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.
117Please respect copyright.ＰＥＮＡＮＡYffZWhooHj
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:117Please respect copyright.ＰＥＮＡＮＡQsoIOSpx9A
---------------http response with etag----------------117Please respect copyright.ＰＥＮＡＮＡ5MwicDL0TS
HTTP/1.1 200 OK117Please respect copyright.ＰＥＮＡＮＡ3IYntfw9KB
Date: Mon, 25 Nov 2024 10:56:12 GMT117Please respect copyright.ＰＥＮＡＮＡVm0mh373H6
accept-ranges: bytes117Please respect copyright.ＰＥＮＡＮＡ6cdSKMEpDw
ETag: "534231200_1732277116"117Please respect copyright.ＰＥＮＡＮＡIDUsnCsMxI
Content-Length: 534231200117Please respect copyright.ＰＥＮＡＮＡBw1haLzGBR
Content-Type: application/octet-stream117Please respect copyright.ＰＥＮＡＮＡkgW3dXjUB3
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)117Please respect copyright.ＰＥＮＡＮＡNzI8vOQwNm
-----------------------------------------------------
117Please respect copyright.ＰＥＮＡＮＡr8j1tVCUc3
Section B. if-none-match
The browser send back the etag to ask server if it has been changed by header like this:117Please respect copyright.ＰＥＮＡＮＡBc3RD188gI
-----http request for a cached resource------117Please respect copyright.ＰＥＮＡＮＡolOw8Bm9ED
GET /zen.iso HTTP/1.1117Please respect copyright.ＰＥＮＡＮＡ7h6y2RrIXX
if-none-match: "534231200_1732277116"117Please respect copyright.ＰＥＮＡＮＡMnOfomSYYj
---------------------------------------------
If the etag is unchanged, the server response like this:
1. for get and head requests:117Please respect copyright.ＰＥＮＡＮＡwrwNOY1QU0
-----------http response for unchanged resource------117Please respect copyright.ＰＥＮＡＮＡq0PJtD78kU
HTTP/1.1 304 Not Modified
(http body canbe empty, the client will used its cache)117Please respect copyright.ＰＥＮＡＮＡjdHrsoGgoD
----------------------------------------------------
2. for other methods requests:117Please respect copyright.ＰＥＮＡＮＡ4XejIZ88kS
-----------http response for unchanged resource------117Please respect copyright.ＰＥＮＡＮＡh6DJAxFDXw
HTTP/1.1 412 Precondition Failed
(http body canbe empty, the client will used its cache)117Please respect copyright.ＰＥＮＡＮＡkbmzzSwLVL
----------------------------------------------------
117Please respect copyright.ＰＥＮＡＮＡLO8AS7TbNJ
If the etag changes, the server response 200 OK with the new file content like this:117Please respect copyright.ＰＥＮＡＮＡrBvyDwLSRV
---------------http response with etag----------------117Please respect copyright.ＰＥＮＡＮＡy3K8bAsUZ0
HTTP/1.1 200 OK117Please respect copyright.ＰＥＮＡＮＡBBHp1KlXMm
Date: Mon, 25 Nov 2024 10:56:12 GMT117Please respect copyright.ＰＥＮＡＮＡgJKAm082nZ
accept-ranges: bytes117Please respect copyright.ＰＥＮＡＮＡn5CHVb9Axj
ETag: "534231200_1732277116"117Please respect copyright.ＰＥＮＡＮＡAgUg7kPWjB
Content-Length: 534231200117Please respect copyright.ＰＥＮＡＮＡKdFWRagqJ2
Content-Type: application/octet-stream117Please respect copyright.ＰＥＮＡＮＡKhJEwDI803
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)117Please respect copyright.ＰＥＮＡＮＡUXNigI6bDO
-----------------------------------------------------
117Please respect copyright.ＰＥＮＡＮＡxt8wislhzL
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:117Please respect copyright.ＰＥＮＡＮＡxTZ0B2y37f
------resume download request-------117Please respect copyright.ＰＥＮＡＮＡpKcEyqg16w
GET /zen.iso HTTP/1.1117Please respect copyright.ＰＥＮＡＮＡqTNmvV37Nk
Range: bytes=287718144-117Please respect copyright.ＰＥＮＡＮＡjfc2HNTGJs
If-Match: "534231200_1732277116"117Please respect copyright.ＰＥＮＡＮＡvqrUF8Rl7l
-----------------------------------117Please respect copyright.ＰＥＮＡＮＡWl4GXv79Be
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:117Please respect copyright.ＰＥＮＡＮＡ9iwxleHuxQ
