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.
150Please respect copyright.ＰＥＮＡＮＡjhDHMSSaae
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:150Please respect copyright.ＰＥＮＡＮＡ3QNdrlwGO9
---------------http response with etag----------------150Please respect copyright.ＰＥＮＡＮＡ3nFQ7d6Xjz
HTTP/1.1 200 OK150Please respect copyright.ＰＥＮＡＮＡrulAG6eX8X
Date: Mon, 25 Nov 2024 10:56:12 GMT150Please respect copyright.ＰＥＮＡＮＡk6bKBgesZg
accept-ranges: bytes150Please respect copyright.ＰＥＮＡＮＡaDh8HajF3y
ETag: "534231200_1732277116"150Please respect copyright.ＰＥＮＡＮＡJTzhH67rtT
Content-Length: 534231200150Please respect copyright.ＰＥＮＡＮＡYEpgtjJAxD
Content-Type: application/octet-stream150Please respect copyright.ＰＥＮＡＮＡcCObcWlMFn
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)150Please respect copyright.ＰＥＮＡＮＡfXpk50aYTE
-----------------------------------------------------
150Please respect copyright.ＰＥＮＡＮＡ2eePG4I9iM
Section B. if-none-match
The browser send back the etag to ask server if it has been changed by header like this:150Please respect copyright.ＰＥＮＡＮＡvFsRT6Nu0B
-----http request for a cached resource------150Please respect copyright.ＰＥＮＡＮＡAPCEjGZVtd
GET /zen.iso HTTP/1.1150Please respect copyright.ＰＥＮＡＮＡfH3oVd42NQ
if-none-match: "534231200_1732277116"150Please respect copyright.ＰＥＮＡＮＡHGb6HTCQAk
---------------------------------------------
If the etag is unchanged, the server response like this:
1. for get and head requests:150Please respect copyright.ＰＥＮＡＮＡRFj0vNnGr1
-----------http response for unchanged resource------150Please respect copyright.ＰＥＮＡＮＡqvOJGY0ob0
HTTP/1.1 304 Not Modified
(http body canbe empty, the client will used its cache)150Please respect copyright.ＰＥＮＡＮＡZabB0LlnpM
----------------------------------------------------
2. for other methods requests:150Please respect copyright.ＰＥＮＡＮＡ7rZMucXEPi
-----------http response for unchanged resource------150Please respect copyright.ＰＥＮＡＮＡPWeWYwUpj1
HTTP/1.1 412 Precondition Failed
(http body canbe empty, the client will used its cache)150Please respect copyright.ＰＥＮＡＮＡmUy2MxNx5F
----------------------------------------------------
150Please respect copyright.ＰＥＮＡＮＡGGs8Qr6cdS
If the etag changes, the server response 200 OK with the new file content like this:150Please respect copyright.ＰＥＮＡＮＡmbXbdrE1QB
---------------http response with etag----------------150Please respect copyright.ＰＥＮＡＮＡ1OkfRYucXE
HTTP/1.1 200 OK150Please respect copyright.ＰＥＮＡＮＡ5330bEMzUh
Date: Mon, 25 Nov 2024 10:56:12 GMT150Please respect copyright.ＰＥＮＡＮＡmACtntwHxT
accept-ranges: bytes150Please respect copyright.ＰＥＮＡＮＡE2W92iJfYo
ETag: "534231200_1732277116"150Please respect copyright.ＰＥＮＡＮＡL52D5eoDGK
Content-Length: 534231200150Please respect copyright.ＰＥＮＡＮＡls5IKDKkmF
Content-Type: application/octet-stream150Please respect copyright.ＰＥＮＡＮＡ4Cbt0PcGo3
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)150Please respect copyright.ＰＥＮＡＮＡKumJLJaO7w
-----------------------------------------------------
150Please respect copyright.ＰＥＮＡＮＡoIeOFUTTeh
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:150Please respect copyright.ＰＥＮＡＮＡM3p5XtxWdo
------resume download request-------150Please respect copyright.ＰＥＮＡＮＡD9mQVfeqOA
GET /zen.iso HTTP/1.1150Please respect copyright.ＰＥＮＡＮＡ5QQ8rBh6Iz
Range: bytes=287718144-150Please respect copyright.ＰＥＮＡＮＡ6jlUSloiPv
If-Match: "534231200_1732277116"150Please respect copyright.ＰＥＮＡＮＡXqCVWzPdT2
-----------------------------------150Please respect copyright.ＰＥＮＡＮＡV6O6UtsYxC
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:150Please respect copyright.ＰＥＮＡＮＡzSbD3edWrQ
