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.
150Please respect copyright.ＰＥＮＡＮＡ4UbSqjhzyV
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:150Please respect copyright.ＰＥＮＡＮＡziShoEiMkZ
---------------http response with etag----------------150Please respect copyright.ＰＥＮＡＮＡBt2U0zzACd
HTTP/1.1 200 OK150Please respect copyright.ＰＥＮＡＮＡI0kvixC07Z
Date: Mon, 25 Nov 2024 10:56:12 GMT150Please respect copyright.ＰＥＮＡＮＡ28By990DxZ
accept-ranges: bytes150Please respect copyright.ＰＥＮＡＮＡRaLcRDB28I
ETag: "534231200_1732277116"150Please respect copyright.ＰＥＮＡＮＡy8du5K2MZC
Content-Length: 534231200150Please respect copyright.ＰＥＮＡＮＡcnwiZqfEpB
Content-Type: application/octet-stream150Please respect copyright.ＰＥＮＡＮＡ3DeMqqi0zB
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)150Please respect copyright.ＰＥＮＡＮＡJ2NrprgqWO
-----------------------------------------------------
150Please respect copyright.ＰＥＮＡＮＡOpXB9AVmNZ
Section B. if-none-match
The browser send back the etag to ask server if it has been changed by header like this:150Please respect copyright.ＰＥＮＡＮＡNfyqzebRHp
-----http request for a cached resource------150Please respect copyright.ＰＥＮＡＮＡpJ1kYtYE4v
GET /zen.iso HTTP/1.1150Please respect copyright.ＰＥＮＡＮＡCtxjloBmiX
if-none-match: "534231200_1732277116"150Please respect copyright.ＰＥＮＡＮＡR2N33RRTLs
---------------------------------------------
If the etag is unchanged, the server response like this:
1. for get and head requests:150Please respect copyright.ＰＥＮＡＮＡeqv2TE4UNN
-----------http response for unchanged resource------150Please respect copyright.ＰＥＮＡＮＡIzyYfdvNHx
HTTP/1.1 304 Not Modified
(http body canbe empty, the client will used its cache)150Please respect copyright.ＰＥＮＡＮＡBdQt0NclBc
----------------------------------------------------
2. for other methods requests:150Please respect copyright.ＰＥＮＡＮＡJGhOWi4PiV
-----------http response for unchanged resource------150Please respect copyright.ＰＥＮＡＮＡ1SYWIR4Y1y
HTTP/1.1 412 Precondition Failed
(http body canbe empty, the client will used its cache)150Please respect copyright.ＰＥＮＡＮＡANKrfcf9oG
----------------------------------------------------
150Please respect copyright.ＰＥＮＡＮＡ56RPjfz7Le
If the etag changes, the server response 200 OK with the new file content like this:150Please respect copyright.ＰＥＮＡＮＡIUGKZjx6FN
---------------http response with etag----------------150Please respect copyright.ＰＥＮＡＮＡC37wS5DaxE
HTTP/1.1 200 OK150Please respect copyright.ＰＥＮＡＮＡeiTyjlBiuQ
Date: Mon, 25 Nov 2024 10:56:12 GMT150Please respect copyright.ＰＥＮＡＮＡgwYYP635pf
accept-ranges: bytes150Please respect copyright.ＰＥＮＡＮＡVpVUS0gLNx
ETag: "534231200_1732277116"150Please respect copyright.ＰＥＮＡＮＡOteGWnM8Qq
Content-Length: 534231200150Please respect copyright.ＰＥＮＡＮＡwBHggsdV54
Content-Type: application/octet-stream150Please respect copyright.ＰＥＮＡＮＡXzIuH2rwjN
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)150Please respect copyright.ＰＥＮＡＮＡHuLnPwwVqN
-----------------------------------------------------
150Please respect copyright.ＰＥＮＡＮＡHiKMf88291
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:150Please respect copyright.ＰＥＮＡＮＡSFdONDP0CV
------resume download request-------150Please respect copyright.ＰＥＮＡＮＡyRK43P3kEb
GET /zen.iso HTTP/1.1150Please respect copyright.ＰＥＮＡＮＡoNJ9KbW1sB
Range: bytes=287718144-150Please respect copyright.ＰＥＮＡＮＡyAyRLlqWuf
If-Match: "534231200_1732277116"150Please respect copyright.ＰＥＮＡＮＡfSYXpA3trw
-----------------------------------150Please respect copyright.ＰＥＮＡＮＡ0Emk6q4aba
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:150Please respect copyright.ＰＥＮＡＮＡ7jO1k5LLXu
