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.
161Please respect copyright.ＰＥＮＡＮＡ80qjPnRJnw
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:161Please respect copyright.ＰＥＮＡＮＡkmWv1kJ9L0
---------------http response with etag----------------161Please respect copyright.ＰＥＮＡＮＡhAWiw2g3k6
HTTP/1.1 200 OK161Please respect copyright.ＰＥＮＡＮＡqpcuAqMsJS
Date: Mon, 25 Nov 2024 10:56:12 GMT161Please respect copyright.ＰＥＮＡＮＡN7hC0DL8rf
accept-ranges: bytes161Please respect copyright.ＰＥＮＡＮＡSKKXKBbfDc
ETag: "534231200_1732277116"161Please respect copyright.ＰＥＮＡＮＡWaK4SQMct1
Content-Length: 534231200161Please respect copyright.ＰＥＮＡＮＡoqhKhQ3Obn
Content-Type: application/octet-stream161Please respect copyright.ＰＥＮＡＮＡ3YErsnECwR
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)161Please respect copyright.ＰＥＮＡＮＡlf8crUBksi
-----------------------------------------------------
161Please respect copyright.ＰＥＮＡＮＡRaN1TzGcgT
Section B. if-none-match
The browser send back the etag to ask server if it has been changed by header like this:161Please respect copyright.ＰＥＮＡＮＡdZNDGatC2v
-----http request for a cached resource------161Please respect copyright.ＰＥＮＡＮＡXOXUOZW4Ey
GET /zen.iso HTTP/1.1161Please respect copyright.ＰＥＮＡＮＡEsf6NLZEgM
if-none-match: "534231200_1732277116"161Please respect copyright.ＰＥＮＡＮＡlwius4wOyV
---------------------------------------------
If the etag is unchanged, the server response like this:
1. for get and head requests:161Please respect copyright.ＰＥＮＡＮＡ9hgSQRtaCt
-----------http response for unchanged resource------161Please respect copyright.ＰＥＮＡＮＡxe91TwxONr
HTTP/1.1 304 Not Modified
(http body canbe empty, the client will used its cache)161Please respect copyright.ＰＥＮＡＮＡWS3Ihvfbd7
----------------------------------------------------
2. for other methods requests:161Please respect copyright.ＰＥＮＡＮＡCsTpPRT3RK
-----------http response for unchanged resource------161Please respect copyright.ＰＥＮＡＮＡTpKBAv77Zz
HTTP/1.1 412 Precondition Failed
(http body canbe empty, the client will used its cache)161Please respect copyright.ＰＥＮＡＮＡDPKD79vMmh
----------------------------------------------------
161Please respect copyright.ＰＥＮＡＮＡpgwjxL0qUy
If the etag changes, the server response 200 OK with the new file content like this:161Please respect copyright.ＰＥＮＡＮＡmkcJbfe1gD
---------------http response with etag----------------161Please respect copyright.ＰＥＮＡＮＡoRe1PDif1i
HTTP/1.1 200 OK161Please respect copyright.ＰＥＮＡＮＡc6CaXdEnYp
Date: Mon, 25 Nov 2024 10:56:12 GMT161Please respect copyright.ＰＥＮＡＮＡVysdyEps4d
accept-ranges: bytes161Please respect copyright.ＰＥＮＡＮＡUiGHTOCC9D
ETag: "534231200_1732277116"161Please respect copyright.ＰＥＮＡＮＡj6Fx8wCNFL
Content-Length: 534231200161Please respect copyright.ＰＥＮＡＮＡndjmyp5BNc
Content-Type: application/octet-stream161Please respect copyright.ＰＥＮＡＮＡ0knFwbivfa
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)161Please respect copyright.ＰＥＮＡＮＡ53rC4Oe9G4
-----------------------------------------------------
161Please respect copyright.ＰＥＮＡＮＡKZ3BLnNVRB
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:161Please respect copyright.ＰＥＮＡＮＡLkUdTgBcdC
------resume download request-------161Please respect copyright.ＰＥＮＡＮＡ7H3hCpSZPK
GET /zen.iso HTTP/1.1161Please respect copyright.ＰＥＮＡＮＡffmEYd3Ciy
Range: bytes=287718144-161Please respect copyright.ＰＥＮＡＮＡqtO7MncZTj
If-Match: "534231200_1732277116"161Please respect copyright.ＰＥＮＡＮＡT8nZOfq5V9
-----------------------------------161Please respect copyright.ＰＥＮＡＮＡjkWPYO0VvU
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:161Please respect copyright.ＰＥＮＡＮＡc4GVoFZ5Sz
