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.
53Please respect copyright.ＰＥＮＡＮＡZDIxAc4HBd
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:53Please respect copyright.ＰＥＮＡＮＡ4ytjRZlUko
---------------http response with etag----------------53Please respect copyright.ＰＥＮＡＮＡJEKhHjeDvO
HTTP/1.1 200 OK53Please respect copyright.ＰＥＮＡＮＡlIN0IcsOdA
Date: Mon, 25 Nov 2024 10:56:12 GMT53Please respect copyright.ＰＥＮＡＮＡAhpapykc5d
accept-ranges: bytes53Please respect copyright.ＰＥＮＡＮＡPVFlWn3Obn
ETag: "534231200_1732277116"53Please respect copyright.ＰＥＮＡＮＡ6mptrYAsO3
Content-Length: 53423120053Please respect copyright.ＰＥＮＡＮＡxm8V0umP9J
Content-Type: application/octet-stream53Please respect copyright.ＰＥＮＡＮＡPv59CXOvUA
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)53Please respect copyright.ＰＥＮＡＮＡB9v6sClaQR
-----------------------------------------------------
53Please respect copyright.ＰＥＮＡＮＡKEH9eK5IFk
Section B. if-none-match
The browser send back the etag to ask server if it has been changed by header like this:53Please respect copyright.ＰＥＮＡＮＡvtLzazOU8z
-----http request for a cached resource------53Please respect copyright.ＰＥＮＡＮＡbDTYACzuV8
GET /zen.iso HTTP/1.153Please respect copyright.ＰＥＮＡＮＡSNdjdcQQwj
if-none-match: "534231200_1732277116"53Please respect copyright.ＰＥＮＡＮＡUHhsP2gpUu
---------------------------------------------
If the etag is unchanged, the server response like this:
1. for get and head requests:53Please respect copyright.ＰＥＮＡＮＡGxs59jVv11
-----------http response for unchanged resource------53Please respect copyright.ＰＥＮＡＮＡTtjmrGJyXx
HTTP/1.1 304 Not Modified
(http body canbe empty, the client will used its cache)53Please respect copyright.ＰＥＮＡＮＡ6ExwLY91Os
----------------------------------------------------
2. for other methods requests:53Please respect copyright.ＰＥＮＡＮＡtVJBC6YztU
-----------http response for unchanged resource------53Please respect copyright.ＰＥＮＡＮＡD4lX6ceUq8
HTTP/1.1 412 Precondition Failed
(http body canbe empty, the client will used its cache)53Please respect copyright.ＰＥＮＡＮＡyOQaAVvguw
----------------------------------------------------
53Please respect copyright.ＰＥＮＡＮＡ2sv74c0h53
If the etag changes, the server response 200 OK with the new file content like this:53Please respect copyright.ＰＥＮＡＮＡJQXUFJXtp2
---------------http response with etag----------------53Please respect copyright.ＰＥＮＡＮＡVOhXSvy6L9
HTTP/1.1 200 OK53Please respect copyright.ＰＥＮＡＮＡs3r9xHpimz
Date: Mon, 25 Nov 2024 10:56:12 GMT53Please respect copyright.ＰＥＮＡＮＡSyhISbp0eL
accept-ranges: bytes53Please respect copyright.ＰＥＮＡＮＡrYZYlFpHfg
ETag: "534231200_1732277116"53Please respect copyright.ＰＥＮＡＮＡSON3MYsWLl
Content-Length: 53423120053Please respect copyright.ＰＥＮＡＮＡnd5WqfvDp8
Content-Type: application/octet-stream53Please respect copyright.ＰＥＮＡＮＡEXb9S0PxgL
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)53Please respect copyright.ＰＥＮＡＮＡYDMiUuT4vx
-----------------------------------------------------
53Please respect copyright.ＰＥＮＡＮＡgRv41ePItj
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:53Please respect copyright.ＰＥＮＡＮＡRY0EbBgbBc
------resume download request-------53Please respect copyright.ＰＥＮＡＮＡgx873Gwnf3
GET /zen.iso HTTP/1.153Please respect copyright.ＰＥＮＡＮＡnAoSS6wjmI
Range: bytes=287718144-53Please respect copyright.ＰＥＮＡＮＡH0VHqDhU17
If-Match: "534231200_1732277116"53Please respect copyright.ＰＥＮＡＮＡZD5lkEWSpn
-----------------------------------53Please respect copyright.ＰＥＮＡＮＡx5FCH5Xht1
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:53Please respect copyright.ＰＥＮＡＮＡONJHDrUIut
