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.
107Please respect copyright.ＰＥＮＡＮＡkfcZdIRpVN
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:107Please respect copyright.ＰＥＮＡＮＡSZw9vGplLO
---------------http response with etag----------------107Please respect copyright.ＰＥＮＡＮＡlviSaKxWLE
HTTP/1.1 200 OK107Please respect copyright.ＰＥＮＡＮＡhL4AZHoR8F
Date: Mon, 25 Nov 2024 10:56:12 GMT107Please respect copyright.ＰＥＮＡＮＡHOBkyKHHov
accept-ranges: bytes107Please respect copyright.ＰＥＮＡＮＡY6cWBxWubN
ETag: "534231200_1732277116"107Please respect copyright.ＰＥＮＡＮＡ3q4QVCtz5V
Content-Length: 534231200107Please respect copyright.ＰＥＮＡＮＡPLRcln1gRj
Content-Type: application/octet-stream107Please respect copyright.ＰＥＮＡＮＡl9ZmLadkLN
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)107Please respect copyright.ＰＥＮＡＮＡSP3h7mmA5b
-----------------------------------------------------
107Please respect copyright.ＰＥＮＡＮＡ0yMToeiXOO
Section B. if-none-match
The browser send back the etag to ask server if it has been changed by header like this:107Please respect copyright.ＰＥＮＡＮＡXUFE20jBz0
-----http request for a cached resource------107Please respect copyright.ＰＥＮＡＮＡL4vtjASXOZ
GET /zen.iso HTTP/1.1107Please respect copyright.ＰＥＮＡＮＡEJ319HbvCf
if-none-match: "534231200_1732277116"107Please respect copyright.ＰＥＮＡＮＡ2dfcnHrZxq
---------------------------------------------
If the etag is unchanged, the server response like this:
1. for get and head requests:107Please respect copyright.ＰＥＮＡＮＡz0Eip5V4th
-----------http response for unchanged resource------107Please respect copyright.ＰＥＮＡＮＡBLarHS2EWS
HTTP/1.1 304 Not Modified
(http body canbe empty, the client will used its cache)107Please respect copyright.ＰＥＮＡＮＡb8T5A8dJ3D
----------------------------------------------------
2. for other methods requests:107Please respect copyright.ＰＥＮＡＮＡeCakd8aJc5
-----------http response for unchanged resource------107Please respect copyright.ＰＥＮＡＮＡb9UW1G1Hdq
HTTP/1.1 412 Precondition Failed
(http body canbe empty, the client will used its cache)107Please respect copyright.ＰＥＮＡＮＡXcjiuXfHGt
----------------------------------------------------
107Please respect copyright.ＰＥＮＡＮＡBE8ixXilWM
If the etag changes, the server response 200 OK with the new file content like this:107Please respect copyright.ＰＥＮＡＮＡVZBsMAzx4D
---------------http response with etag----------------107Please respect copyright.ＰＥＮＡＮＡax0oQmL3L7
HTTP/1.1 200 OK107Please respect copyright.ＰＥＮＡＮＡLN2T3DoAlg
Date: Mon, 25 Nov 2024 10:56:12 GMT107Please respect copyright.ＰＥＮＡＮＡk8FryPAd08
accept-ranges: bytes107Please respect copyright.ＰＥＮＡＮＡLmvJ9ZaskL
ETag: "534231200_1732277116"107Please respect copyright.ＰＥＮＡＮＡEdFTEJdvfy
Content-Length: 534231200107Please respect copyright.ＰＥＮＡＮＡYWkX0NIsDF
Content-Type: application/octet-stream107Please respect copyright.ＰＥＮＡＮＡoGoNMoRL55
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)107Please respect copyright.ＰＥＮＡＮＡvJXsALptRv
-----------------------------------------------------
107Please respect copyright.ＰＥＮＡＮＡsazOYos9V0
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:107Please respect copyright.ＰＥＮＡＮＡHPEvQWcPAu
------resume download request-------107Please respect copyright.ＰＥＮＡＮＡvCD8MNNbmB
GET /zen.iso HTTP/1.1107Please respect copyright.ＰＥＮＡＮＡlHz3GxzDmj
Range: bytes=287718144-107Please respect copyright.ＰＥＮＡＮＡ2nxbTRtP0v
If-Match: "534231200_1732277116"107Please respect copyright.ＰＥＮＡＮＡ2SFJxJhXDJ
-----------------------------------107Please respect copyright.ＰＥＮＡＮＡ4g2tJlTrMT
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:107Please respect copyright.ＰＥＮＡＮＡtfJ2GsF5n8
