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.
159Please respect copyright.ＰＥＮＡＮＡqGNUxT9Mkw
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:159Please respect copyright.ＰＥＮＡＮＡlWfFkePCPg
---------------http response with etag----------------159Please respect copyright.ＰＥＮＡＮＡZ9tkFktVcP
HTTP/1.1 200 OK159Please respect copyright.ＰＥＮＡＮＡCK1mOngIx9
Date: Mon, 25 Nov 2024 10:56:12 GMT159Please respect copyright.ＰＥＮＡＮＡQrxripZu2U
accept-ranges: bytes159Please respect copyright.ＰＥＮＡＮＡ1595B6UdpB
ETag: "534231200_1732277116"159Please respect copyright.ＰＥＮＡＮＡ35LeIjaHjT
Content-Length: 534231200159Please respect copyright.ＰＥＮＡＮＡyk7h8mcAQ1
Content-Type: application/octet-stream159Please respect copyright.ＰＥＮＡＮＡKZnIay8nvP
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)159Please respect copyright.ＰＥＮＡＮＡzPslWePMVQ
-----------------------------------------------------
159Please respect copyright.ＰＥＮＡＮＡueEwQEifhk
Section B. if-none-match
The browser send back the etag to ask server if it has been changed by header like this:159Please respect copyright.ＰＥＮＡＮＡMEZSytnJsb
-----http request for a cached resource------159Please respect copyright.ＰＥＮＡＮＡzfmm1KhO2D
GET /zen.iso HTTP/1.1159Please respect copyright.ＰＥＮＡＮＡgFjDsMiUyJ
if-none-match: "534231200_1732277116"159Please respect copyright.ＰＥＮＡＮＡ1so3nXKaE6
---------------------------------------------
If the etag is unchanged, the server response like this:
1. for get and head requests:159Please respect copyright.ＰＥＮＡＮＡ9KtEype7oE
-----------http response for unchanged resource------159Please respect copyright.ＰＥＮＡＮＡl605iYLI9f
HTTP/1.1 304 Not Modified
(http body canbe empty, the client will used its cache)159Please respect copyright.ＰＥＮＡＮＡnd0SWddJPf
----------------------------------------------------
2. for other methods requests:159Please respect copyright.ＰＥＮＡＮＡAq1lPjGaq6
-----------http response for unchanged resource------159Please respect copyright.ＰＥＮＡＮＡwzmBfVB0TM
HTTP/1.1 412 Precondition Failed
(http body canbe empty, the client will used its cache)159Please respect copyright.ＰＥＮＡＮＡ4jx4BUajms
----------------------------------------------------
159Please respect copyright.ＰＥＮＡＮＡJATLKVC2i8
If the etag changes, the server response 200 OK with the new file content like this:159Please respect copyright.ＰＥＮＡＮＡ9QyiD9ZxVR
---------------http response with etag----------------159Please respect copyright.ＰＥＮＡＮＡdOiNa41ZC2
HTTP/1.1 200 OK159Please respect copyright.ＰＥＮＡＮＡroOHQh7v2g
Date: Mon, 25 Nov 2024 10:56:12 GMT159Please respect copyright.ＰＥＮＡＮＡhk8GEWAXCi
accept-ranges: bytes159Please respect copyright.ＰＥＮＡＮＡW7PkGLriWA
ETag: "534231200_1732277116"159Please respect copyright.ＰＥＮＡＮＡl9izYR4qUM
Content-Length: 534231200159Please respect copyright.ＰＥＮＡＮＡx24CllZNGK
Content-Type: application/octet-stream159Please respect copyright.ＰＥＮＡＮＡrC4lBCo6rU
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)159Please respect copyright.ＰＥＮＡＮＡHnkqpMKLgJ
-----------------------------------------------------
159Please respect copyright.ＰＥＮＡＮＡffaA10aOge
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:159Please respect copyright.ＰＥＮＡＮＡQrCtuiWq6G
------resume download request-------159Please respect copyright.ＰＥＮＡＮＡDTefSt18X7
GET /zen.iso HTTP/1.1159Please respect copyright.ＰＥＮＡＮＡwCr29ty3yI
Range: bytes=287718144-159Please respect copyright.ＰＥＮＡＮＡfn2s7dBiRZ
If-Match: "534231200_1732277116"159Please respect copyright.ＰＥＮＡＮＡ4TaGIn2BP2
-----------------------------------159Please respect copyright.ＰＥＮＡＮＡDmutJ01xsB
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:159Please respect copyright.ＰＥＮＡＮＡ8JqE9tenJP
