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.
78Please respect copyright.ＰＥＮＡＮＡRzXhTHWwbB
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:78Please respect copyright.ＰＥＮＡＮＡEOwoR3Z9Hv
---------------http response with etag----------------78Please respect copyright.ＰＥＮＡＮＡnBoSd4iX4K
HTTP/1.1 200 OK78Please respect copyright.ＰＥＮＡＮＡK7pm97Isvb
Date: Mon, 25 Nov 2024 10:56:12 GMT78Please respect copyright.ＰＥＮＡＮＡr3vqB1DwXN
accept-ranges: bytes78Please respect copyright.ＰＥＮＡＮＡBTJbRXxgw4
ETag: "534231200_1732277116"78Please respect copyright.ＰＥＮＡＮＡNdEuUrUaSB
Content-Length: 53423120078Please respect copyright.ＰＥＮＡＮＡftPrx904W2
Content-Type: application/octet-stream78Please respect copyright.ＰＥＮＡＮＡGM7V7Lf879
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)78Please respect copyright.ＰＥＮＡＮＡJ5neidb2Hd
-----------------------------------------------------
78Please respect copyright.ＰＥＮＡＮＡdHPwvMOswI
Section B. if-none-match
The browser send back the etag to ask server if it has been changed by header like this:78Please respect copyright.ＰＥＮＡＮＡAZEpaq3CBB
-----http request for a cached resource------78Please respect copyright.ＰＥＮＡＮＡbmv0agPDII
GET /zen.iso HTTP/1.178Please respect copyright.ＰＥＮＡＮＡPBzwxNyXvS
if-none-match: "534231200_1732277116"78Please respect copyright.ＰＥＮＡＮＡrDmXoQOWe8
---------------------------------------------
If the etag is unchanged, the server response like this:
1. for get and head requests:78Please respect copyright.ＰＥＮＡＮＡIpSRJSok9F
-----------http response for unchanged resource------78Please respect copyright.ＰＥＮＡＮＡsAArls82yt
HTTP/1.1 304 Not Modified
(http body canbe empty, the client will used its cache)78Please respect copyright.ＰＥＮＡＮＡ9Dngq0UjOn
----------------------------------------------------
2. for other methods requests:78Please respect copyright.ＰＥＮＡＮＡYbhu01HWIa
-----------http response for unchanged resource------78Please respect copyright.ＰＥＮＡＮＡSTDGqais9x
HTTP/1.1 412 Precondition Failed
(http body canbe empty, the client will used its cache)78Please respect copyright.ＰＥＮＡＮＡSx2HomsIB4
----------------------------------------------------
78Please respect copyright.ＰＥＮＡＮＡn4aLlrNp1p
If the etag changes, the server response 200 OK with the new file content like this:78Please respect copyright.ＰＥＮＡＮＡzQGMglMgVJ
---------------http response with etag----------------78Please respect copyright.ＰＥＮＡＮＡ1zltoolt5o
HTTP/1.1 200 OK78Please respect copyright.ＰＥＮＡＮＡWpiIqlXVf8
Date: Mon, 25 Nov 2024 10:56:12 GMT78Please respect copyright.ＰＥＮＡＮＡ2GgW21SyIj
accept-ranges: bytes78Please respect copyright.ＰＥＮＡＮＡhviP6UkkhY
ETag: "534231200_1732277116"78Please respect copyright.ＰＥＮＡＮＡ1Uv001hKx9
Content-Length: 53423120078Please respect copyright.ＰＥＮＡＮＡnYcP2mvnNQ
Content-Type: application/octet-stream78Please respect copyright.ＰＥＮＡＮＡZl6e1uZ86V
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)78Please respect copyright.ＰＥＮＡＮＡL4mgWzYWEs
-----------------------------------------------------
78Please respect copyright.ＰＥＮＡＮＡiVbM9lGFpS
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:78Please respect copyright.ＰＥＮＡＮＡM9eGTBL9Wk
------resume download request-------78Please respect copyright.ＰＥＮＡＮＡWWHOGdVd6R
GET /zen.iso HTTP/1.178Please respect copyright.ＰＥＮＡＮＡlqfLmYU8Kg
Range: bytes=287718144-78Please respect copyright.ＰＥＮＡＮＡb0ydX21ObY
If-Match: "534231200_1732277116"78Please respect copyright.ＰＥＮＡＮＡYcES49ITEu
-----------------------------------78Please respect copyright.ＰＥＮＡＮＡbX7sblqcja
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:78Please respect copyright.ＰＥＮＡＮＡzuKJ1hwCbm
