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.
51Please respect copyright.ＰＥＮＡＮＡVOe1BJTA8U
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:51Please respect copyright.ＰＥＮＡＮＡFvhzpEKAjz
---------------http response with etag----------------51Please respect copyright.ＰＥＮＡＮＡXpTHIWdyVZ
HTTP/1.1 200 OK51Please respect copyright.ＰＥＮＡＮＡQ1Y6mqupIh
Date: Mon, 25 Nov 2024 10:56:12 GMT51Please respect copyright.ＰＥＮＡＮＡO0CW6Q6uhd
accept-ranges: bytes51Please respect copyright.ＰＥＮＡＮＡLFBJvDKTCp
ETag: "534231200_1732277116"51Please respect copyright.ＰＥＮＡＮＡAhlC8ByIDU
Content-Length: 53423120051Please respect copyright.ＰＥＮＡＮＡlXFLHshcuK
Content-Type: application/octet-stream51Please respect copyright.ＰＥＮＡＮＡCCsPckEU6g
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)51Please respect copyright.ＰＥＮＡＮＡQ3xoTzLSiV
-----------------------------------------------------
51Please respect copyright.ＰＥＮＡＮＡB96M6y6qZ8
Section B. if-none-match
The browser send back the etag to ask server if it has been changed by header like this:51Please respect copyright.ＰＥＮＡＮＡnv9qblwnK3
-----http request for a cached resource------51Please respect copyright.ＰＥＮＡＮＡbf4YsiSohN
GET /zen.iso HTTP/1.151Please respect copyright.ＰＥＮＡＮＡDijqPkaOzV
if-none-match: "534231200_1732277116"51Please respect copyright.ＰＥＮＡＮＡ6910xxf3aj
---------------------------------------------
If the etag is unchanged, the server response like this:
1. for get and head requests:51Please respect copyright.ＰＥＮＡＮＡAX4QLYtUdQ
-----------http response for unchanged resource------51Please respect copyright.ＰＥＮＡＮＡQsKtAAHQFn
HTTP/1.1 304 Not Modified
(http body canbe empty, the client will used its cache)51Please respect copyright.ＰＥＮＡＮＡl5ix6HX2JO
----------------------------------------------------
2. for other methods requests:51Please respect copyright.ＰＥＮＡＮＡnwwVvAu9LG
-----------http response for unchanged resource------51Please respect copyright.ＰＥＮＡＮＡABOVJU4LwU
HTTP/1.1 412 Precondition Failed
(http body canbe empty, the client will used its cache)51Please respect copyright.ＰＥＮＡＮＡnhKMJbET3W
----------------------------------------------------
51Please respect copyright.ＰＥＮＡＮＡV4iof9RrYz
If the etag changes, the server response 200 OK with the new file content like this:51Please respect copyright.ＰＥＮＡＮＡpomaMK1n7B
---------------http response with etag----------------51Please respect copyright.ＰＥＮＡＮＡjVB80n8lVN
HTTP/1.1 200 OK51Please respect copyright.ＰＥＮＡＮＡTFUQJePS5Z
Date: Mon, 25 Nov 2024 10:56:12 GMT51Please respect copyright.ＰＥＮＡＮＡLRj7ZfqANo
accept-ranges: bytes51Please respect copyright.ＰＥＮＡＮＡmpufx4iR1O
ETag: "534231200_1732277116"51Please respect copyright.ＰＥＮＡＮＡAGaH0qgqHn
Content-Length: 53423120051Please respect copyright.ＰＥＮＡＮＡNdUpIZzSPS
Content-Type: application/octet-stream51Please respect copyright.ＰＥＮＡＮＡHAITbCDW5m
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)51Please respect copyright.ＰＥＮＡＮＡSv2KOR10Fe
-----------------------------------------------------
51Please respect copyright.ＰＥＮＡＮＡwL66fxYs2w
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:51Please respect copyright.ＰＥＮＡＮＡuzOOnydHD5
------resume download request-------51Please respect copyright.ＰＥＮＡＮＡ1HeACs2wu1
GET /zen.iso HTTP/1.151Please respect copyright.ＰＥＮＡＮＡ0nmoZadJF3
Range: bytes=287718144-51Please respect copyright.ＰＥＮＡＮＡwhLUKntOGE
If-Match: "534231200_1732277116"51Please respect copyright.ＰＥＮＡＮＡTC4UyviKg8
-----------------------------------51Please respect copyright.ＰＥＮＡＮＡpQ5XXRziKJ
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:51Please respect copyright.ＰＥＮＡＮＡg299E0Ss5K