-------------range response-------------------150Please respect copyright.ＰＥＮＡＮＡEuHbaiqWFd
HTTP/1.1 206 Partial Content150Please respect copyright.ＰＥＮＡＮＡ6Z8B8JTe8q
accept-ranges: bytes150Please respect copyright.ＰＥＮＡＮＡOGdxoFQ90b
content-range: bytes 287718144-534231199/534231200150Please respect copyright.ＰＥＮＡＮＡgl2EiE6GC4
ETag: "534231200_1732277116"150Please respect copyright.ＰＥＮＡＮＡWKVrD0McmV
Content-Length: 246513056150Please respect copyright.ＰＥＮＡＮＡ1EaN1R9vWu
Content-Type: application/octet-stream150Please respect copyright.ＰＥＮＡＮＡjtAD0w7UzI
Content-Disposition: attachment; filename="zen.iso"
(http body is the remain file content)150Please respect copyright.ＰＥＮＡＮＡWlRqHvDJ6J
-------------------------------------------
If the etag changes, response 412 like this:150Please respect copyright.ＰＥＮＡＮＡRscY8Iiqcl
-----------http response for changed resource------150Please respect copyright.ＰＥＮＡＮＡKGn6TMEOnL
HTTP/1.1 412 Precondition Failed
(http body canbe empty, the client will stop downloading the resumed file)150Please respect copyright.ＰＥＮＡＮＡN1HzudWV8W
---------------------------------------------------
150Please respect copyright.ＰＥＮＡＮＡiGzp0Wzj5B
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---150Please respect copyright.ＰＥＮＡＮＡIIhqDNTqcb
GET /zen.iso HTTP/1.1150Please respect copyright.ＰＥＮＡＮＡNm1ir4cs2U
Range: bytes=287718144-150Please respect copyright.ＰＥＮＡＮＡa0KPholJkF
If-Range: "534231200_1732277116"150Please respect copyright.ＰＥＮＡＮＡVWqJmgn1dR
------------------------------------150Please respect copyright.ＰＥＮＡＮＡggL6ZCsLxa
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:150Please respect copyright.ＰＥＮＡＮＡWCq31DElDl
-------------range response-------------------150Please respect copyright.ＰＥＮＡＮＡ97hAeRXROq
HTTP/1.1 206 Partial Content150Please respect copyright.ＰＥＮＡＮＡJzsCqvRGJP
accept-ranges: bytes150Please respect copyright.ＰＥＮＡＮＡRWRoGXuUCD
content-range: bytes 287718144-534231199/534231200150Please respect copyright.ＰＥＮＡＮＡ4rdzCbuDLo
ETag: "534231200_1732277116"150Please respect copyright.ＰＥＮＡＮＡWI1lbSXW4a
Content-Length: 246513056150Please respect copyright.ＰＥＮＡＮＡu7PahjDkXE
Content-Type: application/octet-stream150Please respect copyright.ＰＥＮＡＮＡIwJ1JDFFJ0
Content-Disposition: attachment; filename="zen.iso"
(http body is the remain file content)150Please respect copyright.ＰＥＮＡＮＡbvMtfJUBCC
-------------------------------------------
If the etag changes, response 200 from beginning like this:150Please respect copyright.ＰＥＮＡＮＡu1tVb9lLBM
-----------http response from the beginning------150Please respect copyright.ＰＥＮＡＮＡQY7FgLTkZL
HTTP/1.1 200 OK150Please respect copyright.ＰＥＮＡＮＡDBCjvB2cGb
Date: Mon, 25 Nov 2024 10:56:12 GMT150Please respect copyright.ＰＥＮＡＮＡ6spNZUtTOb
accept-ranges: bytes150Please respect copyright.ＰＥＮＡＮＡjPj10zluFI
ETag: "534231200_1732277116"150Please respect copyright.ＰＥＮＡＮＡ1ETZoxFBtz
Content-Length: 534231200150Please respect copyright.ＰＥＮＡＮＡ9kpkZP1Ka8
Content-Type: application/octet-stream150Please respect copyright.ＰＥＮＡＮＡSZJxRA72m6
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)150Please respect copyright.ＰＥＮＡＮＡ9mKHVRnluB
---------------------------------------------------
150Please respect copyright.ＰＥＮＡＮＡiKW4XU1gqf
Section X. Thanks
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/If-None-Match150Please respect copyright.ＰＥＮＡＮＡAgwNIighSn
https://blog.csdn.net/catoop/article/details/134174653150Please respect copyright.ＰＥＮＡＮＡcHoM2ngZvY
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]150Please respect copyright.ＰＥＮＡＮＡ3G1LmZkksY
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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