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.
34Please respect copyright.ＰＥＮＡＮＡILQLE7A497
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:34Please respect copyright.ＰＥＮＡＮＡlK9Uk7WszE
---------------http response with etag----------------34Please respect copyright.ＰＥＮＡＮＡVtzJsIkPU5
HTTP/1.1 200 OK34Please respect copyright.ＰＥＮＡＮＡrCoR6xLjDZ
Date: Mon, 25 Nov 2024 10:56:12 GMT34Please respect copyright.ＰＥＮＡＮＡhFW4aLGn48
accept-ranges: bytes34Please respect copyright.ＰＥＮＡＮＡouzuRzEozz
ETag: "534231200_1732277116"34Please respect copyright.ＰＥＮＡＮＡRK8D1ZA9i0
Content-Length: 53423120034Please respect copyright.ＰＥＮＡＮＡOBswxvRu00
Content-Type: application/octet-stream34Please respect copyright.ＰＥＮＡＮＡ6oiaX3A23y
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)34Please respect copyright.ＰＥＮＡＮＡbDY9OGvTpy
-----------------------------------------------------
34Please respect copyright.ＰＥＮＡＮＡ9omIehHato
Section B. if-none-match
The browser send back the etag to ask server if it has been changed by header like this:34Please respect copyright.ＰＥＮＡＮＡy6ToEBBY4m
-----http request for a cached resource------34Please respect copyright.ＰＥＮＡＮＡZmusBdeiwN
GET /zen.iso HTTP/1.134Please respect copyright.ＰＥＮＡＮＡe1vc1ke2GN
if-none-match: "534231200_1732277116"34Please respect copyright.ＰＥＮＡＮＡ7d0yRIPQDq
---------------------------------------------
If the etag is unchanged, the server response like this:
1. for get and head requests:34Please respect copyright.ＰＥＮＡＮＡRaTd8j2twz
-----------http response for unchanged resource------34Please respect copyright.ＰＥＮＡＮＡCqZfLTijIe
HTTP/1.1 304 Not Modified
(http body canbe empty, the client will used its cache)34Please respect copyright.ＰＥＮＡＮＡ4sTKsYFTKt
----------------------------------------------------
2. for other methods requests:34Please respect copyright.ＰＥＮＡＮＡ83wGGgCCv0
-----------http response for unchanged resource------34Please respect copyright.ＰＥＮＡＮＡqbHWMoLN3m
HTTP/1.1 412 Precondition Failed
(http body canbe empty, the client will used its cache)34Please respect copyright.ＰＥＮＡＮＡJ0i00ghv0a
----------------------------------------------------
34Please respect copyright.ＰＥＮＡＮＡSJgJ9ukQGX
If the etag changes, the server response 200 OK with the new file content like this:34Please respect copyright.ＰＥＮＡＮＡfIULogkAe4
---------------http response with etag----------------34Please respect copyright.ＰＥＮＡＮＡxExnIrTWhA
HTTP/1.1 200 OK34Please respect copyright.ＰＥＮＡＮＡIVRv8JYcj7
Date: Mon, 25 Nov 2024 10:56:12 GMT34Please respect copyright.ＰＥＮＡＮＡ6Ai5LeB2Qv
accept-ranges: bytes34Please respect copyright.ＰＥＮＡＮＡ1oRjhQXtmE
ETag: "534231200_1732277116"34Please respect copyright.ＰＥＮＡＮＡGhiDyCygIb
Content-Length: 53423120034Please respect copyright.ＰＥＮＡＮＡ4n5OM8sWhu
Content-Type: application/octet-stream34Please respect copyright.ＰＥＮＡＮＡlfsjfaUBjX
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)34Please respect copyright.ＰＥＮＡＮＡnwfUTg1CRn
-----------------------------------------------------
34Please respect copyright.ＰＥＮＡＮＡbuuW9piHAy
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:34Please respect copyright.ＰＥＮＡＮＡmOpdvUC1Tr
------resume download request-------34Please respect copyright.ＰＥＮＡＮＡzPMY1JjfOW
GET /zen.iso HTTP/1.134Please respect copyright.ＰＥＮＡＮＡjh5JynlRRz
Range: bytes=287718144-34Please respect copyright.ＰＥＮＡＮＡOOU0XUKvvd
If-Match: "534231200_1732277116"34Please respect copyright.ＰＥＮＡＮＡrrD8k8WRdk
-----------------------------------34Please respect copyright.ＰＥＮＡＮＡ9HWkmcoprb
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:34Please respect copyright.ＰＥＮＡＮＡ4I2CA89Tda
