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.
69Please respect copyright.ＰＥＮＡＮＡ88u6goG0hI
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:69Please respect copyright.ＰＥＮＡＮＡowxBojGVvd
---------------http response with etag----------------69Please respect copyright.ＰＥＮＡＮＡ2a4HOkamGn
HTTP/1.1 200 OK69Please respect copyright.ＰＥＮＡＮＡveaKVED2Au
Date: Mon, 25 Nov 2024 10:56:12 GMT69Please respect copyright.ＰＥＮＡＮＡ4QChlIfXZn
accept-ranges: bytes69Please respect copyright.ＰＥＮＡＮＡ0NhlA6dUpg
ETag: "534231200_1732277116"69Please respect copyright.ＰＥＮＡＮＡbpHu8sqt9M
Content-Length: 53423120069Please respect copyright.ＰＥＮＡＮＡ61duNztAN1
Content-Type: application/octet-stream69Please respect copyright.ＰＥＮＡＮＡPulm4tauIT
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)69Please respect copyright.ＰＥＮＡＮＡPx3okDAUtl
-----------------------------------------------------
69Please respect copyright.ＰＥＮＡＮＡRVh4mKvHE9
Section B. if-none-match
The browser send back the etag to ask server if it has been changed by header like this:69Please respect copyright.ＰＥＮＡＮＡ4Cf6Iasae2
-----http request for a cached resource------69Please respect copyright.ＰＥＮＡＮＡoagjXMrDse
GET /zen.iso HTTP/1.169Please respect copyright.ＰＥＮＡＮＡHCIPaQTnMm
if-none-match: "534231200_1732277116"69Please respect copyright.ＰＥＮＡＮＡF1JjEmQgFn
---------------------------------------------
If the etag is unchanged, the server response like this:
1. for get and head requests:69Please respect copyright.ＰＥＮＡＮＡODnObTFjK3
-----------http response for unchanged resource------69Please respect copyright.ＰＥＮＡＮＡsbIOr8Np9G
HTTP/1.1 304 Not Modified
(http body canbe empty, the client will used its cache)69Please respect copyright.ＰＥＮＡＮＡwVKup7sBoP
----------------------------------------------------
2. for other methods requests:69Please respect copyright.ＰＥＮＡＮＡouMqiq4zjx
-----------http response for unchanged resource------69Please respect copyright.ＰＥＮＡＮＡajarNi3azp
HTTP/1.1 412 Precondition Failed
(http body canbe empty, the client will used its cache)69Please respect copyright.ＰＥＮＡＮＡdnMpvXDn6H
----------------------------------------------------
69Please respect copyright.ＰＥＮＡＮＡZmXzKs1coA
If the etag changes, the server response 200 OK with the new file content like this:69Please respect copyright.ＰＥＮＡＮＡvbdGVTdUfg
---------------http response with etag----------------69Please respect copyright.ＰＥＮＡＮＡzvJEgloQiN
HTTP/1.1 200 OK69Please respect copyright.ＰＥＮＡＮＡbTN3PlOEI2
Date: Mon, 25 Nov 2024 10:56:12 GMT69Please respect copyright.ＰＥＮＡＮＡKTivdj3jQN
accept-ranges: bytes69Please respect copyright.ＰＥＮＡＮＡ4Vn6pWr2MG
ETag: "534231200_1732277116"69Please respect copyright.ＰＥＮＡＮＡbXw1v5gIIC
Content-Length: 53423120069Please respect copyright.ＰＥＮＡＮＡgQOx5g7szm
Content-Type: application/octet-stream69Please respect copyright.ＰＥＮＡＮＡnW9PD5sUbW
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)69Please respect copyright.ＰＥＮＡＮＡf6lj1W8qh4
-----------------------------------------------------
69Please respect copyright.ＰＥＮＡＮＡ37raATekEP
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:69Please respect copyright.ＰＥＮＡＮＡbqLK25tI2s
------resume download request-------69Please respect copyright.ＰＥＮＡＮＡHUv5lD7wUe
GET /zen.iso HTTP/1.169Please respect copyright.ＰＥＮＡＮＡDcARkio06o
Range: bytes=287718144-69Please respect copyright.ＰＥＮＡＮＡQ6AM1CavNh
If-Match: "534231200_1732277116"69Please respect copyright.ＰＥＮＡＮＡMTwcS5c9b3
-----------------------------------69Please respect copyright.ＰＥＮＡＮＡeJjlDNoyQL
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:69Please respect copyright.ＰＥＮＡＮＡ0F1Gl3GGJV