-------------range response-------------------150Please respect copyright.ＰＥＮＡＮＡ9EwrP4aSeK
HTTP/1.1 206 Partial Content150Please respect copyright.ＰＥＮＡＮＡF5Kt1u3HxC
accept-ranges: bytes150Please respect copyright.ＰＥＮＡＮＡlSdCar7mWC
content-range: bytes 287718144-534231199/534231200150Please respect copyright.ＰＥＮＡＮＡc9Tdw2v12u
ETag: "534231200_1732277116"150Please respect copyright.ＰＥＮＡＮＡBDN4dR3eY1
Content-Length: 246513056150Please respect copyright.ＰＥＮＡＮＡLD57nO0wbC
Content-Type: application/octet-stream150Please respect copyright.ＰＥＮＡＮＡXKwwv4Xvn0
Content-Disposition: attachment; filename="zen.iso"
(http body is the remain file content)150Please respect copyright.ＰＥＮＡＮＡxokW270EnN
-------------------------------------------
If the etag changes, response 412 like this:150Please respect copyright.ＰＥＮＡＮＡTW3bQ0gIZx
-----------http response for changed resource------150Please respect copyright.ＰＥＮＡＮＡlmUQMgr7FV
HTTP/1.1 412 Precondition Failed
(http body canbe empty, the client will stop downloading the resumed file)150Please respect copyright.ＰＥＮＡＮＡQf6ILRXFkx
---------------------------------------------------
150Please respect copyright.ＰＥＮＡＮＡWBUWKUUyKj
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---150Please respect copyright.ＰＥＮＡＮＡxwHGfsSzsS
GET /zen.iso HTTP/1.1150Please respect copyright.ＰＥＮＡＮＡO3D087Jd5u
Range: bytes=287718144-150Please respect copyright.ＰＥＮＡＮＡYqaaYtXFKW
If-Range: "534231200_1732277116"150Please respect copyright.ＰＥＮＡＮＡ5W671MMiZS
------------------------------------150Please respect copyright.ＰＥＮＡＮＡvp9SN60FUn
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:150Please respect copyright.ＰＥＮＡＮＡ7XqbWkdxx2
-------------range response-------------------150Please respect copyright.ＰＥＮＡＮＡlo6p0wjvUr
HTTP/1.1 206 Partial Content150Please respect copyright.ＰＥＮＡＮＡcLswyraTKY
accept-ranges: bytes150Please respect copyright.ＰＥＮＡＮＡkVoQZHy8za
content-range: bytes 287718144-534231199/534231200150Please respect copyright.ＰＥＮＡＮＡliBk6xqj8u
ETag: "534231200_1732277116"150Please respect copyright.ＰＥＮＡＮＡ5iMNtrHpNa
Content-Length: 246513056150Please respect copyright.ＰＥＮＡＮＡadQunjdvQ7
Content-Type: application/octet-stream150Please respect copyright.ＰＥＮＡＮＡF2Iyjori2x
Content-Disposition: attachment; filename="zen.iso"
(http body is the remain file content)150Please respect copyright.ＰＥＮＡＮＡIMqjLmkJiB
-------------------------------------------
If the etag changes, response 200 from beginning like this:150Please respect copyright.ＰＥＮＡＮＡnDOylvN0VN
-----------http response from the beginning------150Please respect copyright.ＰＥＮＡＮＡYqRCuxrZij
HTTP/1.1 200 OK150Please respect copyright.ＰＥＮＡＮＡJ2BDgW8hPk
Date: Mon, 25 Nov 2024 10:56:12 GMT150Please respect copyright.ＰＥＮＡＮＡiVB1eR0y4h
accept-ranges: bytes150Please respect copyright.ＰＥＮＡＮＡgxxnfjYrXz
ETag: "534231200_1732277116"150Please respect copyright.ＰＥＮＡＮＡ0eE5AB6KFG
Content-Length: 534231200150Please respect copyright.ＰＥＮＡＮＡK447UeF97n
Content-Type: application/octet-stream150Please respect copyright.ＰＥＮＡＮＡWQMQu2v9aw
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)150Please respect copyright.ＰＥＮＡＮＡOOf7CQKHUi
---------------------------------------------------
150Please respect copyright.ＰＥＮＡＮＡEnOztUeIC9
Section X. Thanks
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/If-None-Match150Please respect copyright.ＰＥＮＡＮＡ6OndS5OFvm
https://blog.csdn.net/catoop/article/details/134174653150Please respect copyright.ＰＥＮＡＮＡBmBv0ZLOEo
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]150Please respect copyright.ＰＥＮＡＮＡRBxYdSR4av
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]
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。
ns216.73.216.180da2
然後按「加至主畫面」