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.
76Please respect copyright.ＰＥＮＡＮＡDaVd4sKjhh
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:76Please respect copyright.ＰＥＮＡＮＡIpke1yvBwc
---------------http response with etag----------------76Please respect copyright.ＰＥＮＡＮＡJkBJgInxhA
HTTP/1.1 200 OK76Please respect copyright.ＰＥＮＡＮＡNhYKsVHft9
Date: Mon, 25 Nov 2024 10:56:12 GMT76Please respect copyright.ＰＥＮＡＮＡ6mHVHHUGHF
accept-ranges: bytes76Please respect copyright.ＰＥＮＡＮＡhROkpKvkn0
ETag: "534231200_1732277116"76Please respect copyright.ＰＥＮＡＮＡy7CpCAsCg8
Content-Length: 53423120076Please respect copyright.ＰＥＮＡＮＡfOwoHfbCE7
Content-Type: application/octet-stream76Please respect copyright.ＰＥＮＡＮＡ9okBQXcwkU
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)76Please respect copyright.ＰＥＮＡＮＡ4ARuT1vvWq
-----------------------------------------------------
76Please respect copyright.ＰＥＮＡＮＡw2gAiSValu
Section B. if-none-match
The browser send back the etag to ask server if it has been changed by header like this:76Please respect copyright.ＰＥＮＡＮＡbGM2Oycsnv
-----http request for a cached resource------76Please respect copyright.ＰＥＮＡＮＡBOt7mkiZ9J
GET /zen.iso HTTP/1.176Please respect copyright.ＰＥＮＡＮＡ087QALLynw
if-none-match: "534231200_1732277116"76Please respect copyright.ＰＥＮＡＮＡ3Km7wBAS9q
---------------------------------------------
If the etag is unchanged, the server response like this:
1. for get and head requests:76Please respect copyright.ＰＥＮＡＮＡ6EI8FWKIfq
-----------http response for unchanged resource------76Please respect copyright.ＰＥＮＡＮＡIxbxFsUAnf
HTTP/1.1 304 Not Modified
(http body canbe empty, the client will used its cache)76Please respect copyright.ＰＥＮＡＮＡoFneUdPcYj
----------------------------------------------------
2. for other methods requests:76Please respect copyright.ＰＥＮＡＮＡx9BGqiysVp
-----------http response for unchanged resource------76Please respect copyright.ＰＥＮＡＮＡGecoPIj4BY
HTTP/1.1 412 Precondition Failed
(http body canbe empty, the client will used its cache)76Please respect copyright.ＰＥＮＡＮＡTFFrlrut6f
----------------------------------------------------
76Please respect copyright.ＰＥＮＡＮＡ1Hdo1vSFio
If the etag changes, the server response 200 OK with the new file content like this:76Please respect copyright.ＰＥＮＡＮＡx4HgZu8BXM
---------------http response with etag----------------76Please respect copyright.ＰＥＮＡＮＡTSIj94qg41
HTTP/1.1 200 OK76Please respect copyright.ＰＥＮＡＮＡkUUtllfkOM
Date: Mon, 25 Nov 2024 10:56:12 GMT76Please respect copyright.ＰＥＮＡＮＡjYPoEUD1sV
accept-ranges: bytes76Please respect copyright.ＰＥＮＡＮＡhXTDQMOevi
ETag: "534231200_1732277116"76Please respect copyright.ＰＥＮＡＮＡOXJJCM0lnO
Content-Length: 53423120076Please respect copyright.ＰＥＮＡＮＡk3WzOZo5ge
Content-Type: application/octet-stream76Please respect copyright.ＰＥＮＡＮＡYORH7bS2nk
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)76Please respect copyright.ＰＥＮＡＮＡQTDh4A8AFZ
-----------------------------------------------------
76Please respect copyright.ＰＥＮＡＮＡV2CPWl1whi
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:76Please respect copyright.ＰＥＮＡＮＡTPtwrvkARK
------resume download request-------76Please respect copyright.ＰＥＮＡＮＡCFvZ8s4GxD
GET /zen.iso HTTP/1.176Please respect copyright.ＰＥＮＡＮＡCmkyIxZI2W
Range: bytes=287718144-76Please respect copyright.ＰＥＮＡＮＡFCEeNnTPzy
If-Match: "534231200_1732277116"76Please respect copyright.ＰＥＮＡＮＡccYybl31Jg
-----------------------------------76Please respect copyright.ＰＥＮＡＮＡSQkpajmv8T
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:76Please respect copyright.ＰＥＮＡＮＡwIDJf6lMNB
