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.
212Please respect copyright.ＰＥＮＡＮＡyvP3LOrdSL
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:212Please respect copyright.ＰＥＮＡＮＡQy2Ss30UOE
---------------http response with etag----------------212Please respect copyright.ＰＥＮＡＮＡcuLYnIoGTZ
HTTP/1.1 200 OK212Please respect copyright.ＰＥＮＡＮＡc0lK4pMFQO
Date: Mon, 25 Nov 2024 10:56:12 GMT212Please respect copyright.ＰＥＮＡＮＡob7D7FbZLI
accept-ranges: bytes212Please respect copyright.ＰＥＮＡＮＡokoPzqP8Ty
ETag: "534231200_1732277116"212Please respect copyright.ＰＥＮＡＮＡ8youKYuBfv
Content-Length: 534231200212Please respect copyright.ＰＥＮＡＮＡpkfDbgcCyX
Content-Type: application/octet-stream212Please respect copyright.ＰＥＮＡＮＡRNUEyGGVLV
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)212Please respect copyright.ＰＥＮＡＮＡrKzHCyxc31
-----------------------------------------------------
212Please respect copyright.ＰＥＮＡＮＡRnuSsGK644
Section B. if-none-match
The browser send back the etag to ask server if it has been changed by header like this:212Please respect copyright.ＰＥＮＡＮＡ922hxSLlLe
-----http request for a cached resource------212Please respect copyright.ＰＥＮＡＮＡHZWHkxFnXM
GET /zen.iso HTTP/1.1212Please respect copyright.ＰＥＮＡＮＡGacRrCE151
if-none-match: "534231200_1732277116"212Please respect copyright.ＰＥＮＡＮＡyvsygEk1Ti
---------------------------------------------
If the etag is unchanged, the server response like this:
1. for get and head requests:212Please respect copyright.ＰＥＮＡＮＡNlSBnL3gku
-----------http response for unchanged resource------212Please respect copyright.ＰＥＮＡＮＡhesmBJrbnP
HTTP/1.1 304 Not Modified
(http body canbe empty, the client will used its cache)212Please respect copyright.ＰＥＮＡＮＡC2l8mkr2vN
----------------------------------------------------
2. for other methods requests:212Please respect copyright.ＰＥＮＡＮＡlv76JcbFze
-----------http response for unchanged resource------212Please respect copyright.ＰＥＮＡＮＡB4enkcQ7mb
HTTP/1.1 412 Precondition Failed
(http body canbe empty, the client will used its cache)212Please respect copyright.ＰＥＮＡＮＡFebXzy1sV0
----------------------------------------------------
212Please respect copyright.ＰＥＮＡＮＡJGM2rN6NHE
If the etag changes, the server response 200 OK with the new file content like this:212Please respect copyright.ＰＥＮＡＮＡzMF3qwhnwy
---------------http response with etag----------------212Please respect copyright.ＰＥＮＡＮＡhZEHfriaZ9
HTTP/1.1 200 OK212Please respect copyright.ＰＥＮＡＮＡ79yjXZvWZ1
Date: Mon, 25 Nov 2024 10:56:12 GMT212Please respect copyright.ＰＥＮＡＮＡfzkO4MtSe7
accept-ranges: bytes212Please respect copyright.ＰＥＮＡＮＡkmeQhGzlbj
ETag: "534231200_1732277116"212Please respect copyright.ＰＥＮＡＮＡ930VZLFxEX
Content-Length: 534231200212Please respect copyright.ＰＥＮＡＮＡwHiPIoEUcd
Content-Type: application/octet-stream212Please respect copyright.ＰＥＮＡＮＡqGgyOOXpVM
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)212Please respect copyright.ＰＥＮＡＮＡkhXYhvt0Kx
-----------------------------------------------------
212Please respect copyright.ＰＥＮＡＮＡROtxiIvWI2
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:212Please respect copyright.ＰＥＮＡＮＡ8OKOXX3vSt
------resume download request-------212Please respect copyright.ＰＥＮＡＮＡY5zzzylKxs
GET /zen.iso HTTP/1.1212Please respect copyright.ＰＥＮＡＮＡ55ISQkTxsq
Range: bytes=287718144-212Please respect copyright.ＰＥＮＡＮＡ8WWnzESG5e
If-Match: "534231200_1732277116"212Please respect copyright.ＰＥＮＡＮＡnBOUxKXVW9
-----------------------------------212Please respect copyright.ＰＥＮＡＮＡ7SaxpYuIsC
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:212Please respect copyright.ＰＥＮＡＮＡaLcCbBYsmE
