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.
227Please respect copyright.ＰＥＮＡＮＡvEOQNhdNTE
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:227Please respect copyright.ＰＥＮＡＮＡenigiCnJss
---------------http response with etag----------------227Please respect copyright.ＰＥＮＡＮＡiQ25YGmZYY
HTTP/1.1 200 OK227Please respect copyright.ＰＥＮＡＮＡV66Za0B1mD
Date: Mon, 25 Nov 2024 10:56:12 GMT227Please respect copyright.ＰＥＮＡＮＡKjss0RRGF5
accept-ranges: bytes227Please respect copyright.ＰＥＮＡＮＡ44eUUAaggT
ETag: "534231200_1732277116"227Please respect copyright.ＰＥＮＡＮＡMbrzFonnST
Content-Length: 534231200227Please respect copyright.ＰＥＮＡＮＡsg6BgeJfkd
Content-Type: application/octet-stream227Please respect copyright.ＰＥＮＡＮＡqPd4hAJIye
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)227Please respect copyright.ＰＥＮＡＮＡvK3FkmmLoC
-----------------------------------------------------
227Please respect copyright.ＰＥＮＡＮＡOygqZ4AC19
Section B. if-none-match
The browser send back the etag to ask server if it has been changed by header like this:227Please respect copyright.ＰＥＮＡＮＡxdgSnJXwZO
-----http request for a cached resource------227Please respect copyright.ＰＥＮＡＮＡhnsxroAg0m
GET /zen.iso HTTP/1.1227Please respect copyright.ＰＥＮＡＮＡ0nUfEYPlxA
if-none-match: "534231200_1732277116"227Please respect copyright.ＰＥＮＡＮＡibSrLUmYRa
---------------------------------------------
If the etag is unchanged, the server response like this:
1. for get and head requests:227Please respect copyright.ＰＥＮＡＮＡjvzzaac3oJ
-----------http response for unchanged resource------227Please respect copyright.ＰＥＮＡＮＡrcPWdUC4li
HTTP/1.1 304 Not Modified
(http body canbe empty, the client will used its cache)227Please respect copyright.ＰＥＮＡＮＡ8O0lCXCkKb
----------------------------------------------------
2. for other methods requests:227Please respect copyright.ＰＥＮＡＮＡaU5Njcabgd
-----------http response for unchanged resource------227Please respect copyright.ＰＥＮＡＮＡpKBh7Vwyna
HTTP/1.1 412 Precondition Failed
(http body canbe empty, the client will used its cache)227Please respect copyright.ＰＥＮＡＮＡIGNNQ2JCWu
----------------------------------------------------
227Please respect copyright.ＰＥＮＡＮＡNfNAHFnhaz
If the etag changes, the server response 200 OK with the new file content like this:227Please respect copyright.ＰＥＮＡＮＡ8rhuGRAXel
---------------http response with etag----------------227Please respect copyright.ＰＥＮＡＮＡhHSEZFvkeC
HTTP/1.1 200 OK227Please respect copyright.ＰＥＮＡＮＡK6UFBSmIRy
Date: Mon, 25 Nov 2024 10:56:12 GMT227Please respect copyright.ＰＥＮＡＮＡX9zeoR6D20
accept-ranges: bytes227Please respect copyright.ＰＥＮＡＮＡjdfdTBGjU1
ETag: "534231200_1732277116"227Please respect copyright.ＰＥＮＡＮＡq6A7zJkt9l
Content-Length: 534231200227Please respect copyright.ＰＥＮＡＮＡCV1zrSck9q
Content-Type: application/octet-stream227Please respect copyright.ＰＥＮＡＮＡfl8OcMrTxL
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)227Please respect copyright.ＰＥＮＡＮＡBxEHzGnZ3y
-----------------------------------------------------
227Please respect copyright.ＰＥＮＡＮＡRnOMv0hVh9
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:227Please respect copyright.ＰＥＮＡＮＡTViNVGBQxj
------resume download request-------227Please respect copyright.ＰＥＮＡＮＡNyzk9U5U1d
GET /zen.iso HTTP/1.1227Please respect copyright.ＰＥＮＡＮＡYqJDH23JfR
Range: bytes=287718144-227Please respect copyright.ＰＥＮＡＮＡu2Yxn7Gqbg
If-Match: "534231200_1732277116"227Please respect copyright.ＰＥＮＡＮＡLW3bSljmE9
-----------------------------------227Please respect copyright.ＰＥＮＡＮＡ0ybNmSh88F
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:227Please respect copyright.ＰＥＮＡＮＡxqymiRx4kK