-------------range response-------------------76Please respect copyright.ＰＥＮＡＮＡyYUDsqY3zw
HTTP/1.1 206 Partial Content76Please respect copyright.ＰＥＮＡＮＡgCUNjhPq5m
accept-ranges: bytes76Please respect copyright.ＰＥＮＡＮＡmQzwA62SAb
content-range: bytes 287718144-534231199/53423120076Please respect copyright.ＰＥＮＡＮＡ8Z3kxTDP2L
ETag: "534231200_1732277116"76Please respect copyright.ＰＥＮＡＮＡGl23vYIJy1
Content-Length: 24651305676Please respect copyright.ＰＥＮＡＮＡZymyXkdwjJ
Content-Type: application/octet-stream76Please respect copyright.ＰＥＮＡＮＡFTNIXctXM7
Content-Disposition: attachment; filename="zen.iso"
(http body is the remain file content)76Please respect copyright.ＰＥＮＡＮＡg0SLuFvnhs
-------------------------------------------
If the etag changes, response 412 like this:76Please respect copyright.ＰＥＮＡＮＡfRDCUulX4u
-----------http response for changed resource------76Please respect copyright.ＰＥＮＡＮＡ5LynQYho71
HTTP/1.1 412 Precondition Failed
(http body canbe empty, the client will stop downloading the resumed file)76Please respect copyright.ＰＥＮＡＮＡQMVHULDmZL
---------------------------------------------------
76Please respect copyright.ＰＥＮＡＮＡ7jB9lIEuQR
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---76Please respect copyright.ＰＥＮＡＮＡiIFxghIIZg
GET /zen.iso HTTP/1.176Please respect copyright.ＰＥＮＡＮＡnr2sAVDW7M
Range: bytes=287718144-76Please respect copyright.ＰＥＮＡＮＡrUP6J0NdoF
If-Range: "534231200_1732277116"76Please respect copyright.ＰＥＮＡＮＡU3HnmsjdCn
------------------------------------76Please respect copyright.ＰＥＮＡＮＡbAMymCeqVe
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:76Please respect copyright.ＰＥＮＡＮＡ71CIWuyNAL
-------------range response-------------------76Please respect copyright.ＰＥＮＡＮＡfKeMkAatAV
HTTP/1.1 206 Partial Content76Please respect copyright.ＰＥＮＡＮＡyvIDYNv9ZB
accept-ranges: bytes76Please respect copyright.ＰＥＮＡＮＡciiWqpy9Dl
content-range: bytes 287718144-534231199/53423120076Please respect copyright.ＰＥＮＡＮＡkPloMng9hQ
ETag: "534231200_1732277116"76Please respect copyright.ＰＥＮＡＮＡVLNPgF9JZu
Content-Length: 24651305676Please respect copyright.ＰＥＮＡＮＡvkpJI6HvjZ
Content-Type: application/octet-stream76Please respect copyright.ＰＥＮＡＮＡr4sPTxXweB
Content-Disposition: attachment; filename="zen.iso"
(http body is the remain file content)76Please respect copyright.ＰＥＮＡＮＡTDAn0EASCU
-------------------------------------------
If the etag changes, response 200 from beginning like this:76Please respect copyright.ＰＥＮＡＮＡ0sRwzpMZn6
-----------http response from the beginning------76Please respect copyright.ＰＥＮＡＮＡykcJZhQHge
HTTP/1.1 200 OK76Please respect copyright.ＰＥＮＡＮＡqypmc6FKA9
Date: Mon, 25 Nov 2024 10:56:12 GMT76Please respect copyright.ＰＥＮＡＮＡzQdloZ8jj5
accept-ranges: bytes76Please respect copyright.ＰＥＮＡＮＡstaWjCMnhQ
ETag: "534231200_1732277116"76Please respect copyright.ＰＥＮＡＮＡ7d3GnKdfcB
Content-Length: 53423120076Please respect copyright.ＰＥＮＡＮＡ9NXf0Evll1
Content-Type: application/octet-stream76Please respect copyright.ＰＥＮＡＮＡWgS3LCzlAa
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)76Please respect copyright.ＰＥＮＡＮＡVsHyYxulnj
---------------------------------------------------
76Please respect copyright.ＰＥＮＡＮＡUITfZlU20Q
Section X. Thanks
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/If-None-Match76Please respect copyright.ＰＥＮＡＮＡQ8uXcA8M16
https://blog.csdn.net/catoop/article/details/13417465376Please respect copyright.ＰＥＮＡＮＡY9aUw8U5d5
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]76Please respect copyright.ＰＥＮＡＮＡl5H4RlYIy4
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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