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.
79Please respect copyright.ＰＥＮＡＮＡ6meIsa5UJh
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:79Please respect copyright.ＰＥＮＡＮＡP1IA72ngtY
---------------http response with etag----------------79Please respect copyright.ＰＥＮＡＮＡ8HVEQJ9QLb
HTTP/1.1 200 OK79Please respect copyright.ＰＥＮＡＮＡ08PKGkJwNH
Date: Mon, 25 Nov 2024 10:56:12 GMT79Please respect copyright.ＰＥＮＡＮＡmBhWt9sXK8
accept-ranges: bytes79Please respect copyright.ＰＥＮＡＮＡybt75ynmrU
ETag: "534231200_1732277116"79Please respect copyright.ＰＥＮＡＮＡZJzESeGCgq
Content-Length: 53423120079Please respect copyright.ＰＥＮＡＮＡOOwwggD4Et
Content-Type: application/octet-stream79Please respect copyright.ＰＥＮＡＮＡ91zcxoJq38
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)79Please respect copyright.ＰＥＮＡＮＡtbPA53fFdv
-----------------------------------------------------
79Please respect copyright.ＰＥＮＡＮＡpwNpQDfLWZ
Section B. if-none-match
The browser send back the etag to ask server if it has been changed by header like this:79Please respect copyright.ＰＥＮＡＮＡiBNpFWDHSp
-----http request for a cached resource------79Please respect copyright.ＰＥＮＡＮＡBlBnNSY9tY
GET /zen.iso HTTP/1.179Please respect copyright.ＰＥＮＡＮＡUFKAa5BLLO
if-none-match: "534231200_1732277116"79Please respect copyright.ＰＥＮＡＮＡzkth1ozu3E
---------------------------------------------
If the etag is unchanged, the server response like this:
1. for get and head requests:79Please respect copyright.ＰＥＮＡＮＡwscTiC90Qo
-----------http response for unchanged resource------79Please respect copyright.ＰＥＮＡＮＡ9dGbJrQDIV
HTTP/1.1 304 Not Modified
(http body canbe empty, the client will used its cache)79Please respect copyright.ＰＥＮＡＮＡPfCxUWEbYX
----------------------------------------------------
2. for other methods requests:79Please respect copyright.ＰＥＮＡＮＡlMFM9Cw7K5
-----------http response for unchanged resource------79Please respect copyright.ＰＥＮＡＮＡ6NqYCIhjpW
HTTP/1.1 412 Precondition Failed
(http body canbe empty, the client will used its cache)79Please respect copyright.ＰＥＮＡＮＡmzaqXJNV8B
----------------------------------------------------
79Please respect copyright.ＰＥＮＡＮＡ7QPGqVWDtN
If the etag changes, the server response 200 OK with the new file content like this:79Please respect copyright.ＰＥＮＡＮＡ8wjnfqQHUI
---------------http response with etag----------------79Please respect copyright.ＰＥＮＡＮＡNsni3v1gKm
HTTP/1.1 200 OK79Please respect copyright.ＰＥＮＡＮＡMmkeLRM7Ct
Date: Mon, 25 Nov 2024 10:56:12 GMT79Please respect copyright.ＰＥＮＡＮＡjS0SW8QttC
accept-ranges: bytes79Please respect copyright.ＰＥＮＡＮＡCKXaoFK41h
ETag: "534231200_1732277116"79Please respect copyright.ＰＥＮＡＮＡUK0rOEOZpW
Content-Length: 53423120079Please respect copyright.ＰＥＮＡＮＡt5hNcTxoYL
Content-Type: application/octet-stream79Please respect copyright.ＰＥＮＡＮＡV8rYDKfrOh
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)79Please respect copyright.ＰＥＮＡＮＡPkUzdEhVqO
-----------------------------------------------------
79Please respect copyright.ＰＥＮＡＮＡnsrHzrB6pV
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:79Please respect copyright.ＰＥＮＡＮＡf9HwpEYYoy
------resume download request-------79Please respect copyright.ＰＥＮＡＮＡvNDjt18TqC
GET /zen.iso HTTP/1.179Please respect copyright.ＰＥＮＡＮＡBY61zp0mCm
Range: bytes=287718144-79Please respect copyright.ＰＥＮＡＮＡTGR7aL3ErK
If-Match: "534231200_1732277116"79Please respect copyright.ＰＥＮＡＮＡdZWB4X6ySq
-----------------------------------79Please respect copyright.ＰＥＮＡＮＡAJK6qn6hMs
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:79Please respect copyright.ＰＥＮＡＮＡNDSLrS9oNW
