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.
203Please respect copyright.ＰＥＮＡＮＡEQQG6kK5rb
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:203Please respect copyright.ＰＥＮＡＮＡnM0UNFlfBb
---------------http response with etag----------------203Please respect copyright.ＰＥＮＡＮＡPsTgWNKEMJ
HTTP/1.1 200 OK203Please respect copyright.ＰＥＮＡＮＡvo4I01EiQf
Date: Mon, 25 Nov 2024 10:56:12 GMT203Please respect copyright.ＰＥＮＡＮＡIoGR0GwKaa
accept-ranges: bytes203Please respect copyright.ＰＥＮＡＮＡ6Lj2cUcbSw
ETag: "534231200_1732277116"203Please respect copyright.ＰＥＮＡＮＡ8OSH9Al3zl
Content-Length: 534231200203Please respect copyright.ＰＥＮＡＮＡe4IiIAhbKb
Content-Type: application/octet-stream203Please respect copyright.ＰＥＮＡＮＡ9YQg4Lbgly
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)203Please respect copyright.ＰＥＮＡＮＡvZh2Oo6okB
-----------------------------------------------------
203Please respect copyright.ＰＥＮＡＮＡ3p7NRvmEE1
Section B. if-none-match
The browser send back the etag to ask server if it has been changed by header like this:203Please respect copyright.ＰＥＮＡＮＡba4kGWSnvp
-----http request for a cached resource------203Please respect copyright.ＰＥＮＡＮＡ0LvaQHUs43
GET /zen.iso HTTP/1.1203Please respect copyright.ＰＥＮＡＮＡ6JEEkNiQAo
if-none-match: "534231200_1732277116"203Please respect copyright.ＰＥＮＡＮＡoVqD1aTaE5
---------------------------------------------
If the etag is unchanged, the server response like this:
1. for get and head requests:203Please respect copyright.ＰＥＮＡＮＡkF3t8fPx9F
-----------http response for unchanged resource------203Please respect copyright.ＰＥＮＡＮＡszFxcWHpLk
HTTP/1.1 304 Not Modified
(http body canbe empty, the client will used its cache)203Please respect copyright.ＰＥＮＡＮＡ7Ig6j0vZUJ
----------------------------------------------------
2. for other methods requests:203Please respect copyright.ＰＥＮＡＮＡi7kqiJRRPF
-----------http response for unchanged resource------203Please respect copyright.ＰＥＮＡＮＡ2G2UukNCXN
HTTP/1.1 412 Precondition Failed
(http body canbe empty, the client will used its cache)203Please respect copyright.ＰＥＮＡＮＡyrsvsiKIyM
----------------------------------------------------
203Please respect copyright.ＰＥＮＡＮＡ8OFA9og8a5
If the etag changes, the server response 200 OK with the new file content like this:203Please respect copyright.ＰＥＮＡＮＡ9ogadYxvFA
---------------http response with etag----------------203Please respect copyright.ＰＥＮＡＮＡ6GkRyyR8YS
HTTP/1.1 200 OK203Please respect copyright.ＰＥＮＡＮＡ39ZzOtpnRe
Date: Mon, 25 Nov 2024 10:56:12 GMT203Please respect copyright.ＰＥＮＡＮＡNyAk3uH1s6
accept-ranges: bytes203Please respect copyright.ＰＥＮＡＮＡGzh70Kc3xy
ETag: "534231200_1732277116"203Please respect copyright.ＰＥＮＡＮＡpFeZrr4U7u
Content-Length: 534231200203Please respect copyright.ＰＥＮＡＮＡix3E1fO0H4
Content-Type: application/octet-stream203Please respect copyright.ＰＥＮＡＮＡxzyWIxoIGb
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)203Please respect copyright.ＰＥＮＡＮＡKmVyUJW3yt
-----------------------------------------------------
203Please respect copyright.ＰＥＮＡＮＡoiMHgaZvJe
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:203Please respect copyright.ＰＥＮＡＮＡNOk0qTMoPR
------resume download request-------203Please respect copyright.ＰＥＮＡＮＡYQGoa60id1
GET /zen.iso HTTP/1.1203Please respect copyright.ＰＥＮＡＮＡxg3jbuxP6W
Range: bytes=287718144-203Please respect copyright.ＰＥＮＡＮＡaGt2SWc678
If-Match: "534231200_1732277116"203Please respect copyright.ＰＥＮＡＮＡrRXmCnttJe
-----------------------------------203Please respect copyright.ＰＥＮＡＮＡiRUhhVd6Qw
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:203Please respect copyright.ＰＥＮＡＮＡiHviNWGtdZ
