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.
148Please respect copyright.ＰＥＮＡＮＡ4LyWpFLD9O
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:148Please respect copyright.ＰＥＮＡＮＡyid9dkQbjt
---------------http response with etag----------------148Please respect copyright.ＰＥＮＡＮＡOi9DWJ9MHI
HTTP/1.1 200 OK148Please respect copyright.ＰＥＮＡＮＡ7lQW6SAuRe
Date: Mon, 25 Nov 2024 10:56:12 GMT148Please respect copyright.ＰＥＮＡＮＡe7Ggsrixtr
accept-ranges: bytes148Please respect copyright.ＰＥＮＡＮＡ9L9a39s5Qc
ETag: "534231200_1732277116"148Please respect copyright.ＰＥＮＡＮＡcCGb4ct0bE
Content-Length: 534231200148Please respect copyright.ＰＥＮＡＮＡDQ2Se12Ax2
Content-Type: application/octet-stream148Please respect copyright.ＰＥＮＡＮＡoAzCauVaBn
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)148Please respect copyright.ＰＥＮＡＮＡ33akb5fLU5
-----------------------------------------------------
148Please respect copyright.ＰＥＮＡＮＡtZhvmLay9r
Section B. if-none-match
The browser send back the etag to ask server if it has been changed by header like this:148Please respect copyright.ＰＥＮＡＮＡPi72TSvEr4
-----http request for a cached resource------148Please respect copyright.ＰＥＮＡＮＡFbVs9fb1d3
GET /zen.iso HTTP/1.1148Please respect copyright.ＰＥＮＡＮＡ8hozpqVtre
if-none-match: "534231200_1732277116"148Please respect copyright.ＰＥＮＡＮＡkStWPs59yg
---------------------------------------------
If the etag is unchanged, the server response like this:
1. for get and head requests:148Please respect copyright.ＰＥＮＡＮＡnLuIYeOryn
-----------http response for unchanged resource------148Please respect copyright.ＰＥＮＡＮＡrhPOPcLKrs
HTTP/1.1 304 Not Modified
(http body canbe empty, the client will used its cache)148Please respect copyright.ＰＥＮＡＮＡa7O6Te3Vdm
----------------------------------------------------
2. for other methods requests:148Please respect copyright.ＰＥＮＡＮＡEWcxWp3NeF
-----------http response for unchanged resource------148Please respect copyright.ＰＥＮＡＮＡzg0tktyCUZ
HTTP/1.1 412 Precondition Failed
(http body canbe empty, the client will used its cache)148Please respect copyright.ＰＥＮＡＮＡAEgpjRb00w
----------------------------------------------------
148Please respect copyright.ＰＥＮＡＮＡqgnsv125cU
If the etag changes, the server response 200 OK with the new file content like this:148Please respect copyright.ＰＥＮＡＮＡBdeFkKM6xJ
---------------http response with etag----------------148Please respect copyright.ＰＥＮＡＮＡfLLIs0owTA
HTTP/1.1 200 OK148Please respect copyright.ＰＥＮＡＮＡF4BfW3a0QB
Date: Mon, 25 Nov 2024 10:56:12 GMT148Please respect copyright.ＰＥＮＡＮＡLgy6jmYJeG
accept-ranges: bytes148Please respect copyright.ＰＥＮＡＮＡnbXM6piCX5
ETag: "534231200_1732277116"148Please respect copyright.ＰＥＮＡＮＡNH2YsJrgdk
Content-Length: 534231200148Please respect copyright.ＰＥＮＡＮＡaUB96Bh7J9
Content-Type: application/octet-stream148Please respect copyright.ＰＥＮＡＮＡ6XkS9OQoMb
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)148Please respect copyright.ＰＥＮＡＮＡcPXrQnrtR7
-----------------------------------------------------
148Please respect copyright.ＰＥＮＡＮＡJ3FHQHPBVl
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:148Please respect copyright.ＰＥＮＡＮＡKGzzwGmZDF
------resume download request-------148Please respect copyright.ＰＥＮＡＮＡB2gIVCjMyH
GET /zen.iso HTTP/1.1148Please respect copyright.ＰＥＮＡＮＡOCwjHLtA1y
Range: bytes=287718144-148Please respect copyright.ＰＥＮＡＮＡ0RH2IoBRNV
If-Match: "534231200_1732277116"148Please respect copyright.ＰＥＮＡＮＡEShqDDnC9f
-----------------------------------148Please respect copyright.ＰＥＮＡＮＡttqmykFHE6
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:148Please respect copyright.ＰＥＮＡＮＡVTjr4mTe4X