-------------range response-------------------227Please respect copyright.ＰＥＮＡＮＡW8oalQzrnx
HTTP/1.1 206 Partial Content227Please respect copyright.ＰＥＮＡＮＡcDVT7wbZkI
accept-ranges: bytes227Please respect copyright.ＰＥＮＡＮＡnpHcpEWVK9
content-range: bytes 287718144-534231199/534231200227Please respect copyright.ＰＥＮＡＮＡBmgdib9FJ4
ETag: "534231200_1732277116"227Please respect copyright.ＰＥＮＡＮＡcn0jG03Gch
Content-Length: 246513056227Please respect copyright.ＰＥＮＡＮＡvUnMifdkLu
Content-Type: application/octet-stream227Please respect copyright.ＰＥＮＡＮＡGISzlaEkHs
Content-Disposition: attachment; filename="zen.iso"
(http body is the remain file content)227Please respect copyright.ＰＥＮＡＮＡgxtEFlxhwG
-------------------------------------------
If the etag changes, response 412 like this:227Please respect copyright.ＰＥＮＡＮＡS2O3HojbPJ
-----------http response for changed resource------227Please respect copyright.ＰＥＮＡＮＡvj7gqhCKuA
HTTP/1.1 412 Precondition Failed
(http body canbe empty, the client will stop downloading the resumed file)227Please respect copyright.ＰＥＮＡＮＡbflqXhTqlO
---------------------------------------------------
227Please respect copyright.ＰＥＮＡＮＡ7yoeDQYqam
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---227Please respect copyright.ＰＥＮＡＮＡxNAI3r0rdI
GET /zen.iso HTTP/1.1227Please respect copyright.ＰＥＮＡＮＡDMKsn1611o
Range: bytes=287718144-227Please respect copyright.ＰＥＮＡＮＡpWbCW1WrP5
If-Range: "534231200_1732277116"227Please respect copyright.ＰＥＮＡＮＡ4gP1J6lKWP
------------------------------------227Please respect copyright.ＰＥＮＡＮＡ6pMzMhYg87
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:227Please respect copyright.ＰＥＮＡＮＡTIX4h0ySs3
-------------range response-------------------227Please respect copyright.ＰＥＮＡＮＡkkoqq54xx8
HTTP/1.1 206 Partial Content227Please respect copyright.ＰＥＮＡＮＡtzYHbDyRKL
accept-ranges: bytes227Please respect copyright.ＰＥＮＡＮＡFMXgo2dSEs
content-range: bytes 287718144-534231199/534231200227Please respect copyright.ＰＥＮＡＮＡtIpBJuvyvX
ETag: "534231200_1732277116"227Please respect copyright.ＰＥＮＡＮＡOx0uj9Q1FK
Content-Length: 246513056227Please respect copyright.ＰＥＮＡＮＡADwdxJdfoy
Content-Type: application/octet-stream227Please respect copyright.ＰＥＮＡＮＡTVepLWKKYM
Content-Disposition: attachment; filename="zen.iso"
(http body is the remain file content)227Please respect copyright.ＰＥＮＡＮＡSJjyOoHGCy
-------------------------------------------
If the etag changes, response 200 from beginning like this:227Please respect copyright.ＰＥＮＡＮＡIZosYIPWDS
-----------http response from the beginning------227Please respect copyright.ＰＥＮＡＮＡURuAYHNwSU
HTTP/1.1 200 OK227Please respect copyright.ＰＥＮＡＮＡrRSkQK0XFh
Date: Mon, 25 Nov 2024 10:56:12 GMT227Please respect copyright.ＰＥＮＡＮＡr9gBJ3vZzu
accept-ranges: bytes227Please respect copyright.ＰＥＮＡＮＡh9IPfXEqhH
ETag: "534231200_1732277116"227Please respect copyright.ＰＥＮＡＮＡiSTKZGhi7c
Content-Length: 534231200227Please respect copyright.ＰＥＮＡＮＡbWRBRmD9LB
Content-Type: application/octet-stream227Please respect copyright.ＰＥＮＡＮＡ8ArtsvpJEr
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)227Please respect copyright.ＰＥＮＡＮＡuLWIG4Gulw
---------------------------------------------------
227Please respect copyright.ＰＥＮＡＮＡQnSphnvEdK
Section X. Thanks
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/If-None-Match227Please respect copyright.ＰＥＮＡＮＡ3NwJwTVsEy
https://blog.csdn.net/catoop/article/details/134174653227Please respect copyright.ＰＥＮＡＮＡwcjEtM6ywm
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]227Please respect copyright.ＰＥＮＡＮＡegLowsLpqc
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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