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.
52Please respect copyright.ＰＥＮＡＮＡwHZGaxZDbA
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:52Please respect copyright.ＰＥＮＡＮＡiwP3o2ZGd1
---------------http response with etag----------------52Please respect copyright.ＰＥＮＡＮＡaGzEffZvEV
HTTP/1.1 200 OK52Please respect copyright.ＰＥＮＡＮＡKDmIhHn4Dv
Date: Mon, 25 Nov 2024 10:56:12 GMT52Please respect copyright.ＰＥＮＡＮＡS78NzNZ4P9
accept-ranges: bytes52Please respect copyright.ＰＥＮＡＮＡdQSkATAyFL
ETag: "534231200_1732277116"52Please respect copyright.ＰＥＮＡＮＡMctPIDeCEh
Content-Length: 53423120052Please respect copyright.ＰＥＮＡＮＡinkTI4MCSG
Content-Type: application/octet-stream52Please respect copyright.ＰＥＮＡＮＡZeMc9F4Uxk
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)52Please respect copyright.ＰＥＮＡＮＡIctRv55HYH
-----------------------------------------------------
52Please respect copyright.ＰＥＮＡＮＡ3wRftBXfaB
Section B. if-none-match
The browser send back the etag to ask server if it has been changed by header like this:52Please respect copyright.ＰＥＮＡＮＡSisRgLDu1k
-----http request for a cached resource------52Please respect copyright.ＰＥＮＡＮＡMM5y2F5bbU
GET /zen.iso HTTP/1.152Please respect copyright.ＰＥＮＡＮＡbKgbCkHMgy
if-none-match: "534231200_1732277116"52Please respect copyright.ＰＥＮＡＮＡfcmd09EyQJ
---------------------------------------------
If the etag is unchanged, the server response like this:
1. for get and head requests:52Please respect copyright.ＰＥＮＡＮＡMHFNRFAerG
-----------http response for unchanged resource------52Please respect copyright.ＰＥＮＡＮＡtKoCYp4p6t
HTTP/1.1 304 Not Modified
(http body canbe empty, the client will used its cache)52Please respect copyright.ＰＥＮＡＮＡ0FExznBgaS
----------------------------------------------------
2. for other methods requests:52Please respect copyright.ＰＥＮＡＮＡTsvifagXMN
-----------http response for unchanged resource------52Please respect copyright.ＰＥＮＡＮＡIoIZ29yg29
HTTP/1.1 412 Precondition Failed
(http body canbe empty, the client will used its cache)52Please respect copyright.ＰＥＮＡＮＡO8a8Ccg9r9
----------------------------------------------------
52Please respect copyright.ＰＥＮＡＮＡMaSg78GXS0
If the etag changes, the server response 200 OK with the new file content like this:52Please respect copyright.ＰＥＮＡＮＡ1zyGijoPTU
---------------http response with etag----------------52Please respect copyright.ＰＥＮＡＮＡxwwWELPi9R
HTTP/1.1 200 OK52Please respect copyright.ＰＥＮＡＮＡMMQsYKDgSq
Date: Mon, 25 Nov 2024 10:56:12 GMT52Please respect copyright.ＰＥＮＡＮＡnMljphQ2aT
accept-ranges: bytes52Please respect copyright.ＰＥＮＡＮＡLmxGb6zbKW
ETag: "534231200_1732277116"52Please respect copyright.ＰＥＮＡＮＡbugHiCpuaH
Content-Length: 53423120052Please respect copyright.ＰＥＮＡＮＡF1oXwej9HK
Content-Type: application/octet-stream52Please respect copyright.ＰＥＮＡＮＡuKca5N6dqC
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)52Please respect copyright.ＰＥＮＡＮＡMFsUSGzc5x
-----------------------------------------------------
52Please respect copyright.ＰＥＮＡＮＡWyien8AkkS
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:52Please respect copyright.ＰＥＮＡＮＡY3jL8VE3VM
------resume download request-------52Please respect copyright.ＰＥＮＡＮＡ74pBoC73rJ
GET /zen.iso HTTP/1.152Please respect copyright.ＰＥＮＡＮＡRmRvrXnHhb
Range: bytes=287718144-52Please respect copyright.ＰＥＮＡＮＡNCGN4by5ZB
If-Match: "534231200_1732277116"52Please respect copyright.ＰＥＮＡＮＡ3h5nl5K8Ah
-----------------------------------52Please respect copyright.ＰＥＮＡＮＡ9pDxsf6OPd
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:52Please respect copyright.ＰＥＮＡＮＡHPME8aRLNy
-------------range response-------------------52Please respect copyright.ＰＥＮＡＮＡQIs36fecCl
