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.
99Please respect copyright.ＰＥＮＡＮＡVrVi7JnklX
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:99Please respect copyright.ＰＥＮＡＮＡKVTYzLgobp
---------------http response with etag----------------99Please respect copyright.ＰＥＮＡＮＡNDs3fcMdTg
HTTP/1.1 200 OK99Please respect copyright.ＰＥＮＡＮＡRUl3scxyjb
Date: Mon, 25 Nov 2024 10:56:12 GMT99Please respect copyright.ＰＥＮＡＮＡyF9OM90ahE
accept-ranges: bytes99Please respect copyright.ＰＥＮＡＮＡnSsro2yMZg
ETag: "534231200_1732277116"99Please respect copyright.ＰＥＮＡＮＡdd3dFmRA1g
Content-Length: 53423120099Please respect copyright.ＰＥＮＡＮＡHEiPayVYlE
Content-Type: application/octet-stream99Please respect copyright.ＰＥＮＡＮＡ9D3a0Jyhw3
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)99Please respect copyright.ＰＥＮＡＮＡXGSbQEU7Lm
-----------------------------------------------------
99Please respect copyright.ＰＥＮＡＮＡm8we3d0gjA
Section B. if-none-match
The browser send back the etag to ask server if it has been changed by header like this:99Please respect copyright.ＰＥＮＡＮＡuiIMbBgwDN
-----http request for a cached resource------99Please respect copyright.ＰＥＮＡＮＡ3prDgBpcKA
GET /zen.iso HTTP/1.199Please respect copyright.ＰＥＮＡＮＡq1XEqhqo9f
if-none-match: "534231200_1732277116"99Please respect copyright.ＰＥＮＡＮＡbBJv0l1Jrq
---------------------------------------------
If the etag is unchanged, the server response like this:
1. for get and head requests:99Please respect copyright.ＰＥＮＡＮＡVLHeWHbHKR
-----------http response for unchanged resource------99Please respect copyright.ＰＥＮＡＮＡok94s9dKBZ
HTTP/1.1 304 Not Modified
(http body canbe empty, the client will used its cache)99Please respect copyright.ＰＥＮＡＮＡKiOEQh9fxC
----------------------------------------------------
2. for other methods requests:99Please respect copyright.ＰＥＮＡＮＡCM5nDbKBAW
-----------http response for unchanged resource------99Please respect copyright.ＰＥＮＡＮＡKrArahMvop
HTTP/1.1 412 Precondition Failed
(http body canbe empty, the client will used its cache)99Please respect copyright.ＰＥＮＡＮＡqZ5QrsAHAt
----------------------------------------------------
99Please respect copyright.ＰＥＮＡＮＡomza14pQIO
If the etag changes, the server response 200 OK with the new file content like this:99Please respect copyright.ＰＥＮＡＮＡNJOVJH0rqW
---------------http response with etag----------------99Please respect copyright.ＰＥＮＡＮＡCTpn6ysFCK
HTTP/1.1 200 OK99Please respect copyright.ＰＥＮＡＮＡlCkTpWzfMa
Date: Mon, 25 Nov 2024 10:56:12 GMT99Please respect copyright.ＰＥＮＡＮＡmRi5i5LDfr
accept-ranges: bytes99Please respect copyright.ＰＥＮＡＮＡkwoLe5R32W
ETag: "534231200_1732277116"99Please respect copyright.ＰＥＮＡＮＡPxRnhomeo8
Content-Length: 53423120099Please respect copyright.ＰＥＮＡＮＡz9FiSWUruF
Content-Type: application/octet-stream99Please respect copyright.ＰＥＮＡＮＡmwqRUcdKro
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)99Please respect copyright.ＰＥＮＡＮＡOeAoRPUZvE
-----------------------------------------------------
99Please respect copyright.ＰＥＮＡＮＡXZqVSJ9IhS
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:99Please respect copyright.ＰＥＮＡＮＡ6k8q6wSAi7
------resume download request-------99Please respect copyright.ＰＥＮＡＮＡFYunpLY69z
GET /zen.iso HTTP/1.199Please respect copyright.ＰＥＮＡＮＡl8RgBWCVST
Range: bytes=287718144-99Please respect copyright.ＰＥＮＡＮＡcwakUoSne1
If-Match: "534231200_1732277116"99Please respect copyright.ＰＥＮＡＮＡz4nsjBRj3u
-----------------------------------99Please respect copyright.ＰＥＮＡＮＡyB3fMSHZUH
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:99Please respect copyright.ＰＥＮＡＮＡSPQOYcfv15