-------------range response-------------------117Please respect copyright.ＰＥＮＡＮＡDBuV3cJ0QY
HTTP/1.1 206 Partial Content117Please respect copyright.ＰＥＮＡＮＡYV52wMFc3x
accept-ranges: bytes117Please respect copyright.ＰＥＮＡＮＡWCFEknWhgl
content-range: bytes 287718144-534231199/534231200117Please respect copyright.ＰＥＮＡＮＡAkpbOd8Vw3
ETag: "534231200_1732277116"117Please respect copyright.ＰＥＮＡＮＡ0OP81utXLR
Content-Length: 246513056117Please respect copyright.ＰＥＮＡＮＡoDF9MzSzxI
Content-Type: application/octet-stream117Please respect copyright.ＰＥＮＡＮＡ6iqELo08F1
Content-Disposition: attachment; filename="zen.iso"
(http body is the remain file content)117Please respect copyright.ＰＥＮＡＮＡPbhnFVEv6g
-------------------------------------------
If the etag changes, response 412 like this:117Please respect copyright.ＰＥＮＡＮＡyn2jGiO19U
-----------http response for changed resource------117Please respect copyright.ＰＥＮＡＮＡCAHgqGpf2x
HTTP/1.1 412 Precondition Failed
(http body canbe empty, the client will stop downloading the resumed file)117Please respect copyright.ＰＥＮＡＮＡMhjRs4Wo46
---------------------------------------------------
117Please respect copyright.ＰＥＮＡＮＡKdqocewJIH
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---117Please respect copyright.ＰＥＮＡＮＡN7fnoEZMUO
GET /zen.iso HTTP/1.1117Please respect copyright.ＰＥＮＡＮＡdF73x9obk4
Range: bytes=287718144-117Please respect copyright.ＰＥＮＡＮＡBFLMQWCH4D
If-Range: "534231200_1732277116"117Please respect copyright.ＰＥＮＡＮＡnQA0jEzVSu
------------------------------------117Please respect copyright.ＰＥＮＡＮＡyNcKcUQblu
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:117Please respect copyright.ＰＥＮＡＮＡFvobgLBWy2
-------------range response-------------------117Please respect copyright.ＰＥＮＡＮＡRR0K7jUIG7
HTTP/1.1 206 Partial Content117Please respect copyright.ＰＥＮＡＮＡprDwDt3vdq
accept-ranges: bytes117Please respect copyright.ＰＥＮＡＮＡCjMnuqQBqf
content-range: bytes 287718144-534231199/534231200117Please respect copyright.ＰＥＮＡＮＡPB0VMlBsP1
ETag: "534231200_1732277116"117Please respect copyright.ＰＥＮＡＮＡFkHJiRSn3B
Content-Length: 246513056117Please respect copyright.ＰＥＮＡＮＡ4jiQHAPZvH
Content-Type: application/octet-stream117Please respect copyright.ＰＥＮＡＮＡbZwIVUF4yf
Content-Disposition: attachment; filename="zen.iso"
(http body is the remain file content)117Please respect copyright.ＰＥＮＡＮＡLfwg8zed1A
-------------------------------------------
If the etag changes, response 200 from beginning like this:117Please respect copyright.ＰＥＮＡＮＡZV4gAlMMfj
-----------http response from the beginning------117Please respect copyright.ＰＥＮＡＮＡR1PlgQ9Cfh
HTTP/1.1 200 OK117Please respect copyright.ＰＥＮＡＮＡFQhitZiznC
Date: Mon, 25 Nov 2024 10:56:12 GMT117Please respect copyright.ＰＥＮＡＮＡZxEmRqkRLj
accept-ranges: bytes117Please respect copyright.ＰＥＮＡＮＡNBpOXt42Ow
ETag: "534231200_1732277116"117Please respect copyright.ＰＥＮＡＮＡtKMI0fkx2C
Content-Length: 534231200117Please respect copyright.ＰＥＮＡＮＡTrIdj9KFti
Content-Type: application/octet-stream117Please respect copyright.ＰＥＮＡＮＡFw9056Fp9S
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)117Please respect copyright.ＰＥＮＡＮＡ7ElpefaSOs
---------------------------------------------------
117Please respect copyright.ＰＥＮＡＮＡsqsGcmS1Xg
Section X. Thanks
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/If-None-Match117Please respect copyright.ＰＥＮＡＮＡ6ULXPw0StD
https://blog.csdn.net/catoop/article/details/134174653117Please respect copyright.ＰＥＮＡＮＡKavJOO2lQF
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.com117Please respect copyright.ＰＥＮＡＮＡBF535F30hr
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。
ns3.17.179.163da2
