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.
231Please respect copyright.ＰＥＮＡＮＡkpA05WUEnj
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:231Please respect copyright.ＰＥＮＡＮＡ3Q7FmohsvK
---------------http response with etag----------------231Please respect copyright.ＰＥＮＡＮＡRYAsWtGQJU
HTTP/1.1 200 OK231Please respect copyright.ＰＥＮＡＮＡLn2S0ELcoF
Date: Mon, 25 Nov 2024 10:56:12 GMT231Please respect copyright.ＰＥＮＡＮＡR3HhANmvhv
accept-ranges: bytes231Please respect copyright.ＰＥＮＡＮＡGzYnlwaZhS
ETag: "534231200_1732277116"231Please respect copyright.ＰＥＮＡＮＡxhV3u8iKxw
Content-Length: 534231200231Please respect copyright.ＰＥＮＡＮＡV6PjztesQK
Content-Type: application/octet-stream231Please respect copyright.ＰＥＮＡＮＡbjKlZQt5aV
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)231Please respect copyright.ＰＥＮＡＮＡxQCoOMBeWA
-----------------------------------------------------
231Please respect copyright.ＰＥＮＡＮＡEezdclhYQZ
Section B. if-none-match
The browser send back the etag to ask server if it has been changed by header like this:231Please respect copyright.ＰＥＮＡＮＡX8zRCCLpIw
-----http request for a cached resource------231Please respect copyright.ＰＥＮＡＮＡ5cnrou9R3w
GET /zen.iso HTTP/1.1231Please respect copyright.ＰＥＮＡＮＡ9jXeeRDnXg
if-none-match: "534231200_1732277116"231Please respect copyright.ＰＥＮＡＮＡOQQTU70xsj
---------------------------------------------
If the etag is unchanged, the server response like this:
1. for get and head requests:231Please respect copyright.ＰＥＮＡＮＡUqOzRvRMVo
-----------http response for unchanged resource------231Please respect copyright.ＰＥＮＡＮＡL31tevQilo
HTTP/1.1 304 Not Modified
(http body canbe empty, the client will used its cache)231Please respect copyright.ＰＥＮＡＮＡv29hli4qyb
----------------------------------------------------
2. for other methods requests:231Please respect copyright.ＰＥＮＡＮＡDg6AKAOGt0
-----------http response for unchanged resource------231Please respect copyright.ＰＥＮＡＮＡU9CAgMZQ9f
HTTP/1.1 412 Precondition Failed
(http body canbe empty, the client will used its cache)231Please respect copyright.ＰＥＮＡＮＡNcV9Kye9KK
----------------------------------------------------
231Please respect copyright.ＰＥＮＡＮＡC7ZIQXPfTJ
If the etag changes, the server response 200 OK with the new file content like this:231Please respect copyright.ＰＥＮＡＮＡSFX9kdDF02
---------------http response with etag----------------231Please respect copyright.ＰＥＮＡＮＡAScg2h6D0K
HTTP/1.1 200 OK231Please respect copyright.ＰＥＮＡＮＡ8aV5weoqz2
Date: Mon, 25 Nov 2024 10:56:12 GMT231Please respect copyright.ＰＥＮＡＮＡptUjftiO0p
accept-ranges: bytes231Please respect copyright.ＰＥＮＡＮＡVC2e8n01aU
ETag: "534231200_1732277116"231Please respect copyright.ＰＥＮＡＮＡrcZhIeGzLy
Content-Length: 534231200231Please respect copyright.ＰＥＮＡＮＡVH0TBNjD2y
Content-Type: application/octet-stream231Please respect copyright.ＰＥＮＡＮＡSF7ITZoYJ2
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)231Please respect copyright.ＰＥＮＡＮＡuPJQ3uC6pq
-----------------------------------------------------
231Please respect copyright.ＰＥＮＡＮＡLPGyLc1Wfw
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:231Please respect copyright.ＰＥＮＡＮＡf6VoXnKuVz
------resume download request-------231Please respect copyright.ＰＥＮＡＮＡpZVYsjnSgv
GET /zen.iso HTTP/1.1231Please respect copyright.ＰＥＮＡＮＡK0K16594kE
Range: bytes=287718144-231Please respect copyright.ＰＥＮＡＮＡqZqAACxYHt
If-Match: "534231200_1732277116"231Please respect copyright.ＰＥＮＡＮＡHBlc20s5e7
-----------------------------------231Please respect copyright.ＰＥＮＡＮＡcfIBrqAGHx
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:231Please respect copyright.ＰＥＮＡＮＡpAo3LbnBcR
