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.
105Please respect copyright.ＰＥＮＡＮＡM8FrVRpWYy
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:105Please respect copyright.ＰＥＮＡＮＡUxDaMRd6sI
---------------http response with etag----------------105Please respect copyright.ＰＥＮＡＮＡBSfKXxsG8X
HTTP/1.1 200 OK105Please respect copyright.ＰＥＮＡＮＡRLiQs42Q8j
Date: Mon, 25 Nov 2024 10:56:12 GMT105Please respect copyright.ＰＥＮＡＮＡ8KQRDM3ykV
accept-ranges: bytes105Please respect copyright.ＰＥＮＡＮＡ2TQPvWPOan
ETag: "534231200_1732277116"105Please respect copyright.ＰＥＮＡＮＡDAfFK8yfrr
Content-Length: 534231200105Please respect copyright.ＰＥＮＡＮＡBzotSbwRTf
Content-Type: application/octet-stream105Please respect copyright.ＰＥＮＡＮＡIuM0g3GZ2D
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)105Please respect copyright.ＰＥＮＡＮＡdvq1oOElIk
-----------------------------------------------------
105Please respect copyright.ＰＥＮＡＮＡKlNV3cQFVK
Section B. if-none-match
The browser send back the etag to ask server if it has been changed by header like this:105Please respect copyright.ＰＥＮＡＮＡHCwGzifWAP
-----http request for a cached resource------105Please respect copyright.ＰＥＮＡＮＡkNTubJWmYi
GET /zen.iso HTTP/1.1105Please respect copyright.ＰＥＮＡＮＡp3yyH2XTeT
if-none-match: "534231200_1732277116"105Please respect copyright.ＰＥＮＡＮＡl7esbt1iIx
---------------------------------------------
If the etag is unchanged, the server response like this:
1. for get and head requests:105Please respect copyright.ＰＥＮＡＮＡgq1fc1O0Jp
-----------http response for unchanged resource------105Please respect copyright.ＰＥＮＡＮＡHEvykzOBME
HTTP/1.1 304 Not Modified
(http body canbe empty, the client will used its cache)105Please respect copyright.ＰＥＮＡＮＡEN3DQlSZCU
----------------------------------------------------
2. for other methods requests:105Please respect copyright.ＰＥＮＡＮＡVuvuUm0h6f
-----------http response for unchanged resource------105Please respect copyright.ＰＥＮＡＮＡ2EZwwQB3Ul
HTTP/1.1 412 Precondition Failed
(http body canbe empty, the client will used its cache)105Please respect copyright.ＰＥＮＡＮＡLZ0o4jqFJb
----------------------------------------------------
105Please respect copyright.ＰＥＮＡＮＡQLYsCaibm5
If the etag changes, the server response 200 OK with the new file content like this:105Please respect copyright.ＰＥＮＡＮＡePG6k98HYe
---------------http response with etag----------------105Please respect copyright.ＰＥＮＡＮＡHL4sdmj3VP
HTTP/1.1 200 OK105Please respect copyright.ＰＥＮＡＮＡUFSRp5Z63f
Date: Mon, 25 Nov 2024 10:56:12 GMT105Please respect copyright.ＰＥＮＡＮＡCWSa4bYm6H
accept-ranges: bytes105Please respect copyright.ＰＥＮＡＮＡZ8COX18EHJ
ETag: "534231200_1732277116"105Please respect copyright.ＰＥＮＡＮＡcyaM10jpfK
Content-Length: 534231200105Please respect copyright.ＰＥＮＡＮＡ8ST7Ln1nkB
Content-Type: application/octet-stream105Please respect copyright.ＰＥＮＡＮＡRCC1wU6ZfF
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)105Please respect copyright.ＰＥＮＡＮＡXc33BLJNXc
-----------------------------------------------------
105Please respect copyright.ＰＥＮＡＮＡ4P1Vp6c8wn
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:105Please respect copyright.ＰＥＮＡＮＡ1a5fzrYjsD
------resume download request-------105Please respect copyright.ＰＥＮＡＮＡSLBoFIijRR
GET /zen.iso HTTP/1.1105Please respect copyright.ＰＥＮＡＮＡ39jA16goWL
Range: bytes=287718144-105Please respect copyright.ＰＥＮＡＮＡyX1QijJRyT
If-Match: "534231200_1732277116"105Please respect copyright.ＰＥＮＡＮＡOVHs6Ve3it
-----------------------------------105Please respect copyright.ＰＥＮＡＮＡN6UDkzhho2
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:105Please respect copyright.ＰＥＮＡＮＡVyZm0wLvWN