-------------range response-------------------99Please respect copyright.ＰＥＮＡＮＡPPWUcgNYLx
HTTP/1.1 206 Partial Content99Please respect copyright.ＰＥＮＡＮＡDw9hAB6SLR
accept-ranges: bytes99Please respect copyright.ＰＥＮＡＮＡ7SthWgXrpc
content-range: bytes 287718144-534231199/53423120099Please respect copyright.ＰＥＮＡＮＡIU11DstYwx
ETag: "534231200_1732277116"99Please respect copyright.ＰＥＮＡＮＡEt93UyXLFa
Content-Length: 24651305699Please respect copyright.ＰＥＮＡＮＡ4ALPxiZ36G
Content-Type: application/octet-stream99Please respect copyright.ＰＥＮＡＮＡMepfgSMkmH
Content-Disposition: attachment; filename="zen.iso"
(http body is the remain file content)99Please respect copyright.ＰＥＮＡＮＡ0YQPrVYsNb
-------------------------------------------
If the etag changes, response 412 like this:99Please respect copyright.ＰＥＮＡＮＡnsEdiZMPmB
-----------http response for changed resource------99Please respect copyright.ＰＥＮＡＮＡuF2O1N0VMp
HTTP/1.1 412 Precondition Failed
(http body canbe empty, the client will stop downloading the resumed file)99Please respect copyright.ＰＥＮＡＮＡI2vjXgBM5y
---------------------------------------------------
99Please respect copyright.ＰＥＮＡＮＡgWXfV1GEjk
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---99Please respect copyright.ＰＥＮＡＮＡGw12v6PlLe
GET /zen.iso HTTP/1.199Please respect copyright.ＰＥＮＡＮＡBRF97wN9wB
Range: bytes=287718144-99Please respect copyright.ＰＥＮＡＮＡi8reMQDZSj
If-Range: "534231200_1732277116"99Please respect copyright.ＰＥＮＡＮＡPYA7PyiUy2
------------------------------------99Please respect copyright.ＰＥＮＡＮＡdgyPje4wMn
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:99Please respect copyright.ＰＥＮＡＮＡegyKWCT6BN
-------------range response-------------------99Please respect copyright.ＰＥＮＡＮＡSLPNRJGbaF
HTTP/1.1 206 Partial Content99Please respect copyright.ＰＥＮＡＮＡbKrxzIM8Xq
accept-ranges: bytes99Please respect copyright.ＰＥＮＡＮＡKI7QNwpN4R
content-range: bytes 287718144-534231199/53423120099Please respect copyright.ＰＥＮＡＮＡeZQoE1rsxS
ETag: "534231200_1732277116"99Please respect copyright.ＰＥＮＡＮＡhdiW2U0U4X
Content-Length: 24651305699Please respect copyright.ＰＥＮＡＮＡXgfPlVbYaT
Content-Type: application/octet-stream99Please respect copyright.ＰＥＮＡＮＡ7Fm1atqhAP
Content-Disposition: attachment; filename="zen.iso"
(http body is the remain file content)99Please respect copyright.ＰＥＮＡＮＡbSnEGPLSYR
-------------------------------------------
If the etag changes, response 200 from beginning like this:99Please respect copyright.ＰＥＮＡＮＡsINt5lLXxJ
-----------http response from the beginning------99Please respect copyright.ＰＥＮＡＮＡTyZsJZKuKI
HTTP/1.1 200 OK99Please respect copyright.ＰＥＮＡＮＡ87TuiIb7Qm
Date: Mon, 25 Nov 2024 10:56:12 GMT99Please respect copyright.ＰＥＮＡＮＡZaiEsadQdy
accept-ranges: bytes99Please respect copyright.ＰＥＮＡＮＡMb2qPwBNQO
ETag: "534231200_1732277116"99Please respect copyright.ＰＥＮＡＮＡVvSlzxXqIf
Content-Length: 53423120099Please respect copyright.ＰＥＮＡＮＡMvN1hS1oly
Content-Type: application/octet-stream99Please respect copyright.ＰＥＮＡＮＡrgsguWds7W
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)99Please respect copyright.ＰＥＮＡＮＡIFyz3uXC0k
---------------------------------------------------
99Please respect copyright.ＰＥＮＡＮＡ3kL6CFje88
Section X. Thanks
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/If-None-Match99Please respect copyright.ＰＥＮＡＮＡIJN1wpIof2
https://blog.csdn.net/catoop/article/details/13417465399Please respect copyright.ＰＥＮＡＮＡ8jAmye3cxr
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]99Please respect copyright.ＰＥＮＡＮＡmedL3rzN2t
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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