥＮＡＮＡOn4QLZxQbg
accept-ranges: bytes87Please respect copyright.ＰＥＮＡＮＡ1jVwrUwiBo
ETag: "534231200_1732277116"87Please respect copyright.ＰＥＮＡＮＡCTWJWH5H1X
Content-Length: 53423120087Please respect copyright.ＰＥＮＡＮＡiGgwEc0dlW
Content-Type: application/octet-stream87Please respect copyright.ＰＥＮＡＮＡDq4VLxzTIA
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)87Please respect copyright.ＰＥＮＡＮＡY7PNhMTsOn
-----------------------------------------------------
87Please respect copyright.ＰＥＮＡＮＡzbLYKTOYLj
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:87Please respect copyright.ＰＥＮＡＮＡ2CxMtNrSwc
------resume download request-------87Please respect copyright.ＰＥＮＡＮＡV49H6EyJeE
GET /zen.iso HTTP/1.187Please respect copyright.ＰＥＮＡＮＡWgY7U47oeb
Range: bytes=287718144-87Please respect copyright.ＰＥＮＡＮＡF0ctxWIiO6
If-Match: "534231200_1732277116"87Please respect copyright.ＰＥＮＡＮＡDZ8wLZgMnM
-----------------------------------87Please respect copyright.ＰＥＮＡＮＡWhAUljPD13
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:87Please respect copyright.ＰＥＮＡＮＡFYCxjFL1t4
-------------range response-------------------87Please respect copyright.ＰＥＮＡＮＡyC323WvGC8
HTTP/1.1 206 Partial Content87Please respect copyright.ＰＥＮＡＮＡpZ6FyObYGV
accept-ranges: bytes87Please respect copyright.ＰＥＮＡＮＡloDfTnryFn
content-range: bytes 287718144-534231199/53423120087Please respect copyright.ＰＥＮＡＮＡGnCU2KFrKw
ETag: "534231200_1732277116"87Please respect copyright.ＰＥＮＡＮＡNIwKPNzbhl
Content-Length: 24651305687Please respect copyright.ＰＥＮＡＮＡIqWQrCR05S
Content-Type: application/octet-stream87Please respect copyright.ＰＥＮＡＮＡLkb3EGOxYq
Content-Disposition: attachment; filename="zen.iso"
(http body is the remain file content)87Please respect copyright.ＰＥＮＡＮＡnsj4ZK5YBz
-------------------------------------------
If the etag changes, response 412 like this:87Please respect copyright.ＰＥＮＡＮＡfeKxSRBZBT
-----------http response for changed resource------87Please respect copyright.ＰＥＮＡＮＡzEUntztnwR
HTTP/1.1 412 Precondition Failed
(http body canbe empty, the client will stop downloading the resumed file)87Please respect copyright.ＰＥＮＡＮＡGMN4R3Gcsi
---------------------------------------------------
87Please respect copyright.ＰＥＮＡＮＡ3YYdHpJn8q
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---87Please respect copyright.ＰＥＮＡＮＡIMIAKhaSGO
GET /zen.iso HTTP/1.187Please respect copyright.ＰＥＮＡＮＡcUvPSnSmYk
Range: bytes=287718144-87Please respect copyright.ＰＥＮＡＮＡHOPrLnb5Ul
If-Range: "534231200_1732277116"87Please respect copyright.ＰＥＮＡＮＡyJVq3KMrOK
------------------------------------87Please respect copyright.ＰＥＮＡＮＡz8yCnZRpKR
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:87Please respect copyright.ＰＥＮＡＮＡrIH31ot49E
-------------range response-------------------87Please respect copyright.ＰＥＮＡＮＡtLN9RYhJFS
HTTP/1.1 206 Partial Content87Please respect copyright.ＰＥＮＡＮＡMG0ioTg0mE
accept-ranges: bytes87Please respect copyright.ＰＥＮＡＮＡbRWlWOd0ZE
content-range: bytes 287718144-534231199/53423120087Please respect copyright.ＰＥＮＡＮＡhaL783Z42p
ETag: "534231200_1732277116"87Please respect copyright.ＰＥＮＡＮＡuq45MsoyyM
Content-Length: 24651305687Please respect copyright.ＰＥＮＡＮＡxiAl2wJkJu
Content-Type: application/octet-stream87Please respect copyright.ＰＥＮＡＮＡ9WAYJJYXz8
Content-Disposition: attachment; filename="zen.iso"
(http body is the remain file content)87Please respect copyright.ＰＥＮＡＮＡ8PAVlADltd
-------------------------------------------
If the etag changes, response 200 from beginning like this:87Please respect copyright.ＰＥＮＡＮＡxTj7OkYBNa
-----------http response from the beginning------87Please respect copyright.ＰＥＮＡＮＡ9Tk2ewK8Zq
HTTP/1.1 200 OK87Please respect copyright.ＰＥＮＡＮＡI4tKWHxBob
Date: Mon, 25 Nov 2024 10:56:12 GMT87Please respect copyright.ＰＥＮＡＮＡQ3JHQvbXMK
accept-ranges: bytes87Please respect copyright.ＰＥＮＡＮＡNDv5mqr1NS
ETag: "534231200_1732277116"87Please respect copyright.ＰＥＮＡＮＡkyan8mHQ4c
Content-Length: 53423120087Please respect copyright.ＰＥＮＡＮＡfSzVX1cxEj
Content-Type: application/octet-stream87Please respect copyright.ＰＥＮＡＮＡdaaSdavgTg
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)87Please respect copyright.ＰＥＮＡＮＡ2ryOqIZyA0
---------------------------------------------------
87Please respect copyright.ＰＥＮＡＮＡCXg3okLlCp
Section X. Thanks
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/If-None-Match87Please respect copyright.ＰＥＮＡＮＡ06e9IijCpq
https://blog.csdn.net/catoop/article/details/13417465387Please respect copyright.ＰＥＮＡＮＡTgduGBOUsv
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]87Please respect copyright.ＰＥＮＡＮＡeGeIx2XYam
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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