-------------range response-------------------203Please respect copyright.ＰＥＮＡＮＡts1rvmfWHe
HTTP/1.1 206 Partial Content203Please respect copyright.ＰＥＮＡＮＡ2IWDxeCMbE
accept-ranges: bytes203Please respect copyright.ＰＥＮＡＮＡsxuHaF6a0q
content-range: bytes 287718144-534231199/534231200203Please respect copyright.ＰＥＮＡＮＡFrSrsT8Ja8
ETag: "534231200_1732277116"203Please respect copyright.ＰＥＮＡＮＡA2qQps0S3k
Content-Length: 246513056203Please respect copyright.ＰＥＮＡＮＡXnoP06wj8w
Content-Type: application/octet-stream203Please respect copyright.ＰＥＮＡＮＡVx6KrDPgIJ
Content-Disposition: attachment; filename="zen.iso"
(http body is the remain file content)203Please respect copyright.ＰＥＮＡＮＡDuFT8pqyD8
-------------------------------------------
If the etag changes, response 412 like this:203Please respect copyright.ＰＥＮＡＮＡBAexOLvpOJ
-----------http response for changed resource------203Please respect copyright.ＰＥＮＡＮＡaCjw5Kj05P
HTTP/1.1 412 Precondition Failed
(http body canbe empty, the client will stop downloading the resumed file)203Please respect copyright.ＰＥＮＡＮＡXjRgB6i8zD
---------------------------------------------------
203Please respect copyright.ＰＥＮＡＮＡTTIyQRsD5I
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---203Please respect copyright.ＰＥＮＡＮＡMa1kiZj6DK
GET /zen.iso HTTP/1.1203Please respect copyright.ＰＥＮＡＮＡAsR8grIrzd
Range: bytes=287718144-203Please respect copyright.ＰＥＮＡＮＡU2ur07ByMw
If-Range: "534231200_1732277116"203Please respect copyright.ＰＥＮＡＮＡyY7gRy2n2P
------------------------------------203Please respect copyright.ＰＥＮＡＮＡ9BQnJHTtCf
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:203Please respect copyright.ＰＥＮＡＮＡVJaDf99GPU
-------------range response-------------------203Please respect copyright.ＰＥＮＡＮＡ8mizATKhRI
HTTP/1.1 206 Partial Content203Please respect copyright.ＰＥＮＡＮＡJGORxXLQ5K
accept-ranges: bytes203Please respect copyright.ＰＥＮＡＮＡxzeIk0jego
content-range: bytes 287718144-534231199/534231200203Please respect copyright.ＰＥＮＡＮＡUGjwOQmt7X
ETag: "534231200_1732277116"203Please respect copyright.ＰＥＮＡＮＡJ5kiP9O10c
Content-Length: 246513056203Please respect copyright.ＰＥＮＡＮＡlnjTck0v7v
Content-Type: application/octet-stream203Please respect copyright.ＰＥＮＡＮＡSJBi8mU7Ho
Content-Disposition: attachment; filename="zen.iso"
(http body is the remain file content)203Please respect copyright.ＰＥＮＡＮＡ0dqp1I6HsH
-------------------------------------------
If the etag changes, response 200 from beginning like this:203Please respect copyright.ＰＥＮＡＮＡv9hKtiSu2j
-----------http response from the beginning------203Please respect copyright.ＰＥＮＡＮＡ2ntDiqMZEm
HTTP/1.1 200 OK203Please respect copyright.ＰＥＮＡＮＡUnqCobXT4m
Date: Mon, 25 Nov 2024 10:56:12 GMT203Please respect copyright.ＰＥＮＡＮＡcOhGKT8cM0
accept-ranges: bytes203Please respect copyright.ＰＥＮＡＮＡr6FlMJKq8m
ETag: "534231200_1732277116"203Please respect copyright.ＰＥＮＡＮＡWtdg59TBFp
Content-Length: 534231200203Please respect copyright.ＰＥＮＡＮＡHLZX2n2ynm
Content-Type: application/octet-stream203Please respect copyright.ＰＥＮＡＮＡbBdedGbnE2
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)203Please respect copyright.ＰＥＮＡＮＡeJZFcLkphm
---------------------------------------------------
203Please respect copyright.ＰＥＮＡＮＡ5L2bU44rcU
Section X. Thanks
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/If-None-Match203Please respect copyright.ＰＥＮＡＮＡhIvEB7q4QE
https://blog.csdn.net/catoop/article/details/134174653203Please respect copyright.ＰＥＮＡＮＡevKGDG7hl1
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]203Please respect copyright.ＰＥＮＡＮＡCYpWKLCPR3
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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