-------------range response-------------------105Please respect copyright.ＰＥＮＡＮＡBnZwOkDHLg
HTTP/1.1 206 Partial Content105Please respect copyright.ＰＥＮＡＮＡFumlS9LCPM
accept-ranges: bytes105Please respect copyright.ＰＥＮＡＮＡkBzZhVyNL6
content-range: bytes 287718144-534231199/534231200105Please respect copyright.ＰＥＮＡＮＡ3abn47SELI
ETag: "534231200_1732277116"105Please respect copyright.ＰＥＮＡＮＡoSIcvUbJ4H
Content-Length: 246513056105Please respect copyright.ＰＥＮＡＮＡxtyX1a5732
Content-Type: application/octet-stream105Please respect copyright.ＰＥＮＡＮＡBEt85tpSxz
Content-Disposition: attachment; filename="zen.iso"
(http body is the remain file content)105Please respect copyright.ＰＥＮＡＮＡ4FFQ8lWIxv
-------------------------------------------
If the etag changes, response 412 like this:105Please respect copyright.ＰＥＮＡＮＡ9BovXcQugm
-----------http response for changed resource------105Please respect copyright.ＰＥＮＡＮＡ6bt3MU3PTL
HTTP/1.1 412 Precondition Failed
(http body canbe empty, the client will stop downloading the resumed file)105Please respect copyright.ＰＥＮＡＮＡV4Vl0hJXN4
---------------------------------------------------
105Please respect copyright.ＰＥＮＡＮＡU6gemGU5cw
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---105Please respect copyright.ＰＥＮＡＮＡ81pRkYkMSz
GET /zen.iso HTTP/1.1105Please respect copyright.ＰＥＮＡＮＡ3UKh7hX8Eq
Range: bytes=287718144-105Please respect copyright.ＰＥＮＡＮＡ35n8HnfdVs
If-Range: "534231200_1732277116"105Please respect copyright.ＰＥＮＡＮＡRQjYLhRD85
------------------------------------105Please respect copyright.ＰＥＮＡＮＡqbCf1Es42s
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:105Please respect copyright.ＰＥＮＡＮＡaFe0cmCnA8
-------------range response-------------------105Please respect copyright.ＰＥＮＡＮＡSB899Uh9sU
HTTP/1.1 206 Partial Content105Please respect copyright.ＰＥＮＡＮＡuzgVNQp28R
accept-ranges: bytes105Please respect copyright.ＰＥＮＡＮＡaEL6F81SMI
content-range: bytes 287718144-534231199/534231200105Please respect copyright.ＰＥＮＡＮＡ86J7xdgsnE
ETag: "534231200_1732277116"105Please respect copyright.ＰＥＮＡＮＡlT3PdflSbp
Content-Length: 246513056105Please respect copyright.ＰＥＮＡＮＡEa0OXV1yo7
Content-Type: application/octet-stream105Please respect copyright.ＰＥＮＡＮＡ5wfXFyJzCW
Content-Disposition: attachment; filename="zen.iso"
(http body is the remain file content)105Please respect copyright.ＰＥＮＡＮＡ59UIRSRSo9
-------------------------------------------
If the etag changes, response 200 from beginning like this:105Please respect copyright.ＰＥＮＡＮＡfeuvClsuGt
-----------http response from the beginning------105Please respect copyright.ＰＥＮＡＮＡk3cxeOi7TQ
HTTP/1.1 200 OK105Please respect copyright.ＰＥＮＡＮＡ8f2ltqscbW
Date: Mon, 25 Nov 2024 10:56:12 GMT105Please respect copyright.ＰＥＮＡＮＡN4qozIFKMt
accept-ranges: bytes105Please respect copyright.ＰＥＮＡＮＡB44mBTRzwf
ETag: "534231200_1732277116"105Please respect copyright.ＰＥＮＡＮＡ7C67GyFDcO
Content-Length: 534231200105Please respect copyright.ＰＥＮＡＮＡqYItGh300T
Content-Type: application/octet-stream105Please respect copyright.ＰＥＮＡＮＡVBp3uNj7HC
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)105Please respect copyright.ＰＥＮＡＮＡOQUxNjldS0
---------------------------------------------------
105Please respect copyright.ＰＥＮＡＮＡBaiV0QFvV0
Section X. Thanks
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/If-None-Match105Please respect copyright.ＰＥＮＡＮＡa506uyIUg8
https://blog.csdn.net/catoop/article/details/134174653105Please respect copyright.ＰＥＮＡＮＡO0vPxDTaCg
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]105Please respect copyright.ＰＥＮＡＮＡendJDsYudY
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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