-------------range response-------------------231Please respect copyright.ＰＥＮＡＮＡYfWpKMZjAd
HTTP/1.1 206 Partial Content231Please respect copyright.ＰＥＮＡＮＡ9Pgcu58x66
accept-ranges: bytes231Please respect copyright.ＰＥＮＡＮＡECzSOGO8po
content-range: bytes 287718144-534231199/534231200231Please respect copyright.ＰＥＮＡＮＡih7hjfDdbH
ETag: "534231200_1732277116"231Please respect copyright.ＰＥＮＡＮＡnOtGOBD0v9
Content-Length: 246513056231Please respect copyright.ＰＥＮＡＮＡtB9KtbUR7k
Content-Type: application/octet-stream231Please respect copyright.ＰＥＮＡＮＡtvHFI09stM
Content-Disposition: attachment; filename="zen.iso"
(http body is the remain file content)231Please respect copyright.ＰＥＮＡＮＡxSxBYSvsAQ
-------------------------------------------
If the etag changes, response 412 like this:231Please respect copyright.ＰＥＮＡＮＡ2iY9xifXTP
-----------http response for changed resource------231Please respect copyright.ＰＥＮＡＮＡA9CnPXE8lb
HTTP/1.1 412 Precondition Failed
(http body canbe empty, the client will stop downloading the resumed file)231Please respect copyright.ＰＥＮＡＮＡA7hoQaxHd7
---------------------------------------------------
231Please respect copyright.ＰＥＮＡＮＡPAnfyWKYsU
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---231Please respect copyright.ＰＥＮＡＮＡnaGltmtktZ
GET /zen.iso HTTP/1.1231Please respect copyright.ＰＥＮＡＮＡxpSaShTLLX
Range: bytes=287718144-231Please respect copyright.ＰＥＮＡＮＡkmSSl7KBWX
If-Range: "534231200_1732277116"231Please respect copyright.ＰＥＮＡＮＡnPJZebQq0d
------------------------------------231Please respect copyright.ＰＥＮＡＮＡjUBBN1Xhmh
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:231Please respect copyright.ＰＥＮＡＮＡae043QmfMR
-------------range response-------------------231Please respect copyright.ＰＥＮＡＮＡV2cMvIdXgA
HTTP/1.1 206 Partial Content231Please respect copyright.ＰＥＮＡＮＡ1Vgt8MlxGl
accept-ranges: bytes231Please respect copyright.ＰＥＮＡＮＡCs6ny7e6WT
content-range: bytes 287718144-534231199/534231200231Please respect copyright.ＰＥＮＡＮＡq5YnDmvIMQ
ETag: "534231200_1732277116"231Please respect copyright.ＰＥＮＡＮＡkmz6rRSnQ4
Content-Length: 246513056231Please respect copyright.ＰＥＮＡＮＡAQa2Pxn6Ju
Content-Type: application/octet-stream231Please respect copyright.ＰＥＮＡＮＡQjdsgFFPjn
Content-Disposition: attachment; filename="zen.iso"
(http body is the remain file content)231Please respect copyright.ＰＥＮＡＮＡZhHHzFgLqr
-------------------------------------------
If the etag changes, response 200 from beginning like this:231Please respect copyright.ＰＥＮＡＮＡ5k09xymyqp
-----------http response from the beginning------231Please respect copyright.ＰＥＮＡＮＡ2kSuy3pfR7
HTTP/1.1 200 OK231Please respect copyright.ＰＥＮＡＮＡ9afIj1hCEF
Date: Mon, 25 Nov 2024 10:56:12 GMT231Please respect copyright.ＰＥＮＡＮＡ8rosGyN3sT
accept-ranges: bytes231Please respect copyright.ＰＥＮＡＮＡSoW088wURu
ETag: "534231200_1732277116"231Please respect copyright.ＰＥＮＡＮＡCUcLFVYs0W
Content-Length: 534231200231Please respect copyright.ＰＥＮＡＮＡ6OPtA4ZdvA
Content-Type: application/octet-stream231Please respect copyright.ＰＥＮＡＮＡpGacJonScw
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)231Please respect copyright.ＰＥＮＡＮＡoEhc6U3GoE
---------------------------------------------------
231Please respect copyright.ＰＥＮＡＮＡT0ikc5jGY7
Section X. Thanks
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/If-None-Match231Please respect copyright.ＰＥＮＡＮＡT9qtmjRlWg
https://blog.csdn.net/catoop/article/details/134174653231Please respect copyright.ＰＥＮＡＮＡexW3T3w8Ux
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]231Please respect copyright.ＰＥＮＡＮＡLWXMOLDcDt
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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