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.
208Please respect copyright.ＰＥＮＡＮＡWA43WvCC5r
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:208Please respect copyright.ＰＥＮＡＮＡeOnUVUeLpt
---------------http response with etag----------------208Please respect copyright.ＰＥＮＡＮＡcYmOK80ypF
HTTP/1.1 200 OK208Please respect copyright.ＰＥＮＡＮＡhQ35SmAd11
Date: Mon, 25 Nov 2024 10:56:12 GMT208Please respect copyright.ＰＥＮＡＮＡYJO5n9vcfp
accept-ranges: bytes208Please respect copyright.ＰＥＮＡＮＡMevQD4VsEO
ETag: "534231200_1732277116"208Please respect copyright.ＰＥＮＡＮＡXaGOBh1Qy7
Content-Length: 534231200208Please respect copyright.ＰＥＮＡＮＡCkONRsHljb
Content-Type: application/octet-stream208Please respect copyright.ＰＥＮＡＮＡowxS38bUQN
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)208Please respect copyright.ＰＥＮＡＮＡR5ZmAjPpsa
-----------------------------------------------------
208Please respect copyright.ＰＥＮＡＮＡfRsbuIcm7r
Section B. if-none-match
The browser send back the etag to ask server if it has been changed by header like this:208Please respect copyright.ＰＥＮＡＮＡRU16ek5Pqq
-----http request for a cached resource------208Please respect copyright.ＰＥＮＡＮＡsDp3UnERkf
GET /zen.iso HTTP/1.1208Please respect copyright.ＰＥＮＡＮＡhwJuUgWnXa
if-none-match: "534231200_1732277116"208Please respect copyright.ＰＥＮＡＮＡkNfpNeXvFK
---------------------------------------------
If the etag is unchanged, the server response like this:
1. for get and head requests:208Please respect copyright.ＰＥＮＡＮＡrsDEpIKE2k
-----------http response for unchanged resource------208Please respect copyright.ＰＥＮＡＮＡdWYdPLTWAm
HTTP/1.1 304 Not Modified
(http body canbe empty, the client will used its cache)208Please respect copyright.ＰＥＮＡＮＡQUY9T7WRrF
----------------------------------------------------
2. for other methods requests:208Please respect copyright.ＰＥＮＡＮＡJMecVe6T2i
-----------http response for unchanged resource------208Please respect copyright.ＰＥＮＡＮＡHIezfqjZia
HTTP/1.1 412 Precondition Failed
(http body canbe empty, the client will used its cache)208Please respect copyright.ＰＥＮＡＮＡ3vPTscfp92
----------------------------------------------------
208Please respect copyright.ＰＥＮＡＮＡh4ZVDGZQO4
If the etag changes, the server response 200 OK with the new file content like this:208Please respect copyright.ＰＥＮＡＮＡeOkIlJtRMu
---------------http response with etag----------------208Please respect copyright.ＰＥＮＡＮＡzL1jwN84VU
HTTP/1.1 200 OK208Please respect copyright.ＰＥＮＡＮＡeR0xnTgBv9
Date: Mon, 25 Nov 2024 10:56:12 GMT208Please respect copyright.ＰＥＮＡＮＡpzCUl7ZWd9
accept-ranges: bytes208Please respect copyright.ＰＥＮＡＮＡpaCAOd0U5V
ETag: "534231200_1732277116"208Please respect copyright.ＰＥＮＡＮＡeKVEjlDBgo
Content-Length: 534231200208Please respect copyright.ＰＥＮＡＮＡndfbZ2Uevk
Content-Type: application/octet-stream208Please respect copyright.ＰＥＮＡＮＡIr8ezKOYcH
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)208Please respect copyright.ＰＥＮＡＮＡbB57SulXtR
-----------------------------------------------------
208Please respect copyright.ＰＥＮＡＮＡWiocLa4XFx
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:208Please respect copyright.ＰＥＮＡＮＡCKyEZs1tIG
------resume download request-------208Please respect copyright.ＰＥＮＡＮＡ7p7TAjAXW9
GET /zen.iso HTTP/1.1208Please respect copyright.ＰＥＮＡＮＡVkvN6SaTAx
Range: bytes=287718144-208Please respect copyright.ＰＥＮＡＮＡDoMUpOJIax
If-Match: "534231200_1732277116"208Please respect copyright.ＰＥＮＡＮＡ4pV5hGZ28k
-----------------------------------208Please respect copyright.ＰＥＮＡＮＡHW0y5Q0TZm
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:208Please respect copyright.ＰＥＮＡＮＡFOSnGYjDtR
