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.
183Please respect copyright.ＰＥＮＡＮＡ4MIldaB4nx
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:183Please respect copyright.ＰＥＮＡＮＡR8tNlP9aPN
---------------http response with etag----------------183Please respect copyright.ＰＥＮＡＮＡnJiVM0cgyE
HTTP/1.1 200 OK183Please respect copyright.ＰＥＮＡＮＡt9BZufNsQS
Date: Mon, 25 Nov 2024 10:56:12 GMT183Please respect copyright.ＰＥＮＡＮＡozCdkZUSmq
accept-ranges: bytes183Please respect copyright.ＰＥＮＡＮＡCq39U1SO5p
ETag: "534231200_1732277116"183Please respect copyright.ＰＥＮＡＮＡwxY5Hmk11R
Content-Length: 534231200183Please respect copyright.ＰＥＮＡＮＡvDShmn84OC
Content-Type: application/octet-stream183Please respect copyright.ＰＥＮＡＮＡhoOojQklFF
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)183Please respect copyright.ＰＥＮＡＮＡxlOPhbEcDh
-----------------------------------------------------
183Please respect copyright.ＰＥＮＡＮＡd88et10UU5
Section B. if-none-match
The browser send back the etag to ask server if it has been changed by header like this:183Please respect copyright.ＰＥＮＡＮＡG9q4US78Bb
-----http request for a cached resource------183Please respect copyright.ＰＥＮＡＮＡF7X5VhNJTn
GET /zen.iso HTTP/1.1183Please respect copyright.ＰＥＮＡＮＡ19XmaBIAgR
if-none-match: "534231200_1732277116"183Please respect copyright.ＰＥＮＡＮＡK7FNrM609s
---------------------------------------------
If the etag is unchanged, the server response like this:
1. for get and head requests:183Please respect copyright.ＰＥＮＡＮＡBdChhiUylr
-----------http response for unchanged resource------183Please respect copyright.ＰＥＮＡＮＡYFUrzNPYS5
HTTP/1.1 304 Not Modified
(http body canbe empty, the client will used its cache)183Please respect copyright.ＰＥＮＡＮＡgKcZtMpInu
----------------------------------------------------
2. for other methods requests:183Please respect copyright.ＰＥＮＡＮＡQ8IE5FFpdG
-----------http response for unchanged resource------183Please respect copyright.ＰＥＮＡＮＡdrJUU5uHIZ
HTTP/1.1 412 Precondition Failed
(http body canbe empty, the client will used its cache)183Please respect copyright.ＰＥＮＡＮＡ8zbwKOocAb
----------------------------------------------------
183Please respect copyright.ＰＥＮＡＮＡotzmM3M7nb
If the etag changes, the server response 200 OK with the new file content like this:183Please respect copyright.ＰＥＮＡＮＡW6VXL0PqyO
---------------http response with etag----------------183Please respect copyright.ＰＥＮＡＮＡ06If5EfVKU
HTTP/1.1 200 OK183Please respect copyright.ＰＥＮＡＮＡObTfEtjNDb
Date: Mon, 25 Nov 2024 10:56:12 GMT183Please respect copyright.ＰＥＮＡＮＡ5b9wWh8FIC
accept-ranges: bytes183Please respect copyright.ＰＥＮＡＮＡKm7ANsbiAp
ETag: "534231200_1732277116"183Please respect copyright.ＰＥＮＡＮＡxbvYSKb5A3
Content-Length: 534231200183Please respect copyright.ＰＥＮＡＮＡiHrnL54mwL
Content-Type: application/octet-stream183Please respect copyright.ＰＥＮＡＮＡwV2pAvsUlL
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)183Please respect copyright.ＰＥＮＡＮＡDkDslOiemv
-----------------------------------------------------
183Please respect copyright.ＰＥＮＡＮＡTc4GUl216S
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:183Please respect copyright.ＰＥＮＡＮＡ8YDPNzdgYK
------resume download request-------183Please respect copyright.ＰＥＮＡＮＡvlN24cmTDh
GET /zen.iso HTTP/1.1183Please respect copyright.ＰＥＮＡＮＡrsy6wDROxl
Range: bytes=287718144-183Please respect copyright.ＰＥＮＡＮＡWF9sPnltA3
If-Match: "534231200_1732277116"183Please respect copyright.ＰＥＮＡＮＡrYHU85Ofno
-----------------------------------183Please respect copyright.ＰＥＮＡＮＡKG97Q1MZpz
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:183Please respect copyright.ＰＥＮＡＮＡFkfznAQZhn
