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.
132Please respect copyright.ＰＥＮＡＮＡ19GtUHqJUp
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:132Please respect copyright.ＰＥＮＡＮＡRTLHHN0oe1
---------------http response with etag----------------132Please respect copyright.ＰＥＮＡＮＡq73vGxFvfs
HTTP/1.1 200 OK132Please respect copyright.ＰＥＮＡＮＡJ6xPM7JUVk
Date: Mon, 25 Nov 2024 10:56:12 GMT132Please respect copyright.ＰＥＮＡＮＡCVITZdoQWK
accept-ranges: bytes132Please respect copyright.ＰＥＮＡＮＡDlPaMbF9jK
ETag: "534231200_1732277116"132Please respect copyright.ＰＥＮＡＮＡluK23cQmZp
Content-Length: 534231200132Please respect copyright.ＰＥＮＡＮＡD4euo3UyMr
Content-Type: application/octet-stream132Please respect copyright.ＰＥＮＡＮＡWUY9IdV2MS
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)132Please respect copyright.ＰＥＮＡＮＡSK8An9i3SH
-----------------------------------------------------
132Please respect copyright.ＰＥＮＡＮＡTMe6foDdvm
Section B. if-none-match
The browser send back the etag to ask server if it has been changed by header like this:132Please respect copyright.ＰＥＮＡＮＡepDPp1q8Sf
-----http request for a cached resource------132Please respect copyright.ＰＥＮＡＮＡrZYC1qgWua
GET /zen.iso HTTP/1.1132Please respect copyright.ＰＥＮＡＮＡDkIBjLA8Wb
if-none-match: "534231200_1732277116"132Please respect copyright.ＰＥＮＡＮＡlHrIDdLrRp
---------------------------------------------
If the etag is unchanged, the server response like this:
1. for get and head requests:132Please respect copyright.ＰＥＮＡＮＡ1wWxONDlrJ
-----------http response for unchanged resource------132Please respect copyright.ＰＥＮＡＮＡd8UCKn9M8V
HTTP/1.1 304 Not Modified
(http body canbe empty, the client will used its cache)132Please respect copyright.ＰＥＮＡＮＡIkTvV3CHAq
----------------------------------------------------
2. for other methods requests:132Please respect copyright.ＰＥＮＡＮＡLFlqRqM3Gl
-----------http response for unchanged resource------132Please respect copyright.ＰＥＮＡＮＡObhErTNJn8
HTTP/1.1 412 Precondition Failed
(http body canbe empty, the client will used its cache)132Please respect copyright.ＰＥＮＡＮＡX30n9g0pRK
----------------------------------------------------
132Please respect copyright.ＰＥＮＡＮＡScKPHdTtCD
If the etag changes, the server response 200 OK with the new file content like this:132Please respect copyright.ＰＥＮＡＮＡx8Qi9Q1f3Q
---------------http response with etag----------------132Please respect copyright.ＰＥＮＡＮＡfIfGJDkNdJ
HTTP/1.1 200 OK132Please respect copyright.ＰＥＮＡＮＡS6TNy2PGbu
Date: Mon, 25 Nov 2024 10:56:12 GMT132Please respect copyright.ＰＥＮＡＮＡ6bCxI5s0DI
accept-ranges: bytes132Please respect copyright.ＰＥＮＡＮＡjTVDdeSTLP
ETag: "534231200_1732277116"132Please respect copyright.ＰＥＮＡＮＡX6Yf2Q91k6
Content-Length: 534231200132Please respect copyright.ＰＥＮＡＮＡ4BCdZmOyDd
Content-Type: application/octet-stream132Please respect copyright.ＰＥＮＡＮＡOmw8Bhln6O
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)132Please respect copyright.ＰＥＮＡＮＡwZe73d4fxr
-----------------------------------------------------
132Please respect copyright.ＰＥＮＡＮＡAytaYKHfLd
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:132Please respect copyright.ＰＥＮＡＮＡD2Mf2vnSet
------resume download request-------132Please respect copyright.ＰＥＮＡＮＡdcwLeyZKGa
GET /zen.iso HTTP/1.1132Please respect copyright.ＰＥＮＡＮＡsMwWc809eK
Range: bytes=287718144-132Please respect copyright.ＰＥＮＡＮＡ2kyxAMWjwQ
If-Match: "534231200_1732277116"132Please respect copyright.ＰＥＮＡＮＡKvITMesCIC
-----------------------------------132Please respect copyright.ＰＥＮＡＮＡtjiJ7DBdS1
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:132Please respect copyright.ＰＥＮＡＮＡdVaiR7uBv2