-------------range response-------------------34Please respect copyright.ＰＥＮＡＮＡzhTbVnszBi
HTTP/1.1 206 Partial Content34Please respect copyright.ＰＥＮＡＮＡB7ljXAkFvm
accept-ranges: bytes34Please respect copyright.ＰＥＮＡＮＡ495ugs7smR
content-range: bytes 287718144-534231199/53423120034Please respect copyright.ＰＥＮＡＮＡPWxPO05vF9
ETag: "534231200_1732277116"34Please respect copyright.ＰＥＮＡＮＡrHBkovc6nr
Content-Length: 24651305634Please respect copyright.ＰＥＮＡＮＡ9gO1Mh2f02
Content-Type: application/octet-stream34Please respect copyright.ＰＥＮＡＮＡIZtzQftOMb
Content-Disposition: attachment; filename="zen.iso"
(http body is the remain file content)34Please respect copyright.ＰＥＮＡＮＡSzkN4Yav5p
-------------------------------------------
If the etag changes, response 412 like this:34Please respect copyright.ＰＥＮＡＮＡVHK2FrGj8t
-----------http response for changed resource------34Please respect copyright.ＰＥＮＡＮＡ6dJzFNb03p
HTTP/1.1 412 Precondition Failed
(http body canbe empty, the client will stop downloading the resumed file)34Please respect copyright.ＰＥＮＡＮＡYt2OTPV2W5
---------------------------------------------------
34Please respect copyright.ＰＥＮＡＮＡ9eY6bNdHDB
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---34Please respect copyright.ＰＥＮＡＮＡkACmlcfplF
GET /zen.iso HTTP/1.134Please respect copyright.ＰＥＮＡＮＡidoH0Ce6aK
Range: bytes=287718144-34Please respect copyright.ＰＥＮＡＮＡf2cR7FTGgI
If-Range: "534231200_1732277116"34Please respect copyright.ＰＥＮＡＮＡy56AQoJav3
------------------------------------34Please respect copyright.ＰＥＮＡＮＡb1sPfyAmuH
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:34Please respect copyright.ＰＥＮＡＮＡrgedKLe7Ts
-------------range response-------------------34Please respect copyright.ＰＥＮＡＮＡFw6VfCCj4u
HTTP/1.1 206 Partial Content34Please respect copyright.ＰＥＮＡＮＡoWL65X8YeH
accept-ranges: bytes34Please respect copyright.ＰＥＮＡＮＡZtwGK6MdAY
content-range: bytes 287718144-534231199/53423120034Please respect copyright.ＰＥＮＡＮＡwKwG5cknUS
ETag: "534231200_1732277116"34Please respect copyright.ＰＥＮＡＮＡW8GZDb3QjJ
Content-Length: 24651305634Please respect copyright.ＰＥＮＡＮＡEkQc9qXAZJ
Content-Type: application/octet-stream34Please respect copyright.ＰＥＮＡＮＡhPTEBP7m1y
Content-Disposition: attachment; filename="zen.iso"
(http body is the remain file content)34Please respect copyright.ＰＥＮＡＮＡ4fNMp7Ljv0
-------------------------------------------
If the etag changes, response 200 from beginning like this:34Please respect copyright.ＰＥＮＡＮＡ4g2BFbWkZa
-----------http response from the beginning------34Please respect copyright.ＰＥＮＡＮＡSvSTWG54MS
HTTP/1.1 200 OK34Please respect copyright.ＰＥＮＡＮＡgLJsiQUFbx
Date: Mon, 25 Nov 2024 10:56:12 GMT34Please respect copyright.ＰＥＮＡＮＡTC0HH5IUXG
accept-ranges: bytes34Please respect copyright.ＰＥＮＡＮＡJCcXbXK6qZ
ETag: "534231200_1732277116"34Please respect copyright.ＰＥＮＡＮＡsFMDcDmfTj
Content-Length: 53423120034Please respect copyright.ＰＥＮＡＮＡCyUuszGgrx
Content-Type: application/octet-stream34Please respect copyright.ＰＥＮＡＮＡWYJePM4awM
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)34Please respect copyright.ＰＥＮＡＮＡGilIh3xseB
---------------------------------------------------
34Please respect copyright.ＰＥＮＡＮＡAiEx1c2eGe
Section X. Thanks
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/If-None-Match34Please respect copyright.ＰＥＮＡＮＡKlSGygxApM
https://blog.csdn.net/catoop/article/details/13417465334Please respect copyright.ＰＥＮＡＮＡ5VflICnQ7p
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: si.jinmin@gmail.com34Please respect copyright.ＰＥＮＡＮＡiHlDm7byRk
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: si.jinmin@gmail.com
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