-------------range response-------------------69Please respect copyright.ＰＥＮＡＮＡNn1UGRclOX
HTTP/1.1 206 Partial Content69Please respect copyright.ＰＥＮＡＮＡKNE0G0QdYV
accept-ranges: bytes69Please respect copyright.ＰＥＮＡＮＡAillZPouOr
content-range: bytes 287718144-534231199/53423120069Please respect copyright.ＰＥＮＡＮＡwhEJNkbIfz
ETag: "534231200_1732277116"69Please respect copyright.ＰＥＮＡＮＡnUIVCfFW11
Content-Length: 24651305669Please respect copyright.ＰＥＮＡＮＡUHugRkxCck
Content-Type: application/octet-stream69Please respect copyright.ＰＥＮＡＮＡGMFQYJ8azj
Content-Disposition: attachment; filename="zen.iso"
(http body is the remain file content)69Please respect copyright.ＰＥＮＡＮＡuxC4FqL5fP
-------------------------------------------
If the etag changes, response 412 like this:69Please respect copyright.ＰＥＮＡＮＡIGkGKAi8P7
-----------http response for changed resource------69Please respect copyright.ＰＥＮＡＮＡKOOKiAv0OD
HTTP/1.1 412 Precondition Failed
(http body canbe empty, the client will stop downloading the resumed file)69Please respect copyright.ＰＥＮＡＮＡSuPm2LOrpX
---------------------------------------------------
69Please respect copyright.ＰＥＮＡＮＡ7AAuBS999W
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---69Please respect copyright.ＰＥＮＡＮＡr9swkGIeKC
GET /zen.iso HTTP/1.169Please respect copyright.ＰＥＮＡＮＡiurI7roVSl
Range: bytes=287718144-69Please respect copyright.ＰＥＮＡＮＡzWg4f7tmkZ
If-Range: "534231200_1732277116"69Please respect copyright.ＰＥＮＡＮＡq3NP6kAtI5
------------------------------------69Please respect copyright.ＰＥＮＡＮＡHTd4LwLBry
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:69Please respect copyright.ＰＥＮＡＮＡ6xKAKzrrpT
-------------range response-------------------69Please respect copyright.ＰＥＮＡＮＡLBB0nHAAJL
HTTP/1.1 206 Partial Content69Please respect copyright.ＰＥＮＡＮＡvQYyPJucwc
accept-ranges: bytes69Please respect copyright.ＰＥＮＡＮＡuPAPJXO8U7
content-range: bytes 287718144-534231199/53423120069Please respect copyright.ＰＥＮＡＮＡue8B6cPTZk
ETag: "534231200_1732277116"69Please respect copyright.ＰＥＮＡＮＡYz49UTLity
Content-Length: 24651305669Please respect copyright.ＰＥＮＡＮＡva1ppJwikx
Content-Type: application/octet-stream69Please respect copyright.ＰＥＮＡＮＡR0UWjAf6KC
Content-Disposition: attachment; filename="zen.iso"
(http body is the remain file content)69Please respect copyright.ＰＥＮＡＮＡmAekh23qc8
-------------------------------------------
If the etag changes, response 200 from beginning like this:69Please respect copyright.ＰＥＮＡＮＡDYMtLkQJTo
-----------http response from the beginning------69Please respect copyright.ＰＥＮＡＮＡaIqJMW618x
HTTP/1.1 200 OK69Please respect copyright.ＰＥＮＡＮＡn5z0aWFAQH
Date: Mon, 25 Nov 2024 10:56:12 GMT69Please respect copyright.ＰＥＮＡＮＡq867YkqNpW
accept-ranges: bytes69Please respect copyright.ＰＥＮＡＮＡSyi1yvyhsn
ETag: "534231200_1732277116"69Please respect copyright.ＰＥＮＡＮＡkGEEfOWZkr
Content-Length: 53423120069Please respect copyright.ＰＥＮＡＮＡloI8a3zB4x
Content-Type: application/octet-stream69Please respect copyright.ＰＥＮＡＮＡJnZEXCLeSM
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)69Please respect copyright.ＰＥＮＡＮＡY1hTsXzzxU
---------------------------------------------------
69Please respect copyright.ＰＥＮＡＮＡI9Di5tdyBG
Section X. Thanks
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/If-None-Match69Please respect copyright.ＰＥＮＡＮＡNBlDbyijBB
https://blog.csdn.net/catoop/article/details/13417465369Please respect copyright.ＰＥＮＡＮＡYj9tN33iRY
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]69Please respect copyright.ＰＥＮＡＮＡQOTJJiGZNe
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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