-------------range response-------------------183Please respect copyright.ＰＥＮＡＮＡC0ybnrUFQA
HTTP/1.1 206 Partial Content183Please respect copyright.ＰＥＮＡＮＡUnVCxQ4Wam
accept-ranges: bytes183Please respect copyright.ＰＥＮＡＮＡKJNKDoC5Jz
content-range: bytes 287718144-534231199/534231200183Please respect copyright.ＰＥＮＡＮＡublMKFP81k
ETag: "534231200_1732277116"183Please respect copyright.ＰＥＮＡＮＡG7XRzn47Fr
Content-Length: 246513056183Please respect copyright.ＰＥＮＡＮＡ90Pfeez2Sf
Content-Type: application/octet-stream183Please respect copyright.ＰＥＮＡＮＡILXBaHy4gj
Content-Disposition: attachment; filename="zen.iso"
(http body is the remain file content)183Please respect copyright.ＰＥＮＡＮＡQ9n1mVLQC2
-------------------------------------------
If the etag changes, response 412 like this:183Please respect copyright.ＰＥＮＡＮＡHffDwBQeH3
-----------http response for changed resource------183Please respect copyright.ＰＥＮＡＮＡtn1Ieg3CcE
HTTP/1.1 412 Precondition Failed
(http body canbe empty, the client will stop downloading the resumed file)183Please respect copyright.ＰＥＮＡＮＡlWiftPaN3L
---------------------------------------------------
183Please respect copyright.ＰＥＮＡＮＡ74pddkI4Bw
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---183Please respect copyright.ＰＥＮＡＮＡy1S8y1yHc8
GET /zen.iso HTTP/1.1183Please respect copyright.ＰＥＮＡＮＡecR1QAfgZa
Range: bytes=287718144-183Please respect copyright.ＰＥＮＡＮＡ7pNCekRGtk
If-Range: "534231200_1732277116"183Please respect copyright.ＰＥＮＡＮＡ0HehAtl9lN
------------------------------------183Please respect copyright.ＰＥＮＡＮＡTvWQgrMHh4
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:183Please respect copyright.ＰＥＮＡＮＡezGxLOtZRx
-------------range response-------------------183Please respect copyright.ＰＥＮＡＮＡySnZXi517B
HTTP/1.1 206 Partial Content183Please respect copyright.ＰＥＮＡＮＡFeRijmU3vU
accept-ranges: bytes183Please respect copyright.ＰＥＮＡＮＡB3fdiq6fNB
content-range: bytes 287718144-534231199/534231200183Please respect copyright.ＰＥＮＡＮＡ6rfrgz9VNE
ETag: "534231200_1732277116"183Please respect copyright.ＰＥＮＡＮＡuOQLSVTaJr
Content-Length: 246513056183Please respect copyright.ＰＥＮＡＮＡuyxBncOq3d
Content-Type: application/octet-stream183Please respect copyright.ＰＥＮＡＮＡU2e2oeVNiu
Content-Disposition: attachment; filename="zen.iso"
(http body is the remain file content)183Please respect copyright.ＰＥＮＡＮＡGnn4DIRNPk
-------------------------------------------
If the etag changes, response 200 from beginning like this:183Please respect copyright.ＰＥＮＡＮＡuDrGFENA7g
-----------http response from the beginning------183Please respect copyright.ＰＥＮＡＮＡVUlL0sgCBa
HTTP/1.1 200 OK183Please respect copyright.ＰＥＮＡＮＡMNdK1ULsbD
Date: Mon, 25 Nov 2024 10:56:12 GMT183Please respect copyright.ＰＥＮＡＮＡXVOAW9Wayh
accept-ranges: bytes183Please respect copyright.ＰＥＮＡＮＡeuMQnky5Hk
ETag: "534231200_1732277116"183Please respect copyright.ＰＥＮＡＮＡ9x8aFYaW4R
Content-Length: 534231200183Please respect copyright.ＰＥＮＡＮＡBBZOYujuiT
Content-Type: application/octet-stream183Please respect copyright.ＰＥＮＡＮＡANY60kiuSs
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)183Please respect copyright.ＰＥＮＡＮＡWt37ySThNS
---------------------------------------------------
183Please respect copyright.ＰＥＮＡＮＡp9ZVi8kjsA
Section X. Thanks
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/If-None-Match183Please respect copyright.ＰＥＮＡＮＡ5r0cDbGHpj
https://blog.csdn.net/catoop/article/details/134174653183Please respect copyright.ＰＥＮＡＮＡ9kxnHqdi8q
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]183Please respect copyright.ＰＥＮＡＮＡvX3hu7WyvQ
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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