-------------range response-------------------51Please respect copyright.ＰＥＮＡＮＡR1M5aOGmhF
HTTP/1.1 206 Partial Content51Please respect copyright.ＰＥＮＡＮＡ9CxewETAWa
accept-ranges: bytes51Please respect copyright.ＰＥＮＡＮＡRDITSGmWj4
content-range: bytes 287718144-534231199/53423120051Please respect copyright.ＰＥＮＡＮＡpKJ6tmUeau
ETag: "534231200_1732277116"51Please respect copyright.ＰＥＮＡＮＡFqiOBCEu9S
Content-Length: 24651305651Please respect copyright.ＰＥＮＡＮＡINqkuQLm9W
Content-Type: application/octet-stream51Please respect copyright.ＰＥＮＡＮＡxERGjzSFt9
Content-Disposition: attachment; filename="zen.iso"
(http body is the remain file content)51Please respect copyright.ＰＥＮＡＮＡTPzvEAkUSD
-------------------------------------------
If the etag changes, response 412 like this:51Please respect copyright.ＰＥＮＡＮＡGPFaR7wFed
-----------http response for changed resource------51Please respect copyright.ＰＥＮＡＮＡZEXJStHHaP
HTTP/1.1 412 Precondition Failed
(http body canbe empty, the client will stop downloading the resumed file)51Please respect copyright.ＰＥＮＡＮＡYCNozNzbev
---------------------------------------------------
51Please respect copyright.ＰＥＮＡＮＡE7Zo88Cu6n
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---51Please respect copyright.ＰＥＮＡＮＡ72nTtPKK4x
GET /zen.iso HTTP/1.151Please respect copyright.ＰＥＮＡＮＡJ5h1zNsWqg
Range: bytes=287718144-51Please respect copyright.ＰＥＮＡＮＡmtTDSgvbUb
If-Range: "534231200_1732277116"51Please respect copyright.ＰＥＮＡＮＡzY5Kz9Djsr
------------------------------------51Please respect copyright.ＰＥＮＡＮＡZ8embcGEeB
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:51Please respect copyright.ＰＥＮＡＮＡF1aauytrxG
-------------range response-------------------51Please respect copyright.ＰＥＮＡＮＡ2lsySlsHl5
HTTP/1.1 206 Partial Content51Please respect copyright.ＰＥＮＡＮＡ8qexAQzmgh
accept-ranges: bytes51Please respect copyright.ＰＥＮＡＮＡUd1k9UkWAY
content-range: bytes 287718144-534231199/53423120051Please respect copyright.ＰＥＮＡＮＡc5uScM7Hhp
ETag: "534231200_1732277116"51Please respect copyright.ＰＥＮＡＮＡVINJHb6Fca
Content-Length: 24651305651Please respect copyright.ＰＥＮＡＮＡ6wCEeuHBHU
Content-Type: application/octet-stream51Please respect copyright.ＰＥＮＡＮＡldHmSSnt8r
Content-Disposition: attachment; filename="zen.iso"
(http body is the remain file content)51Please respect copyright.ＰＥＮＡＮＡxSfSCGHgEY
-------------------------------------------
If the etag changes, response 200 from beginning like this:51Please respect copyright.ＰＥＮＡＮＡL2dVplDbjt
-----------http response from the beginning------51Please respect copyright.ＰＥＮＡＮＡGK7H1noZT7
HTTP/1.1 200 OK51Please respect copyright.ＰＥＮＡＮＡwihG3Ij5TJ
Date: Mon, 25 Nov 2024 10:56:12 GMT51Please respect copyright.ＰＥＮＡＮＡfbwVAmWZAI
accept-ranges: bytes51Please respect copyright.ＰＥＮＡＮＡJ6WzdFRhJ0
ETag: "534231200_1732277116"51Please respect copyright.ＰＥＮＡＮＡjCN8pJU3xK
Content-Length: 53423120051Please respect copyright.ＰＥＮＡＮＡEqb2ip34pb
Content-Type: application/octet-stream51Please respect copyright.ＰＥＮＡＮＡemKy6wRLgR
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)51Please respect copyright.ＰＥＮＡＮＡVoRRa98jbb
---------------------------------------------------
51Please respect copyright.ＰＥＮＡＮＡmM2Cj5TsXv
Section X. Thanks
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/If-None-Match51Please respect copyright.ＰＥＮＡＮＡqc3bd1wTdH
https://blog.csdn.net/catoop/article/details/13417465351Please respect copyright.ＰＥＮＡＮＡYMSWWFVRcS
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.com51Please respect copyright.ＰＥＮＡＮＡ3s0XMzQ90h
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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