-------------range response-------------------78Please respect copyright.ＰＥＮＡＮＡN5EtIPLWn7
HTTP/1.1 206 Partial Content78Please respect copyright.ＰＥＮＡＮＡp7JmZtRG8R
accept-ranges: bytes78Please respect copyright.ＰＥＮＡＮＡU7Q1UtjwaZ
content-range: bytes 287718144-534231199/53423120078Please respect copyright.ＰＥＮＡＮＡPp9vPSrKcs
ETag: "534231200_1732277116"78Please respect copyright.ＰＥＮＡＮＡciM6478utM
Content-Length: 24651305678Please respect copyright.ＰＥＮＡＮＡNkhrm00K9P
Content-Type: application/octet-stream78Please respect copyright.ＰＥＮＡＮＡtY84ISSKRU
Content-Disposition: attachment; filename="zen.iso"
(http body is the remain file content)78Please respect copyright.ＰＥＮＡＮＡ9v9tP80JTw
-------------------------------------------
If the etag changes, response 412 like this:78Please respect copyright.ＰＥＮＡＮＡs6nj8epmT3
-----------http response for changed resource------78Please respect copyright.ＰＥＮＡＮＡQC0JHduCI1
HTTP/1.1 412 Precondition Failed
(http body canbe empty, the client will stop downloading the resumed file)78Please respect copyright.ＰＥＮＡＮＡXtAcYNdvhc
---------------------------------------------------
78Please respect copyright.ＰＥＮＡＮＡ25Jdd50q7T
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---78Please respect copyright.ＰＥＮＡＮＡbobbHAtraP
GET /zen.iso HTTP/1.178Please respect copyright.ＰＥＮＡＮＡY1r3RjhSVJ
Range: bytes=287718144-78Please respect copyright.ＰＥＮＡＮＡZvnkEkMPDT
If-Range: "534231200_1732277116"78Please respect copyright.ＰＥＮＡＮＡPdfpSpFB2A
------------------------------------78Please respect copyright.ＰＥＮＡＮＡHNHevEtLBz
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:78Please respect copyright.ＰＥＮＡＮＡpW5sSsqNUP
-------------range response-------------------78Please respect copyright.ＰＥＮＡＮＡkw0rz87yQT
HTTP/1.1 206 Partial Content78Please respect copyright.ＰＥＮＡＮＡEFsaOo9O1k
accept-ranges: bytes78Please respect copyright.ＰＥＮＡＮＡPU8GsPCtTh
content-range: bytes 287718144-534231199/53423120078Please respect copyright.ＰＥＮＡＮＡRK8YD0Djmn
ETag: "534231200_1732277116"78Please respect copyright.ＰＥＮＡＮＡZrQAkvyy5B
Content-Length: 24651305678Please respect copyright.ＰＥＮＡＮＡ3ZVfyCCQpS
Content-Type: application/octet-stream78Please respect copyright.ＰＥＮＡＮＡTHg0bOCuVS
Content-Disposition: attachment; filename="zen.iso"
(http body is the remain file content)78Please respect copyright.ＰＥＮＡＮＡFOhJk3Q7eV
-------------------------------------------
If the etag changes, response 200 from beginning like this:78Please respect copyright.ＰＥＮＡＮＡFzc4lr8bZ5
-----------http response from the beginning------78Please respect copyright.ＰＥＮＡＮＡ9v5LUGYD4a
HTTP/1.1 200 OK78Please respect copyright.ＰＥＮＡＮＡFOg0QTnu04
Date: Mon, 25 Nov 2024 10:56:12 GMT78Please respect copyright.ＰＥＮＡＮＡZ2mytsICIp
accept-ranges: bytes78Please respect copyright.ＰＥＮＡＮＡ8LJCqtzqVe
ETag: "534231200_1732277116"78Please respect copyright.ＰＥＮＡＮＡBE1ZiIFcE3
Content-Length: 53423120078Please respect copyright.ＰＥＮＡＮＡTZ7f8utTnG
Content-Type: application/octet-stream78Please respect copyright.ＰＥＮＡＮＡZtWY96Mc3J
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)78Please respect copyright.ＰＥＮＡＮＡcvZL3bTjL2
---------------------------------------------------
78Please respect copyright.ＰＥＮＡＮＡ6Hkq62C7vY
Section X. Thanks
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/If-None-Match78Please respect copyright.ＰＥＮＡＮＡO17UHTSqiA
https://blog.csdn.net/catoop/article/details/13417465378Please respect copyright.ＰＥＮＡＮＡr7x53JntyR
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]78Please respect copyright.ＰＥＮＡＮＡJ4HYeDluIA
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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