-------------range response-------------------53Please respect copyright.ＰＥＮＡＮＡNs8ywCyDVW
HTTP/1.1 206 Partial Content53Please respect copyright.ＰＥＮＡＮＡThHsmq99wY
accept-ranges: bytes53Please respect copyright.ＰＥＮＡＮＡ57LnI7egID
content-range: bytes 287718144-534231199/53423120053Please respect copyright.ＰＥＮＡＮＡH1loa5DJng
ETag: "534231200_1732277116"53Please respect copyright.ＰＥＮＡＮＡ7NtNlivU1P
Content-Length: 24651305653Please respect copyright.ＰＥＮＡＮＡld7EDHbfuX
Content-Type: application/octet-stream53Please respect copyright.ＰＥＮＡＮＡop1SzrUCnS
Content-Disposition: attachment; filename="zen.iso"
(http body is the remain file content)53Please respect copyright.ＰＥＮＡＮＡwa1PirNnRx
-------------------------------------------
If the etag changes, response 412 like this:53Please respect copyright.ＰＥＮＡＮＡr0JcHAIaZs
-----------http response for changed resource------53Please respect copyright.ＰＥＮＡＮＡFI7kBMA6oL
HTTP/1.1 412 Precondition Failed
(http body canbe empty, the client will stop downloading the resumed file)53Please respect copyright.ＰＥＮＡＮＡ9ATGnaq43z
---------------------------------------------------
53Please respect copyright.ＰＥＮＡＮＡi91iN7WRJX
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---53Please respect copyright.ＰＥＮＡＮＡimVQdBXWEt
GET /zen.iso HTTP/1.153Please respect copyright.ＰＥＮＡＮＡL0hHu5LYu2
Range: bytes=287718144-53Please respect copyright.ＰＥＮＡＮＡTVmjQbII1A
If-Range: "534231200_1732277116"53Please respect copyright.ＰＥＮＡＮＡxLgMOu0gK1
------------------------------------53Please respect copyright.ＰＥＮＡＮＡ06Sef164i0
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:53Please respect copyright.ＰＥＮＡＮＡV7WT0N93aB
-------------range response-------------------53Please respect copyright.ＰＥＮＡＮＡ6wnSdshjm3
HTTP/1.1 206 Partial Content53Please respect copyright.ＰＥＮＡＮＡZivkXMjjcG
accept-ranges: bytes53Please respect copyright.ＰＥＮＡＮＡ9GzIb6EawS
content-range: bytes 287718144-534231199/53423120053Please respect copyright.ＰＥＮＡＮＡm3IMjpVEWy
ETag: "534231200_1732277116"53Please respect copyright.ＰＥＮＡＮＡilyCzxfKOW
Content-Length: 24651305653Please respect copyright.ＰＥＮＡＮＡNCpcTLheH1
Content-Type: application/octet-stream53Please respect copyright.ＰＥＮＡＮＡhcE4DslA61
Content-Disposition: attachment; filename="zen.iso"
(http body is the remain file content)53Please respect copyright.ＰＥＮＡＮＡ2n1H9uv3cB
-------------------------------------------
If the etag changes, response 200 from beginning like this:53Please respect copyright.ＰＥＮＡＮＡafBOs2FtSm
-----------http response from the beginning------53Please respect copyright.ＰＥＮＡＮＡ9Pc9VOdBRF
HTTP/1.1 200 OK53Please respect copyright.ＰＥＮＡＮＡHhko7m76Vw
Date: Mon, 25 Nov 2024 10:56:12 GMT53Please respect copyright.ＰＥＮＡＮＡX6gNL5sGTA
accept-ranges: bytes53Please respect copyright.ＰＥＮＡＮＡOHFDa6Vdop
ETag: "534231200_1732277116"53Please respect copyright.ＰＥＮＡＮＡstcDrVxVXA
Content-Length: 53423120053Please respect copyright.ＰＥＮＡＮＡn6vwourXLD
Content-Type: application/octet-stream53Please respect copyright.ＰＥＮＡＮＡRJI1ANffA8
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)53Please respect copyright.ＰＥＮＡＮＡlnpfAc1Vr7
---------------------------------------------------
53Please respect copyright.ＰＥＮＡＮＡv1KQSwXUag
Section X. Thanks
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/If-None-Match53Please respect copyright.ＰＥＮＡＮＡHrhncXxK56
https://blog.csdn.net/catoop/article/details/13417465353Please respect copyright.ＰＥＮＡＮＡMwqZLeK49N
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: si.jinmin@gmail.com53Please respect copyright.ＰＥＮＡＮＡpl13S3OvMq
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: si.jinmin@gmail.com
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