-------------range response-------------------212Please respect copyright.ＰＥＮＡＮＡPoRQcWyGv6
HTTP/1.1 206 Partial Content212Please respect copyright.ＰＥＮＡＮＡZUf9dqrbsF
accept-ranges: bytes212Please respect copyright.ＰＥＮＡＮＡnrsjNZiRog
content-range: bytes 287718144-534231199/534231200212Please respect copyright.ＰＥＮＡＮＡSTdE6XgIka
ETag: "534231200_1732277116"212Please respect copyright.ＰＥＮＡＮＡfe6gQmHopO
Content-Length: 246513056212Please respect copyright.ＰＥＮＡＮＡVGd3oFvcc6
Content-Type: application/octet-stream212Please respect copyright.ＰＥＮＡＮＡ1AH89dbwZG
Content-Disposition: attachment; filename="zen.iso"
(http body is the remain file content)212Please respect copyright.ＰＥＮＡＮＡXWwim0d77p
-------------------------------------------
If the etag changes, response 412 like this:212Please respect copyright.ＰＥＮＡＮＡ39MSFP24mY
-----------http response for changed resource------212Please respect copyright.ＰＥＮＡＮＡkQBhAZ9IE4
HTTP/1.1 412 Precondition Failed
(http body canbe empty, the client will stop downloading the resumed file)212Please respect copyright.ＰＥＮＡＮＡOnHjEGyNgA
---------------------------------------------------
212Please respect copyright.ＰＥＮＡＮＡZC2ngtljEB
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---212Please respect copyright.ＰＥＮＡＮＡifmTqVA3uC
GET /zen.iso HTTP/1.1212Please respect copyright.ＰＥＮＡＮＡH5ln4YxAOV
Range: bytes=287718144-212Please respect copyright.ＰＥＮＡＮＡcVR2McbNtu
If-Range: "534231200_1732277116"212Please respect copyright.ＰＥＮＡＮＡQ4CyRETujf
------------------------------------212Please respect copyright.ＰＥＮＡＮＡmcNk9hl1xE
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:212Please respect copyright.ＰＥＮＡＮＡ7vkFzHl120
-------------range response-------------------212Please respect copyright.ＰＥＮＡＮＡdRdYDDopKT
HTTP/1.1 206 Partial Content212Please respect copyright.ＰＥＮＡＮＡ61EcGsRxRj
accept-ranges: bytes212Please respect copyright.ＰＥＮＡＮＡxSEcUCBHgi
content-range: bytes 287718144-534231199/534231200212Please respect copyright.ＰＥＮＡＮＡFhN0egM3WQ
ETag: "534231200_1732277116"212Please respect copyright.ＰＥＮＡＮＡV1rohzNnZ0
Content-Length: 246513056212Please respect copyright.ＰＥＮＡＮＡvuUuuUNnB2
Content-Type: application/octet-stream212Please respect copyright.ＰＥＮＡＮＡSOpBgr8X90
Content-Disposition: attachment; filename="zen.iso"
(http body is the remain file content)212Please respect copyright.ＰＥＮＡＮＡxCC1S5GMyv
-------------------------------------------
If the etag changes, response 200 from beginning like this:212Please respect copyright.ＰＥＮＡＮＡ4f0Uk1hJvj
-----------http response from the beginning------212Please respect copyright.ＰＥＮＡＮＡbIzOorEEfH
HTTP/1.1 200 OK212Please respect copyright.ＰＥＮＡＮＡGxkSRJwssN
Date: Mon, 25 Nov 2024 10:56:12 GMT212Please respect copyright.ＰＥＮＡＮＡkWcBzUoBTM
accept-ranges: bytes212Please respect copyright.ＰＥＮＡＮＡfjBwq6Okqv
ETag: "534231200_1732277116"212Please respect copyright.ＰＥＮＡＮＡG8KPKPu2Q6
Content-Length: 534231200212Please respect copyright.ＰＥＮＡＮＡTSbD7ot0uE
Content-Type: application/octet-stream212Please respect copyright.ＰＥＮＡＮＡTUW9I4O51d
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)212Please respect copyright.ＰＥＮＡＮＡr68aNxRjT7
---------------------------------------------------
212Please respect copyright.ＰＥＮＡＮＡM944T29nNi
Section X. Thanks
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/If-None-Match212Please respect copyright.ＰＥＮＡＮＡY4UAecbP9Z
https://blog.csdn.net/catoop/article/details/134174653212Please respect copyright.ＰＥＮＡＮＡXsJsOLeJSV
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]212Please respect copyright.ＰＥＮＡＮＡcQ71097zKn
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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