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.
68Please respect copyright.ＰＥＮＡＮＡpngjlWWBnX
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:68Please respect copyright.ＰＥＮＡＮＡklJKpUmYIK
---------------http response with etag----------------68Please respect copyright.ＰＥＮＡＮＡ7VQ2hGQlh1
HTTP/1.1 200 OK68Please respect copyright.ＰＥＮＡＮＡ1et6QQBM1w
Date: Mon, 25 Nov 2024 10:56:12 GMT68Please respect copyright.ＰＥＮＡＮＡX7CGQwMeng
accept-ranges: bytes68Please respect copyright.ＰＥＮＡＮＡ3MAGRlk2MM
ETag: "534231200_1732277116"68Please respect copyright.ＰＥＮＡＮＡjDdwKN0Tak
Content-Length: 53423120068Please respect copyright.ＰＥＮＡＮＡawD5kXEdYr
Content-Type: application/octet-stream68Please respect copyright.ＰＥＮＡＮＡmzaFJlBaLe
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)68Please respect copyright.ＰＥＮＡＮＡ840HMagq5M
-----------------------------------------------------
68Please respect copyright.ＰＥＮＡＮＡExJwTWCycM
Section B. if-none-match
The browser send back the etag to ask server if it has been changed by header like this:68Please respect copyright.ＰＥＮＡＮＡT9mYFev5SE
-----http request for a cached resource------68Please respect copyright.ＰＥＮＡＮＡ2FHc8tXgw9
GET /zen.iso HTTP/1.168Please respect copyright.ＰＥＮＡＮＡpSiCoFCOFv
if-none-match: "534231200_1732277116"68Please respect copyright.ＰＥＮＡＮＡVcK2bpu4QM
---------------------------------------------
If the etag is unchanged, the server response like this:
1. for get and head requests:68Please respect copyright.ＰＥＮＡＮＡjuBQ2bSto6
-----------http response for unchanged resource------68Please respect copyright.ＰＥＮＡＮＡdmY6hxJA2m
HTTP/1.1 304 Not Modified
(http body canbe empty, the client will used its cache)68Please respect copyright.ＰＥＮＡＮＡac8LChcvKT
----------------------------------------------------
2. for other methods requests:68Please respect copyright.ＰＥＮＡＮＡdVBlkedbq1
-----------http response for unchanged resource------68Please respect copyright.ＰＥＮＡＮＡqDwA8cK2Xo
HTTP/1.1 412 Precondition Failed
(http body canbe empty, the client will used its cache)68Please respect copyright.ＰＥＮＡＮＡk45fcLqhJM
----------------------------------------------------
68Please respect copyright.ＰＥＮＡＮＡ7kxPyVxhW7
If the etag changes, the server response 200 OK with the new file content like this:68Please respect copyright.ＰＥＮＡＮＡzL1g7PgP4W
---------------http response with etag----------------68Please respect copyright.ＰＥＮＡＮＡ89QxLfy98a
HTTP/1.1 200 OK68Please respect copyright.ＰＥＮＡＮＡNzRrK6j5c8
Date: Mon, 25 Nov 2024 10:56:12 GMT68Please respect copyright.ＰＥＮＡＮＡhERGdyGKLA
accept-ranges: bytes68Please respect copyright.ＰＥＮＡＮＡJmMNrjaJcy
ETag: "534231200_1732277116"68Please respect copyright.ＰＥＮＡＮＡBULuhZLuci
Content-Length: 53423120068Please respect copyright.ＰＥＮＡＮＡOqEJpX65Gl
Content-Type: application/octet-stream68Please respect copyright.ＰＥＮＡＮＡNi269sraFn
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)68Please respect copyright.ＰＥＮＡＮＡbPCJ1ZeXyh
-----------------------------------------------------
68Please respect copyright.ＰＥＮＡＮＡLr7gjNq9fK
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:68Please respect copyright.ＰＥＮＡＮＡvnqYgSzUfa
------resume download request-------68Please respect copyright.ＰＥＮＡＮＡGJdBga4oDH
GET /zen.iso HTTP/1.168Please respect copyright.ＰＥＮＡＮＡDh0fm3lMB9
Range: bytes=287718144-68Please respect copyright.ＰＥＮＡＮＡINnMsV4MwP
If-Match: "534231200_1732277116"68Please respect copyright.ＰＥＮＡＮＡBEGnVmOAFp
-----------------------------------68Please respect copyright.ＰＥＮＡＮＡZ71ADUqQ0r
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:68Please respect copyright.ＰＥＮＡＮＡqCzzoySDg1
