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.
71Please respect copyright.ＰＥＮＡＮＡSMU66I9vTS
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:71Please respect copyright.ＰＥＮＡＮＡiUJJ00nJwD
---------------http response with etag----------------71Please respect copyright.ＰＥＮＡＮＡ4v1zF3q4Ck
HTTP/1.1 200 OK71Please respect copyright.ＰＥＮＡＮＡjVJ0yj92aJ
Date: Mon, 25 Nov 2024 10:56:12 GMT71Please respect copyright.ＰＥＮＡＮＡnAzykjMyCm
accept-ranges: bytes71Please respect copyright.ＰＥＮＡＮＡAha5V77rEw
ETag: "534231200_1732277116"71Please respect copyright.ＰＥＮＡＮＡtR7nWALarw
Content-Length: 53423120071Please respect copyright.ＰＥＮＡＮＡX5Qr1gpwPc
Content-Type: application/octet-stream71Please respect copyright.ＰＥＮＡＮＡuE0sPMWl6n
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)71Please respect copyright.ＰＥＮＡＮＡPKEKNxmtjd
-----------------------------------------------------
71Please respect copyright.ＰＥＮＡＮＡQLhg90gwUp
Section B. if-none-match
The browser send back the etag to ask server if it has been changed by header like this:71Please respect copyright.ＰＥＮＡＮＡQ8hTIvSY0T
-----http request for a cached resource------71Please respect copyright.ＰＥＮＡＮＡiNxkhfKDm6
GET /zen.iso HTTP/1.171Please respect copyright.ＰＥＮＡＮＡve5g1pFkOZ
if-none-match: "534231200_1732277116"71Please respect copyright.ＰＥＮＡＮＡeoH1vVay6J
---------------------------------------------
If the etag is unchanged, the server response like this:
1. for get and head requests:71Please respect copyright.ＰＥＮＡＮＡ7rFq18pq94
-----------http response for unchanged resource------71Please respect copyright.ＰＥＮＡＮＡkUEZdvA3zB
HTTP/1.1 304 Not Modified
(http body canbe empty, the client will used its cache)71Please respect copyright.ＰＥＮＡＮＡcqSC3BR2BP
----------------------------------------------------
2. for other methods requests:71Please respect copyright.ＰＥＮＡＮＡ97loO0prUa
-----------http response for unchanged resource------71Please respect copyright.ＰＥＮＡＮＡ7O0oL7o37j
HTTP/1.1 412 Precondition Failed
(http body canbe empty, the client will used its cache)71Please respect copyright.ＰＥＮＡＮＡhGvw9eakwl
----------------------------------------------------
71Please respect copyright.ＰＥＮＡＮＡ0n6Ku8xIor
If the etag changes, the server response 200 OK with the new file content like this:71Please respect copyright.ＰＥＮＡＮＡhQwqS6y31t
---------------http response with etag----------------71Please respect copyright.ＰＥＮＡＮＡujf0E2qfFk
HTTP/1.1 200 OK71Please respect copyright.ＰＥＮＡＮＡX62tsq3Mte
Date: Mon, 25 Nov 2024 10:56:12 GMT71Please respect copyright.ＰＥＮＡＮＡ7LbgbTk1Y7
accept-ranges: bytes71Please respect copyright.ＰＥＮＡＮＡPMMLeYks8j
ETag: "534231200_1732277116"71Please respect copyright.ＰＥＮＡＮＡxhGVissCDI
Content-Length: 53423120071Please respect copyright.ＰＥＮＡＮＡ8SKP1HoUHi
Content-Type: application/octet-stream71Please respect copyright.ＰＥＮＡＮＡYtw4B83FWx
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)71Please respect copyright.ＰＥＮＡＮＡPKnwNtqxAA
-----------------------------------------------------
71Please respect copyright.ＰＥＮＡＮＡNwcOJMMOwS
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:71Please respect copyright.ＰＥＮＡＮＡ1PV0KapeLN
------resume download request-------71Please respect copyright.ＰＥＮＡＮＡY3SxfrHCN0
GET /zen.iso HTTP/1.171Please respect copyright.ＰＥＮＡＮＡHPTJozkMTR
Range: bytes=287718144-71Please respect copyright.ＰＥＮＡＮＡ6J84eLPGNU
If-Match: "534231200_1732277116"71Please respect copyright.ＰＥＮＡＮＡKGAV0fy34R
-----------------------------------71Please respect copyright.ＰＥＮＡＮＡCvTOjWBuqj
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:71Please respect copyright.ＰＥＮＡＮＡZbKV5ZhFPY