-------------range response-------------------159Please respect copyright.ＰＥＮＡＮＡ0t7Fps2Cy5
HTTP/1.1 206 Partial Content159Please respect copyright.ＰＥＮＡＮＡY1cBGpW56F
accept-ranges: bytes159Please respect copyright.ＰＥＮＡＮＡRpBVdSGxLi
content-range: bytes 287718144-534231199/534231200159Please respect copyright.ＰＥＮＡＮＡ5mooIKLpdd
ETag: "534231200_1732277116"159Please respect copyright.ＰＥＮＡＮＡ0UNqKG1Txu
Content-Length: 246513056159Please respect copyright.ＰＥＮＡＮＡ1XcMZpJQ5y
Content-Type: application/octet-stream159Please respect copyright.ＰＥＮＡＮＡZjqLSSgWnv
Content-Disposition: attachment; filename="zen.iso"
(http body is the remain file content)159Please respect copyright.ＰＥＮＡＮＡy7K8I35ef6
-------------------------------------------
If the etag changes, response 412 like this:159Please respect copyright.ＰＥＮＡＮＡkm1MpbuR72
-----------http response for changed resource------159Please respect copyright.ＰＥＮＡＮＡLJqPdeM4sq
HTTP/1.1 412 Precondition Failed
(http body canbe empty, the client will stop downloading the resumed file)159Please respect copyright.ＰＥＮＡＮＡ7yY4Zm2SHv
---------------------------------------------------
159Please respect copyright.ＰＥＮＡＮＡO48ZyLdTWA
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---159Please respect copyright.ＰＥＮＡＮＡRzV0L1r1Yg
GET /zen.iso HTTP/1.1159Please respect copyright.ＰＥＮＡＮＡ9UUXG48APG
Range: bytes=287718144-159Please respect copyright.ＰＥＮＡＮＡkni7Z4LrKz
If-Range: "534231200_1732277116"159Please respect copyright.ＰＥＮＡＮＡPUv829xZh5
------------------------------------159Please respect copyright.ＰＥＮＡＮＡwMAqmeLVd7
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:159Please respect copyright.ＰＥＮＡＮＡ3ng1L8CMSD
-------------range response-------------------159Please respect copyright.ＰＥＮＡＮＡiHP3TNkvid
HTTP/1.1 206 Partial Content159Please respect copyright.ＰＥＮＡＮＡVhJp1VTe0j
accept-ranges: bytes159Please respect copyright.ＰＥＮＡＮＡBhajyVAtYq
content-range: bytes 287718144-534231199/534231200159Please respect copyright.ＰＥＮＡＮＡQVcJZTuF6w
ETag: "534231200_1732277116"159Please respect copyright.ＰＥＮＡＮＡMvEqzhLRmC
Content-Length: 246513056159Please respect copyright.ＰＥＮＡＮＡcfEXtgM18g
Content-Type: application/octet-stream159Please respect copyright.ＰＥＮＡＮＡ6yySMYS4Mm
Content-Disposition: attachment; filename="zen.iso"
(http body is the remain file content)159Please respect copyright.ＰＥＮＡＮＡZL3OgDPr8W
-------------------------------------------
If the etag changes, response 200 from beginning like this:159Please respect copyright.ＰＥＮＡＮＡ8WEDTzbEqM
-----------http response from the beginning------159Please respect copyright.ＰＥＮＡＮＡ8iJTfoDN6p
HTTP/1.1 200 OK159Please respect copyright.ＰＥＮＡＮＡQuMxHBCoJ3
Date: Mon, 25 Nov 2024 10:56:12 GMT159Please respect copyright.ＰＥＮＡＮＡiEN4jb56gu
accept-ranges: bytes159Please respect copyright.ＰＥＮＡＮＡh2DCT7SNtQ
ETag: "534231200_1732277116"159Please respect copyright.ＰＥＮＡＮＡlLhsAv3NOV
Content-Length: 534231200159Please respect copyright.ＰＥＮＡＮＡjkmPrbYwTZ
Content-Type: application/octet-stream159Please respect copyright.ＰＥＮＡＮＡieegDKZztd
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)159Please respect copyright.ＰＥＮＡＮＡYnFa6w1y82
---------------------------------------------------
159Please respect copyright.ＰＥＮＡＮＡM0EXgGHrEH
Section X. Thanks
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/If-None-Match159Please respect copyright.ＰＥＮＡＮＡ5dsF5dnBBM
https://blog.csdn.net/catoop/article/details/134174653159Please respect copyright.ＰＥＮＡＮＡj6XUJOADNi
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]159Please respect copyright.ＰＥＮＡＮＡSsV57Eajxo
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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