-------------range response-------------------132Please respect copyright.ＰＥＮＡＮＡkfacsQQJIz
HTTP/1.1 206 Partial Content132Please respect copyright.ＰＥＮＡＮＡtX8jStBnNx
accept-ranges: bytes132Please respect copyright.ＰＥＮＡＮＡ2OwQH9jOeE
content-range: bytes 287718144-534231199/534231200132Please respect copyright.ＰＥＮＡＮＡ1onwAlFBNu
ETag: "534231200_1732277116"132Please respect copyright.ＰＥＮＡＮＡT6vkU2YOd8
Content-Length: 246513056132Please respect copyright.ＰＥＮＡＮＡkksMhKADnj
Content-Type: application/octet-stream132Please respect copyright.ＰＥＮＡＮＡlYC2jAQZAR
Content-Disposition: attachment; filename="zen.iso"
(http body is the remain file content)132Please respect copyright.ＰＥＮＡＮＡjGYlyhreZI
-------------------------------------------
If the etag changes, response 412 like this:132Please respect copyright.ＰＥＮＡＮＡmp59AGtLPg
-----------http response for changed resource------132Please respect copyright.ＰＥＮＡＮＡujYWBbvx6M
HTTP/1.1 412 Precondition Failed
(http body canbe empty, the client will stop downloading the resumed file)132Please respect copyright.ＰＥＮＡＮＡE5b0QkrtaM
---------------------------------------------------
132Please respect copyright.ＰＥＮＡＮＡZa05c3TFgz
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---132Please respect copyright.ＰＥＮＡＮＡS50IAe6E3P
GET /zen.iso HTTP/1.1132Please respect copyright.ＰＥＮＡＮＡvHJbWYzM9z
Range: bytes=287718144-132Please respect copyright.ＰＥＮＡＮＡor3KKbNUa2
If-Range: "534231200_1732277116"132Please respect copyright.ＰＥＮＡＮＡhh9Lz9vH9R
------------------------------------132Please respect copyright.ＰＥＮＡＮＡpUFTBVzVra
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:132Please respect copyright.ＰＥＮＡＮＡ7nQ2op6LH7
-------------range response-------------------132Please respect copyright.ＰＥＮＡＮＡpt6R9dhNeO
HTTP/1.1 206 Partial Content132Please respect copyright.ＰＥＮＡＮＡ643HEm9Fo9
accept-ranges: bytes132Please respect copyright.ＰＥＮＡＮＡZ8W7I2vOrz
content-range: bytes 287718144-534231199/534231200132Please respect copyright.ＰＥＮＡＮＡH4eUIEhcVI
ETag: "534231200_1732277116"132Please respect copyright.ＰＥＮＡＮＡFThKPdtrg2
Content-Length: 246513056132Please respect copyright.ＰＥＮＡＮＡO9mW084MC9
Content-Type: application/octet-stream132Please respect copyright.ＰＥＮＡＮＡB315m3at1W
Content-Disposition: attachment; filename="zen.iso"
(http body is the remain file content)132Please respect copyright.ＰＥＮＡＮＡWts10W8sUP
-------------------------------------------
If the etag changes, response 200 from beginning like this:132Please respect copyright.ＰＥＮＡＮＡTeAmo0vgmN
-----------http response from the beginning------132Please respect copyright.ＰＥＮＡＮＡCWNafixt1T
HTTP/1.1 200 OK132Please respect copyright.ＰＥＮＡＮＡESXApOLjj0
Date: Mon, 25 Nov 2024 10:56:12 GMT132Please respect copyright.ＰＥＮＡＮＡAz9eAdGRRs
accept-ranges: bytes132Please respect copyright.ＰＥＮＡＮＡrUE2fKwMYO
ETag: "534231200_1732277116"132Please respect copyright.ＰＥＮＡＮＡ4Ysg7oz5Mt
Content-Length: 534231200132Please respect copyright.ＰＥＮＡＮＡmVzhxwXng1
Content-Type: application/octet-stream132Please respect copyright.ＰＥＮＡＮＡrS6jnUU2R2
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)132Please respect copyright.ＰＥＮＡＮＡfo9kYoV7i6
---------------------------------------------------
132Please respect copyright.ＰＥＮＡＮＡ6Y43Keiefo
Section X. Thanks
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/If-None-Match132Please respect copyright.ＰＥＮＡＮＡWvIULPimtj
https://blog.csdn.net/catoop/article/details/134174653132Please respect copyright.ＰＥＮＡＮＡXrqm3ACCuP
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]132Please respect copyright.ＰＥＮＡＮＡFyc0tnBamC
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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