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.
165Please respect copyright.ＰＥＮＡＮＡ2CKoXtgELc
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:165Please respect copyright.ＰＥＮＡＮＡEPxp4QIwo6
---------------http response with etag----------------165Please respect copyright.ＰＥＮＡＮＡI38v28XrWx
HTTP/1.1 200 OK165Please respect copyright.ＰＥＮＡＮＡ8Ay4vBRVkq
Date: Mon, 25 Nov 2024 10:56:12 GMT165Please respect copyright.ＰＥＮＡＮＡYCetgOz7et
accept-ranges: bytes165Please respect copyright.ＰＥＮＡＮＡn0G65Ducz6
ETag: "534231200_1732277116"165Please respect copyright.ＰＥＮＡＮＡXGXU5e7MNK
Content-Length: 534231200165Please respect copyright.ＰＥＮＡＮＡmk0QNlPayy
Content-Type: application/octet-stream165Please respect copyright.ＰＥＮＡＮＡhvF7bxrIOP
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)165Please respect copyright.ＰＥＮＡＮＡtBvh6Iyc7o
-----------------------------------------------------
165Please respect copyright.ＰＥＮＡＮＡwXLFLO6BgJ
Section B. if-none-match
The browser send back the etag to ask server if it has been changed by header like this:165Please respect copyright.ＰＥＮＡＮＡKTGHv6pypA
-----http request for a cached resource------165Please respect copyright.ＰＥＮＡＮＡfmmnsWLybA
GET /zen.iso HTTP/1.1165Please respect copyright.ＰＥＮＡＮＡ2sJEjaFUBp
if-none-match: "534231200_1732277116"165Please respect copyright.ＰＥＮＡＮＡbUP0x5KCrE
---------------------------------------------
If the etag is unchanged, the server response like this:
1. for get and head requests:165Please respect copyright.ＰＥＮＡＮＡRj9GQU8bdD
-----------http response for unchanged resource------165Please respect copyright.ＰＥＮＡＮＡJe0CjprIQh
HTTP/1.1 304 Not Modified
(http body canbe empty, the client will used its cache)165Please respect copyright.ＰＥＮＡＮＡdPCXHL2J6c
----------------------------------------------------
2. for other methods requests:165Please respect copyright.ＰＥＮＡＮＡhrG7L7B0Ye
-----------http response for unchanged resource------165Please respect copyright.ＰＥＮＡＮＡgfNABTJe5N
HTTP/1.1 412 Precondition Failed
(http body canbe empty, the client will used its cache)165Please respect copyright.ＰＥＮＡＮＡXCaQ97UfSx
----------------------------------------------------
165Please respect copyright.ＰＥＮＡＮＡlwK0w15p6M
If the etag changes, the server response 200 OK with the new file content like this:165Please respect copyright.ＰＥＮＡＮＡ7RB82eiLTb
---------------http response with etag----------------165Please respect copyright.ＰＥＮＡＮＡ9fiYzU0Y2Q
HTTP/1.1 200 OK165Please respect copyright.ＰＥＮＡＮＡRkBqhgofqP
Date: Mon, 25 Nov 2024 10:56:12 GMT165Please respect copyright.ＰＥＮＡＮＡqZREuVUl8G
accept-ranges: bytes165Please respect copyright.ＰＥＮＡＮＡutMRhlYoXc
ETag: "534231200_1732277116"165Please respect copyright.ＰＥＮＡＮＡrtAVwtdve5
Content-Length: 534231200165Please respect copyright.ＰＥＮＡＮＡM6ZETI9N4h
Content-Type: application/octet-stream165Please respect copyright.ＰＥＮＡＮＡf6Q8GNRDZq
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)165Please respect copyright.ＰＥＮＡＮＡnvvfPcwiXY
-----------------------------------------------------
165Please respect copyright.ＰＥＮＡＮＡtYI4J8Zgax
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:165Please respect copyright.ＰＥＮＡＮＡgIxOErofwk
------resume download request-------165Please respect copyright.ＰＥＮＡＮＡrhglkwot4a
GET /zen.iso HTTP/1.1165Please respect copyright.ＰＥＮＡＮＡT6VNNoTJ0a
Range: bytes=287718144-165Please respect copyright.ＰＥＮＡＮＡ7W4jaiNbSL
If-Match: "534231200_1732277116"165Please respect copyright.ＰＥＮＡＮＡdAoUv4VNIx
-----------------------------------165Please respect copyright.ＰＥＮＡＮＡkn1Zeualqe
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:165Please respect copyright.ＰＥＮＡＮＡK6rRiogO7i