-------------range response-------------------79Please respect copyright.ＰＥＮＡＮＡmjiSnNYCrw
HTTP/1.1 206 Partial Content79Please respect copyright.ＰＥＮＡＮＡARe63p3hSe
accept-ranges: bytes79Please respect copyright.ＰＥＮＡＮＡ0bba3VpNsW
content-range: bytes 287718144-534231199/53423120079Please respect copyright.ＰＥＮＡＮＡNkWySOLMHO
ETag: "534231200_1732277116"79Please respect copyright.ＰＥＮＡＮＡJKFfliJjmK
Content-Length: 24651305679Please respect copyright.ＰＥＮＡＮＡeDmeD1cCVr
Content-Type: application/octet-stream79Please respect copyright.ＰＥＮＡＮＡssjM73jG5h
Content-Disposition: attachment; filename="zen.iso"
(http body is the remain file content)79Please respect copyright.ＰＥＮＡＮＡMh3yC22xN0
-------------------------------------------
If the etag changes, response 412 like this:79Please respect copyright.ＰＥＮＡＮＡU72kAkphoF
-----------http response for changed resource------79Please respect copyright.ＰＥＮＡＮＡIqPvzbRbDT
HTTP/1.1 412 Precondition Failed
(http body canbe empty, the client will stop downloading the resumed file)79Please respect copyright.ＰＥＮＡＮＡySkIFDFTPO
---------------------------------------------------
79Please respect copyright.ＰＥＮＡＮＡTBcQsCpK5K
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---79Please respect copyright.ＰＥＮＡＮＡZNEztmA1Bg
GET /zen.iso HTTP/1.179Please respect copyright.ＰＥＮＡＮＡ636qvuFS4r
Range: bytes=287718144-79Please respect copyright.ＰＥＮＡＮＡP3guMdm2To
If-Range: "534231200_1732277116"79Please respect copyright.ＰＥＮＡＮＡBIkGzFYkS0
------------------------------------79Please respect copyright.ＰＥＮＡＮＡjxGmU7dOlx
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:79Please respect copyright.ＰＥＮＡＮＡEvcVNr5pWp
-------------range response-------------------79Please respect copyright.ＰＥＮＡＮＡLq6unESWOh
HTTP/1.1 206 Partial Content79Please respect copyright.ＰＥＮＡＮＡzZI78g2oTU
accept-ranges: bytes79Please respect copyright.ＰＥＮＡＮＡOeb34ilSI7
content-range: bytes 287718144-534231199/53423120079Please respect copyright.ＰＥＮＡＮＡFA0yakcR0e
ETag: "534231200_1732277116"79Please respect copyright.ＰＥＮＡＮＡ7JB524umTA
Content-Length: 24651305679Please respect copyright.ＰＥＮＡＮＡ75GJOQZUkn
Content-Type: application/octet-stream79Please respect copyright.ＰＥＮＡＮＡRE3z0BbHie
Content-Disposition: attachment; filename="zen.iso"
(http body is the remain file content)79Please respect copyright.ＰＥＮＡＮＡGitIvgqVZu
-------------------------------------------
If the etag changes, response 200 from beginning like this:79Please respect copyright.ＰＥＮＡＮＡ7OhqRQOO3k
-----------http response from the beginning------79Please respect copyright.ＰＥＮＡＮＡkYsYdDHv5t
HTTP/1.1 200 OK79Please respect copyright.ＰＥＮＡＮＡebFhzOskoU
Date: Mon, 25 Nov 2024 10:56:12 GMT79Please respect copyright.ＰＥＮＡＮＡrNp3orwGVz
accept-ranges: bytes79Please respect copyright.ＰＥＮＡＮＡ2I5D2bp0o5
ETag: "534231200_1732277116"79Please respect copyright.ＰＥＮＡＮＡ20avM5uF2w
Content-Length: 53423120079Please respect copyright.ＰＥＮＡＮＡwn9eyUPsdq
Content-Type: application/octet-stream79Please respect copyright.ＰＥＮＡＮＡFsH0TU5rb0
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)79Please respect copyright.ＰＥＮＡＮＡAcp0qhfPqe
---------------------------------------------------
79Please respect copyright.ＰＥＮＡＮＡEjdewt3vnp
Section X. Thanks
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/If-None-Match79Please respect copyright.ＰＥＮＡＮＡanbIl7J5Bs
https://blog.csdn.net/catoop/article/details/13417465379Please respect copyright.ＰＥＮＡＮＡPFo6tiHzvH
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]79Please respect copyright.ＰＥＮＡＮＡD6ZgNyl1GT
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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