-------------range response-------------------68Please respect copyright.ＰＥＮＡＮＡQc2ByPJP3x
HTTP/1.1 206 Partial Content68Please respect copyright.ＰＥＮＡＮＡIYOJ8Nuwxq
accept-ranges: bytes68Please respect copyright.ＰＥＮＡＮＡju1IkEyauc
content-range: bytes 287718144-534231199/53423120068Please respect copyright.ＰＥＮＡＮＡt3AR2OcDL3
ETag: "534231200_1732277116"68Please respect copyright.ＰＥＮＡＮＡXOFNgHbDPB
Content-Length: 24651305668Please respect copyright.ＰＥＮＡＮＡIXUyk643NF
Content-Type: application/octet-stream68Please respect copyright.ＰＥＮＡＮＡtivsWttnZz
Content-Disposition: attachment; filename="zen.iso"
(http body is the remain file content)68Please respect copyright.ＰＥＮＡＮＡUn20lUj1Jp
-------------------------------------------
If the etag changes, response 412 like this:68Please respect copyright.ＰＥＮＡＮＡ7MjmOvj8jb
-----------http response for changed resource------68Please respect copyright.ＰＥＮＡＮＡp8TpguZr3m
HTTP/1.1 412 Precondition Failed
(http body canbe empty, the client will stop downloading the resumed file)68Please respect copyright.ＰＥＮＡＮＡVxu3tXF7aT
---------------------------------------------------
68Please respect copyright.ＰＥＮＡＮＡNvP5n5flxC
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---68Please respect copyright.ＰＥＮＡＮＡwfGh3fCPpy
GET /zen.iso HTTP/1.168Please respect copyright.ＰＥＮＡＮＡoeLnbDmlkS
Range: bytes=287718144-68Please respect copyright.ＰＥＮＡＮＡ30qyIthN1F
If-Range: "534231200_1732277116"68Please respect copyright.ＰＥＮＡＮＡAMtGyHavHy
------------------------------------68Please respect copyright.ＰＥＮＡＮＡX8vTEJXtGS
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:68Please respect copyright.ＰＥＮＡＮＡTzzvW6DIGw
-------------range response-------------------68Please respect copyright.ＰＥＮＡＮＡ7wgQCwpBRa
HTTP/1.1 206 Partial Content68Please respect copyright.ＰＥＮＡＮＡu0CdTjx9cr
accept-ranges: bytes68Please respect copyright.ＰＥＮＡＮＡRkaKM7CkLE
content-range: bytes 287718144-534231199/53423120068Please respect copyright.ＰＥＮＡＮＡy6b8xovVJr
ETag: "534231200_1732277116"68Please respect copyright.ＰＥＮＡＮＡqxaWUvJ2XK
Content-Length: 24651305668Please respect copyright.ＰＥＮＡＮＡe0hPgSP1fe
Content-Type: application/octet-stream68Please respect copyright.ＰＥＮＡＮＡF20GjWD4Oi
Content-Disposition: attachment; filename="zen.iso"
(http body is the remain file content)68Please respect copyright.ＰＥＮＡＮＡHaPBXYd7Iu
-------------------------------------------
If the etag changes, response 200 from beginning like this:68Please respect copyright.ＰＥＮＡＮＡLvSeXOQDHs
-----------http response from the beginning------68Please respect copyright.ＰＥＮＡＮＡ4GxKUqxcZH
HTTP/1.1 200 OK68Please respect copyright.ＰＥＮＡＮＡDCgIaSRCWr
Date: Mon, 25 Nov 2024 10:56:12 GMT68Please respect copyright.ＰＥＮＡＮＡAnpzqwiXn9
accept-ranges: bytes68Please respect copyright.ＰＥＮＡＮＡegzx4mWS6o
ETag: "534231200_1732277116"68Please respect copyright.ＰＥＮＡＮＡ6jpnyKmzav
Content-Length: 53423120068Please respect copyright.ＰＥＮＡＮＡp7aqemtO0g
Content-Type: application/octet-stream68Please respect copyright.ＰＥＮＡＮＡrHUyMB6dU4
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)68Please respect copyright.ＰＥＮＡＮＡ1Azqtas4Gy
---------------------------------------------------
68Please respect copyright.ＰＥＮＡＮＡ4kqQjDHlJM
Section X. Thanks
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/If-None-Match68Please respect copyright.ＰＥＮＡＮＡIlipiECYLj
https://blog.csdn.net/catoop/article/details/13417465368Please respect copyright.ＰＥＮＡＮＡNVw2qWDCmr
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]68Please respect copyright.ＰＥＮＡＮＡBmRlqAo0LS
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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