-------------range response-------------------71Please respect copyright.ＰＥＮＡＮＡLBHIoMUgV4
HTTP/1.1 206 Partial Content71Please respect copyright.ＰＥＮＡＮＡtxK5v6cVdd
accept-ranges: bytes71Please respect copyright.ＰＥＮＡＮＡROSPfEbaZn
content-range: bytes 287718144-534231199/53423120071Please respect copyright.ＰＥＮＡＮＡfGBRJcDvZP
ETag: "534231200_1732277116"71Please respect copyright.ＰＥＮＡＮＡMRYoxu9Pja
Content-Length: 24651305671Please respect copyright.ＰＥＮＡＮＡxjyhmlJzyT
Content-Type: application/octet-stream71Please respect copyright.ＰＥＮＡＮＡeopf5iVRMu
Content-Disposition: attachment; filename="zen.iso"
(http body is the remain file content)71Please respect copyright.ＰＥＮＡＮＡmiagQsF6ju
-------------------------------------------
If the etag changes, response 412 like this:71Please respect copyright.ＰＥＮＡＮＡD1lQbiyVOE
-----------http response for changed resource------71Please respect copyright.ＰＥＮＡＮＡXQzpL69MBH
HTTP/1.1 412 Precondition Failed
(http body canbe empty, the client will stop downloading the resumed file)71Please respect copyright.ＰＥＮＡＮＡKzGAG7Ti7V
---------------------------------------------------
71Please respect copyright.ＰＥＮＡＮＡ2N3f3X07yb
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---71Please respect copyright.ＰＥＮＡＮＡdCjqCwVh8u
GET /zen.iso HTTP/1.171Please respect copyright.ＰＥＮＡＮＡMDC4aAx7vd
Range: bytes=287718144-71Please respect copyright.ＰＥＮＡＮＡ8XfoKy7bZE
If-Range: "534231200_1732277116"71Please respect copyright.ＰＥＮＡＮＡvCj8y7rntd
------------------------------------71Please respect copyright.ＰＥＮＡＮＡ5NJemqJapq
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:71Please respect copyright.ＰＥＮＡＮＡUyF92niEAn
-------------range response-------------------71Please respect copyright.ＰＥＮＡＮＡBvDwRSqpJX
HTTP/1.1 206 Partial Content71Please respect copyright.ＰＥＮＡＮＡfXYzF5Yxg5
accept-ranges: bytes71Please respect copyright.ＰＥＮＡＮＡ2prPXmDHSB
content-range: bytes 287718144-534231199/53423120071Please respect copyright.ＰＥＮＡＮＡ80OruNW2LL
ETag: "534231200_1732277116"71Please respect copyright.ＰＥＮＡＮＡQtHlNtEmwW
Content-Length: 24651305671Please respect copyright.ＰＥＮＡＮＡBPBJUS9L39
Content-Type: application/octet-stream71Please respect copyright.ＰＥＮＡＮＡn0FLvKqlON
Content-Disposition: attachment; filename="zen.iso"
(http body is the remain file content)71Please respect copyright.ＰＥＮＡＮＡJP8ABhXjyr
-------------------------------------------
If the etag changes, response 200 from beginning like this:71Please respect copyright.ＰＥＮＡＮＡjSSMSl7QGc
-----------http response from the beginning------71Please respect copyright.ＰＥＮＡＮＡoWTYVGetyc
HTTP/1.1 200 OK71Please respect copyright.ＰＥＮＡＮＡ0W5Do9fT61
Date: Mon, 25 Nov 2024 10:56:12 GMT71Please respect copyright.ＰＥＮＡＮＡgwRC4nV2QL
accept-ranges: bytes71Please respect copyright.ＰＥＮＡＮＡpBBeLmoPIU
ETag: "534231200_1732277116"71Please respect copyright.ＰＥＮＡＮＡATvrFS1oDV
Content-Length: 53423120071Please respect copyright.ＰＥＮＡＮＡ7ifgfjC8Xw
Content-Type: application/octet-stream71Please respect copyright.ＰＥＮＡＮＡwhVdDeQXWc
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)71Please respect copyright.ＰＥＮＡＮＡW6EujjjzmS
---------------------------------------------------
71Please respect copyright.ＰＥＮＡＮＡJ1tW0wcqGn
Section X. Thanks
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/If-None-Match71Please respect copyright.ＰＥＮＡＮＡOrjTrtofAP
https://blog.csdn.net/catoop/article/details/13417465371Please respect copyright.ＰＥＮＡＮＡjVcahkTSyX
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]71Please respect copyright.ＰＥＮＡＮＡTs9nVeAZ3N
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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