-------------range response-------------------208Please respect copyright.ＰＥＮＡＮＡPvXRO8P78h
HTTP/1.1 206 Partial Content208Please respect copyright.ＰＥＮＡＮＡuY49mZ7GC2
accept-ranges: bytes208Please respect copyright.ＰＥＮＡＮＡoeb7L9ycKM
content-range: bytes 287718144-534231199/534231200208Please respect copyright.ＰＥＮＡＮＡBA4Zwuh14Q
ETag: "534231200_1732277116"208Please respect copyright.ＰＥＮＡＮＡmipjau3WrP
Content-Length: 246513056208Please respect copyright.ＰＥＮＡＮＡRlfd5rwuDq
Content-Type: application/octet-stream208Please respect copyright.ＰＥＮＡＮＡviBY39w39E
Content-Disposition: attachment; filename="zen.iso"
(http body is the remain file content)208Please respect copyright.ＰＥＮＡＮＡQbihdfGiqb
-------------------------------------------
If the etag changes, response 412 like this:208Please respect copyright.ＰＥＮＡＮＡmFJM21pPpQ
-----------http response for changed resource------208Please respect copyright.ＰＥＮＡＮＡg3kPZvjmPH
HTTP/1.1 412 Precondition Failed
(http body canbe empty, the client will stop downloading the resumed file)208Please respect copyright.ＰＥＮＡＮＡ4YVLPW4wsq
---------------------------------------------------
208Please respect copyright.ＰＥＮＡＮＡfUu3GRXaL2
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---208Please respect copyright.ＰＥＮＡＮＡhqfOCUXo2M
GET /zen.iso HTTP/1.1208Please respect copyright.ＰＥＮＡＮＡ8wgl3FrSvI
Range: bytes=287718144-208Please respect copyright.ＰＥＮＡＮＡxWE9uvhVSY
If-Range: "534231200_1732277116"208Please respect copyright.ＰＥＮＡＮＡwm9s82Q50J
------------------------------------208Please respect copyright.ＰＥＮＡＮＡAvUIk0y829
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:208Please respect copyright.ＰＥＮＡＮＡNq5aY32eY7
-------------range response-------------------208Please respect copyright.ＰＥＮＡＮＡYtlBAyzw2N
HTTP/1.1 206 Partial Content208Please respect copyright.ＰＥＮＡＮＡBwMFqTvEwg
accept-ranges: bytes208Please respect copyright.ＰＥＮＡＮＡb9hbXgnrj5
content-range: bytes 287718144-534231199/534231200208Please respect copyright.ＰＥＮＡＮＡnf2atrpS7n
ETag: "534231200_1732277116"208Please respect copyright.ＰＥＮＡＮＡsjTQZ851Dg
Content-Length: 246513056208Please respect copyright.ＰＥＮＡＮＡ27Y4JNCuma
Content-Type: application/octet-stream208Please respect copyright.ＰＥＮＡＮＡW8bN4KgYsf
Content-Disposition: attachment; filename="zen.iso"
(http body is the remain file content)208Please respect copyright.ＰＥＮＡＮＡcetnap29mE
-------------------------------------------
If the etag changes, response 200 from beginning like this:208Please respect copyright.ＰＥＮＡＮＡC79BMyXFnR
-----------http response from the beginning------208Please respect copyright.ＰＥＮＡＮＡJzfTzwtsn3
HTTP/1.1 200 OK208Please respect copyright.ＰＥＮＡＮＡKyeDQpFkNz
Date: Mon, 25 Nov 2024 10:56:12 GMT208Please respect copyright.ＰＥＮＡＮＡ3g2WEidDkS
accept-ranges: bytes208Please respect copyright.ＰＥＮＡＮＡMNaSwR97hF
ETag: "534231200_1732277116"208Please respect copyright.ＰＥＮＡＮＡdhQUBieqxL
Content-Length: 534231200208Please respect copyright.ＰＥＮＡＮＡkmwSJDs3iO
Content-Type: application/octet-stream208Please respect copyright.ＰＥＮＡＮＡayqCzkILOi
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)208Please respect copyright.ＰＥＮＡＮＡvcmjVISW9A
---------------------------------------------------
208Please respect copyright.ＰＥＮＡＮＡsPzimOsZMG
Section X. Thanks
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/If-None-Match208Please respect copyright.ＰＥＮＡＮＡFJ4BaH2aXh
https://blog.csdn.net/catoop/article/details/134174653208Please respect copyright.ＰＥＮＡＮＡriM8vLxmGV
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]208Please respect copyright.ＰＥＮＡＮＡoOZOEfgajG
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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