-------------range response-------------------161Please respect copyright.ＰＥＮＡＮＡ6GM0C7hUhY
HTTP/1.1 206 Partial Content161Please respect copyright.ＰＥＮＡＮＡyy7HWT9Llp
accept-ranges: bytes161Please respect copyright.ＰＥＮＡＮＡTFMA9STlu6
content-range: bytes 287718144-534231199/534231200161Please respect copyright.ＰＥＮＡＮＡtrJPkTgZKt
ETag: "534231200_1732277116"161Please respect copyright.ＰＥＮＡＮＡ1O6pFfsfBS
Content-Length: 246513056161Please respect copyright.ＰＥＮＡＮＡRRDsGjwXo0
Content-Type: application/octet-stream161Please respect copyright.ＰＥＮＡＮＡOcvGdM4Bru
Content-Disposition: attachment; filename="zen.iso"
(http body is the remain file content)161Please respect copyright.ＰＥＮＡＮＡxt86bjEnL2
-------------------------------------------
If the etag changes, response 412 like this:161Please respect copyright.ＰＥＮＡＮＡwFFdI4AB4k
-----------http response for changed resource------161Please respect copyright.ＰＥＮＡＮＡgWoHpZinPL
HTTP/1.1 412 Precondition Failed
(http body canbe empty, the client will stop downloading the resumed file)161Please respect copyright.ＰＥＮＡＮＡrbVXxpP4FT
---------------------------------------------------
161Please respect copyright.ＰＥＮＡＮＡDI8GK3A7oi
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---161Please respect copyright.ＰＥＮＡＮＡ4V588lwZGb
GET /zen.iso HTTP/1.1161Please respect copyright.ＰＥＮＡＮＡCxeOdfR6c6
Range: bytes=287718144-161Please respect copyright.ＰＥＮＡＮＡJ6hCzwDcuF
If-Range: "534231200_1732277116"161Please respect copyright.ＰＥＮＡＮＡbiPOXpkQUs
------------------------------------161Please respect copyright.ＰＥＮＡＮＡkkey3L4elK
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:161Please respect copyright.ＰＥＮＡＮＡuoY4RJl2ur
-------------range response-------------------161Please respect copyright.ＰＥＮＡＮＡJaf4Cydn59
HTTP/1.1 206 Partial Content161Please respect copyright.ＰＥＮＡＮＡxIhZtQOJjx
accept-ranges: bytes161Please respect copyright.ＰＥＮＡＮＡkwBTeyNAJI
content-range: bytes 287718144-534231199/534231200161Please respect copyright.ＰＥＮＡＮＡ6Uwhp9T31F
ETag: "534231200_1732277116"161Please respect copyright.ＰＥＮＡＮＡG6BD8CoTjR
Content-Length: 246513056161Please respect copyright.ＰＥＮＡＮＡwcnavqMMpw
Content-Type: application/octet-stream161Please respect copyright.ＰＥＮＡＮＡ9eoVUQuWs4
Content-Disposition: attachment; filename="zen.iso"
(http body is the remain file content)161Please respect copyright.ＰＥＮＡＮＡnzU2uT6YON
-------------------------------------------
If the etag changes, response 200 from beginning like this:161Please respect copyright.ＰＥＮＡＮＡ3NzBySid4V
-----------http response from the beginning------161Please respect copyright.ＰＥＮＡＮＡLit1xE0FOJ
HTTP/1.1 200 OK161Please respect copyright.ＰＥＮＡＮＡfm4zg6RSor
Date: Mon, 25 Nov 2024 10:56:12 GMT161Please respect copyright.ＰＥＮＡＮＡ6ZSAPG5Kgw
accept-ranges: bytes161Please respect copyright.ＰＥＮＡＮＡ1c3PeM4iC1
ETag: "534231200_1732277116"161Please respect copyright.ＰＥＮＡＮＡKEhy33S1j2
Content-Length: 534231200161Please respect copyright.ＰＥＮＡＮＡvomJWeHaWI
Content-Type: application/octet-stream161Please respect copyright.ＰＥＮＡＮＡeaz67X5Dbv
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)161Please respect copyright.ＰＥＮＡＮＡBMMmJ6oYmP
---------------------------------------------------
161Please respect copyright.ＰＥＮＡＮＡLnVEwaYvnt
Section X. Thanks
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/If-None-Match161Please respect copyright.ＰＥＮＡＮＡafbW4X5kJD
https://blog.csdn.net/catoop/article/details/134174653161Please respect copyright.ＰＥＮＡＮＡugXWlG4Xzm
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]161Please respect copyright.ＰＥＮＡＮＡNl1uM1aqTR
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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