-------------range response-------------------165Please respect copyright.ＰＥＮＡＮＡZJwgbRECnD
HTTP/1.1 206 Partial Content165Please respect copyright.ＰＥＮＡＮＡE6HRp20GPN
accept-ranges: bytes165Please respect copyright.ＰＥＮＡＮＡJJomzAH13I
content-range: bytes 287718144-534231199/534231200165Please respect copyright.ＰＥＮＡＮＡ9SA6keYc81
ETag: "534231200_1732277116"165Please respect copyright.ＰＥＮＡＮＡ2Xto0sxgEA
Content-Length: 246513056165Please respect copyright.ＰＥＮＡＮＡpJoLlmrRU6
Content-Type: application/octet-stream165Please respect copyright.ＰＥＮＡＮＡLmqMy4cNNd
Content-Disposition: attachment; filename="zen.iso"
(http body is the remain file content)165Please respect copyright.ＰＥＮＡＮＡh1XEUv2d5z
-------------------------------------------
If the etag changes, response 412 like this:165Please respect copyright.ＰＥＮＡＮＡDk51du5jbe
-----------http response for changed resource------165Please respect copyright.ＰＥＮＡＮＡAcWF7UC0r3
HTTP/1.1 412 Precondition Failed
(http body canbe empty, the client will stop downloading the resumed file)165Please respect copyright.ＰＥＮＡＮＡdBhHRsWZIo
---------------------------------------------------
165Please respect copyright.ＰＥＮＡＮＡ2CRIKVzC9s
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---165Please respect copyright.ＰＥＮＡＮＡ3LVbHjQUcp
GET /zen.iso HTTP/1.1165Please respect copyright.ＰＥＮＡＮＡtBH1K6WrEw
Range: bytes=287718144-165Please respect copyright.ＰＥＮＡＮＡjlhsxPK4bI
If-Range: "534231200_1732277116"165Please respect copyright.ＰＥＮＡＮＡGa3RAaVL1g
------------------------------------165Please respect copyright.ＰＥＮＡＮＡmZddlmDMgq
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:165Please respect copyright.ＰＥＮＡＮＡA8IhhsrCFU
-------------range response-------------------165Please respect copyright.ＰＥＮＡＮＡiaeTqWwWKk
HTTP/1.1 206 Partial Content165Please respect copyright.ＰＥＮＡＮＡLMqIulKqdY
accept-ranges: bytes165Please respect copyright.ＰＥＮＡＮＡSoSwbqsIwJ
content-range: bytes 287718144-534231199/534231200165Please respect copyright.ＰＥＮＡＮＡfwPlv07U34
ETag: "534231200_1732277116"165Please respect copyright.ＰＥＮＡＮＡft8Zfn68sJ
Content-Length: 246513056165Please respect copyright.ＰＥＮＡＮＡLMaCey8w5u
Content-Type: application/octet-stream165Please respect copyright.ＰＥＮＡＮＡt0F7HDoxLh
Content-Disposition: attachment; filename="zen.iso"
(http body is the remain file content)165Please respect copyright.ＰＥＮＡＮＡmzGjeUtSLN
-------------------------------------------
If the etag changes, response 200 from beginning like this:165Please respect copyright.ＰＥＮＡＮＡANoez2IZ6W
-----------http response from the beginning------165Please respect copyright.ＰＥＮＡＮＡ7Qs1LYXc4W
HTTP/1.1 200 OK165Please respect copyright.ＰＥＮＡＮＡty3fePqg97
Date: Mon, 25 Nov 2024 10:56:12 GMT165Please respect copyright.ＰＥＮＡＮＡKhDBT67sg4
accept-ranges: bytes165Please respect copyright.ＰＥＮＡＮＡLmXtk9LPA7
ETag: "534231200_1732277116"165Please respect copyright.ＰＥＮＡＮＡNUloZvOaAw
Content-Length: 534231200165Please respect copyright.ＰＥＮＡＮＡg1okw0gjt9
Content-Type: application/octet-stream165Please respect copyright.ＰＥＮＡＮＡZkqsIKsvM7
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)165Please respect copyright.ＰＥＮＡＮＡmNAtDyOcG8
---------------------------------------------------
165Please respect copyright.ＰＥＮＡＮＡwGQEDc0BYk
Section X. Thanks
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/If-None-Match165Please respect copyright.ＰＥＮＡＮＡ3oWw73d3GA
https://blog.csdn.net/catoop/article/details/134174653165Please respect copyright.ＰＥＮＡＮＡ97JPUf9Mlh
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]165Please respect copyright.ＰＥＮＡＮＡDlHB9bcKxq
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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