-------------range response-------------------148Please respect copyright.ＰＥＮＡＮＡ7av8EKivna
HTTP/1.1 206 Partial Content148Please respect copyright.ＰＥＮＡＮＡV8pjkmpU3e
accept-ranges: bytes148Please respect copyright.ＰＥＮＡＮＡcR7xk52Hij
content-range: bytes 287718144-534231199/534231200148Please respect copyright.ＰＥＮＡＮＡOJsvLhumF7
ETag: "534231200_1732277116"148Please respect copyright.ＰＥＮＡＮＡta0QLkd57X
Content-Length: 246513056148Please respect copyright.ＰＥＮＡＮＡyqzOXzvzP7
Content-Type: application/octet-stream148Please respect copyright.ＰＥＮＡＮＡOARtHbGehl
Content-Disposition: attachment; filename="zen.iso"
(http body is the remain file content)148Please respect copyright.ＰＥＮＡＮＡ9uSjC8mImM
-------------------------------------------
If the etag changes, response 412 like this:148Please respect copyright.ＰＥＮＡＮＡc27IjycNrG
-----------http response for changed resource------148Please respect copyright.ＰＥＮＡＮＡCwKFfavCac
HTTP/1.1 412 Precondition Failed
(http body canbe empty, the client will stop downloading the resumed file)148Please respect copyright.ＰＥＮＡＮＡYkJ3xO3Gza
---------------------------------------------------
148Please respect copyright.ＰＥＮＡＮＡFY7eZ1EO0t
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---148Please respect copyright.ＰＥＮＡＮＡ8z4wzDlauE
GET /zen.iso HTTP/1.1148Please respect copyright.ＰＥＮＡＮＡzjo356owym
Range: bytes=287718144-148Please respect copyright.ＰＥＮＡＮＡxXLS6Z38kA
If-Range: "534231200_1732277116"148Please respect copyright.ＰＥＮＡＮＡmRMys7Mop5
------------------------------------148Please respect copyright.ＰＥＮＡＮＡcdzodf9D6b
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:148Please respect copyright.ＰＥＮＡＮＡYRjeMA8QMS
-------------range response-------------------148Please respect copyright.ＰＥＮＡＮＡsqRFKbELYV
HTTP/1.1 206 Partial Content148Please respect copyright.ＰＥＮＡＮＡr5MXfw8RRE
accept-ranges: bytes148Please respect copyright.ＰＥＮＡＮＡY3mdKwXLPD
content-range: bytes 287718144-534231199/534231200148Please respect copyright.ＰＥＮＡＮＡZafxhiAMqb
ETag: "534231200_1732277116"148Please respect copyright.ＰＥＮＡＮＡ7DZX9NTdcZ
Content-Length: 246513056148Please respect copyright.ＰＥＮＡＮＡ1cKiDyzKOj
Content-Type: application/octet-stream148Please respect copyright.ＰＥＮＡＮＡKwXjRWYcdb
Content-Disposition: attachment; filename="zen.iso"
(http body is the remain file content)148Please respect copyright.ＰＥＮＡＮＡUAzwYIohu7
-------------------------------------------
If the etag changes, response 200 from beginning like this:148Please respect copyright.ＰＥＮＡＮＡNNIYabWMoj
-----------http response from the beginning------148Please respect copyright.ＰＥＮＡＮＡy4YImNFgdR
HTTP/1.1 200 OK148Please respect copyright.ＰＥＮＡＮＡqwtXzyeTRQ
Date: Mon, 25 Nov 2024 10:56:12 GMT148Please respect copyright.ＰＥＮＡＮＡZs8frpydSF
accept-ranges: bytes148Please respect copyright.ＰＥＮＡＮＡqkBpGJ4WBy
ETag: "534231200_1732277116"148Please respect copyright.ＰＥＮＡＮＡtorDrMvvIe
Content-Length: 534231200148Please respect copyright.ＰＥＮＡＮＡhjhzF7nxCK
Content-Type: application/octet-stream148Please respect copyright.ＰＥＮＡＮＡifLc3xCebi
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)148Please respect copyright.ＰＥＮＡＮＡbaYzPEprUq
---------------------------------------------------
148Please respect copyright.ＰＥＮＡＮＡ3ilBRFt57b
Section X. Thanks
developer.mozilla.org/en-US/docs/Web/HTTP/Headers/If-None-Match148Please respect copyright.ＰＥＮＡＮＡPgPPg2hAcD
blog.csdn.net/catoop/article/details/134174653148Please respect copyright.ＰＥＮＡＮＡy5jLvjgUbD
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.com148Please respect copyright.ＰＥＮＡＮＡHlDArJl0hq
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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