HTTP/1.1 206 Partial Content52Please respect copyright.ＰＥＮＡＮＡv1QdH50RUH
accept-ranges: bytes52Please respect copyright.ＰＥＮＡＮＡoTzxe2CNY4
content-range: bytes 287718144-534231199/53423120052Please respect copyright.ＰＥＮＡＮＡnyzDqmwGL9
ETag: "534231200_1732277116"52Please respect copyright.ＰＥＮＡＮＡR3SzkbcLnS
Content-Length: 24651305652Please respect copyright.ＰＥＮＡＮＡkdqBsqbHxN
Content-Type: application/octet-stream52Please respect copyright.ＰＥＮＡＮＡ5xTwczKzHT
Content-Disposition: attachment; filename="zen.iso"
(http body is the remain file content)52Please respect copyright.ＰＥＮＡＮＡwL4TjRYvrr
-------------------------------------------
If the etag changes, response 412 like this:52Please respect copyright.ＰＥＮＡＮＡIQmVACp8YN
-----------http response for changed resource------52Please respect copyright.ＰＥＮＡＮＡPC3MUsM3VF
HTTP/1.1 412 Precondition Failed
(http body canbe empty, the client will stop downloading the resumed file)52Please respect copyright.ＰＥＮＡＮＡ4iq64u7EcI
---------------------------------------------------
52Please respect copyright.ＰＥＮＡＮＡBg5rHgwWm3
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---52Please respect copyright.ＰＥＮＡＮＡd8QQDWt5Hn
GET /zen.iso HTTP/1.152Please respect copyright.ＰＥＮＡＮＡYfBUPPAGQW
Range: bytes=287718144-52Please respect copyright.ＰＥＮＡＮＡIISds0ws8e
If-Range: "534231200_1732277116"52Please respect copyright.ＰＥＮＡＮＡ9nX8ArwvZs
------------------------------------52Please respect copyright.ＰＥＮＡＮＡP5WqEkxR4c
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:52Please respect copyright.ＰＥＮＡＮＡGZnBEJwxdZ
-------------range response-------------------52Please respect copyright.ＰＥＮＡＮＡaKXhdLNovZ
HTTP/1.1 206 Partial Content52Please respect copyright.ＰＥＮＡＮＡWHTaWgQgSw
accept-ranges: bytes52Please respect copyright.ＰＥＮＡＮＡuZPP3KNkDv
content-range: bytes 287718144-534231199/53423120052Please respect copyright.ＰＥＮＡＮＡ6NxQFS4B5h
ETag: "534231200_1732277116"52Please respect copyright.ＰＥＮＡＮＡo0iRU1UNHa
Content-Length: 24651305652Please respect copyright.ＰＥＮＡＮＡiUc9hRc1pF
Content-Type: application/octet-stream52Please respect copyright.ＰＥＮＡＮＡBRvqDRzVG4
Content-Disposition: attachment; filename="zen.iso"
(http body is the remain file content)52Please respect copyright.ＰＥＮＡＮＡRTdFvwuunE
-------------------------------------------
If the etag changes, response 200 from beginning like this:52Please respect copyright.ＰＥＮＡＮＡfoLaxpj6nO
-----------http response from the beginning------52Please respect copyright.ＰＥＮＡＮＡSr3X9Z2aJ9
HTTP/1.1 200 OK52Please respect copyright.ＰＥＮＡＮＡjLBvvoBwLR
Date: Mon, 25 Nov 2024 10:56:12 GMT52Please respect copyright.ＰＥＮＡＮＡKEUD8m071V
accept-ranges: bytes52Please respect copyright.ＰＥＮＡＮＡLyvabsK3vI
ETag: "534231200_1732277116"52Please respect copyright.ＰＥＮＡＮＡZiB8Y8zJHr
Content-Length: 53423120052Please respect copyright.ＰＥＮＡＮＡh07sqs4yUf
Content-Type: application/octet-stream52Please respect copyright.ＰＥＮＡＮＡhwSNiBc79J
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)52Please respect copyright.ＰＥＮＡＮＡLSqDX7XA2k
---------------------------------------------------
52Please respect copyright.ＰＥＮＡＮＡWB0coqOZk4
Section X. Thanks
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/If-None-Match52Please respect copyright.ＰＥＮＡＮＡT8LjJ6NRMh
https://blog.csdn.net/catoop/article/details/13417465352Please respect copyright.ＰＥＮＡＮＡoKMqtY1Db2
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.com52Please respect copyright.ＰＥＮＡＮＡGQaySSRXVN
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
如果您對C/C++ Programming, Linux, HTTP Protocol, Website Development, Vue, Git, VsCode感興趣，邀請您加入「Linux/C/C++ Website Development」微信群，請加我的微信（si_jinmin）以便拉您进群。
ns 15.158.61.20da2