-------------range response-------------------107Please respect copyright.ＰＥＮＡＮＡQZ2n8k02MA
HTTP/1.1 206 Partial Content107Please respect copyright.ＰＥＮＡＮＡjHcBi51kob
accept-ranges: bytes107Please respect copyright.ＰＥＮＡＮＡ5uFtzMmUks
content-range: bytes 287718144-534231199/534231200107Please respect copyright.ＰＥＮＡＮＡv6tUEdnIgF
ETag: "534231200_1732277116"107Please respect copyright.ＰＥＮＡＮＡk73bYzKEU7
Content-Length: 246513056107Please respect copyright.ＰＥＮＡＮＡ4auIvifoEC
Content-Type: application/octet-stream107Please respect copyright.ＰＥＮＡＮＡ5gM8W1oVcp
Content-Disposition: attachment; filename="zen.iso"
(http body is the remain file content)107Please respect copyright.ＰＥＮＡＮＡ1aV7J3BzEh
-------------------------------------------
If the etag changes, response 412 like this:107Please respect copyright.ＰＥＮＡＮＡR0tCO7IXPc
-----------http response for changed resource------107Please respect copyright.ＰＥＮＡＮＡgdDcd6NDPd
HTTP/1.1 412 Precondition Failed
(http body canbe empty, the client will stop downloading the resumed file)107Please respect copyright.ＰＥＮＡＮＡMuSj3Nf47h
---------------------------------------------------
107Please respect copyright.ＰＥＮＡＮＡN4ehXqdpJm
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---107Please respect copyright.ＰＥＮＡＮＡVwX3zWlUwM
GET /zen.iso HTTP/1.1107Please respect copyright.ＰＥＮＡＮＡrSzuYWtvxF
Range: bytes=287718144-107Please respect copyright.ＰＥＮＡＮＡfyFHbWoPqG
If-Range: "534231200_1732277116"107Please respect copyright.ＰＥＮＡＮＡI8nEszZCBj
------------------------------------107Please respect copyright.ＰＥＮＡＮＡtcufTYujSZ
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:107Please respect copyright.ＰＥＮＡＮＡmO9ZNqOlbh
-------------range response-------------------107Please respect copyright.ＰＥＮＡＮＡ2h0QqXX9eB
HTTP/1.1 206 Partial Content107Please respect copyright.ＰＥＮＡＮＡ8dHHPbmhau
accept-ranges: bytes107Please respect copyright.ＰＥＮＡＮＡKqDIndwmO0
content-range: bytes 287718144-534231199/534231200107Please respect copyright.ＰＥＮＡＮＡEh9srp9Uwv
ETag: "534231200_1732277116"107Please respect copyright.ＰＥＮＡＮＡAA6ncl7aJZ
Content-Length: 246513056107Please respect copyright.ＰＥＮＡＮＡT0z8xe9txR
Content-Type: application/octet-stream107Please respect copyright.ＰＥＮＡＮＡuDExSH7AaJ
Content-Disposition: attachment; filename="zen.iso"
(http body is the remain file content)107Please respect copyright.ＰＥＮＡＮＡGLRcrsxEkZ
-------------------------------------------
If the etag changes, response 200 from beginning like this:107Please respect copyright.ＰＥＮＡＮＡ83nq8w77ZU
-----------http response from the beginning------107Please respect copyright.ＰＥＮＡＮＡ7PNvoAKoOP
HTTP/1.1 200 OK107Please respect copyright.ＰＥＮＡＮＡ7cRLQEvk1Y
Date: Mon, 25 Nov 2024 10:56:12 GMT107Please respect copyright.ＰＥＮＡＮＡ852ycBM0dx
accept-ranges: bytes107Please respect copyright.ＰＥＮＡＮＡnuQ3eyynoM
ETag: "534231200_1732277116"107Please respect copyright.ＰＥＮＡＮＡ6ldvCWcpQ7
Content-Length: 534231200107Please respect copyright.ＰＥＮＡＮＡWd0DwPnP6t
Content-Type: application/octet-stream107Please respect copyright.ＰＥＮＡＮＡsQC17TMqPH
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)107Please respect copyright.ＰＥＮＡＮＡtYhb1M6em0
---------------------------------------------------
107Please respect copyright.ＰＥＮＡＮＡX7OsqDh7uL
Section X. Thanks
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/If-None-Match107Please respect copyright.ＰＥＮＡＮＡvNVa25oDLL
https://blog.csdn.net/catoop/article/details/134174653107Please respect copyright.ＰＥＮＡＮＡUIYbaAXfqn
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]107Please respect copyright.ＰＥＮＡＮＡoi1lV3LhrQ
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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