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.
80Please respect copyright.ＰＥＮＡＮＡlj9gdoeYMH
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:80Please respect copyright.ＰＥＮＡＮＡ7sblm6uq4o
---------------http response with etag----------------80Please respect copyright.ＰＥＮＡＮＡe3dleTRSW0
HTTP/1.1 200 OK80Please respect copyright.ＰＥＮＡＮＡ3djUziIn51
Date: Mon, 25 Nov 2024 10:56:12 GMT80Please respect copyright.ＰＥＮＡＮＡ0E9QCQ4b2o
accept-ranges: bytes80Please respect copyright.ＰＥＮＡＮＡCawrtFkN1h
ETag: "534231200_1732277116"80Please respect copyright.ＰＥＮＡＮＡmyrNVM2Qug
Content-Length: 53423120080Please respect copyright.ＰＥＮＡＮＡF58ErqMeby
Content-Type: application/octet-stream80Please respect copyright.ＰＥＮＡＮＡaiDyPHKA6N
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)80Please respect copyright.ＰＥＮＡＮＡVVvkUJClGL
-----------------------------------------------------
80Please respect copyright.ＰＥＮＡＮＡBh4DMkaezA
Section B. if-none-match
The browser send back the etag to ask server if it has been changed by header like this:80Please respect copyright.ＰＥＮＡＮＡPQd4v5Kiap
-----http request for a cached resource------80Please respect copyright.ＰＥＮＡＮＡKB7335pHJP
GET /zen.iso HTTP/1.180Please respect copyright.ＰＥＮＡＮＡYOpBG1LlCF
if-none-match: "534231200_1732277116"80Please respect copyright.ＰＥＮＡＮＡK3taRcupGz
---------------------------------------------
If the etag is unchanged, the server response like this:
1. for get and head requests:80Please respect copyright.ＰＥＮＡＮＡqMiItYiZzl
-----------http response for unchanged resource------80Please respect copyright.ＰＥＮＡＮＡffR4FZpNGZ
HTTP/1.1 304 Not Modified
(http body canbe empty, the client will used its cache)80Please respect copyright.ＰＥＮＡＮＡP0JXN2APPe
----------------------------------------------------
2. for other methods requests:80Please respect copyright.ＰＥＮＡＮＡHAUjNhkHN2
-----------http response for unchanged resource------80Please respect copyright.ＰＥＮＡＮＡK0ixJOoxEJ
HTTP/1.1 412 Precondition Failed
(http body canbe empty, the client will used its cache)80Please respect copyright.ＰＥＮＡＮＡ2dgyRKOZVr
----------------------------------------------------
80Please respect copyright.ＰＥＮＡＮＡ5Lm46fF2lr
If the etag changes, the server response 200 OK with the new file content like this:80Please respect copyright.ＰＥＮＡＮＡFIgSks9UbW
---------------http response with etag----------------80Please respect copyright.ＰＥＮＡＮＡu6D0WHypYX
HTTP/1.1 200 OK80Please respect copyright.ＰＥＮＡＮＡhzrL7o3iRN
Date: Mon, 25 Nov 2024 10:56:12 GMT80Please respect copyright.ＰＥＮＡＮＡuP5Kb1C9ZF
accept-ranges: bytes80Please respect copyright.ＰＥＮＡＮＡLztlClCr0E
ETag: "534231200_1732277116"80Please respect copyright.ＰＥＮＡＮＡYurOKkqDUi
Content-Length: 53423120080Please respect copyright.ＰＥＮＡＮＡZsL59xNWEb
Content-Type: application/octet-stream80Please respect copyright.ＰＥＮＡＮＡ1HNVXrjfHR
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)80Please respect copyright.ＰＥＮＡＮＡ1wA5OYPYsf
-----------------------------------------------------
80Please respect copyright.ＰＥＮＡＮＡTQ65hKtaVQ
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:80Please respect copyright.ＰＥＮＡＮＡYTCwHwBo9Q
------resume download request-------80Please respect copyright.ＰＥＮＡＮＡU8XR3O0qen
GET /zen.iso HTTP/1.180Please respect copyright.ＰＥＮＡＮＡru3BZBNOSu
Range: bytes=287718144-80Please respect copyright.ＰＥＮＡＮＡH6KjnyEivJ
If-Match: "534231200_1732277116"80Please respect copyright.ＰＥＮＡＮＡ9NGTvYkjua
-----------------------------------80Please respect copyright.ＰＥＮＡＮＡ4gt1PyBIT0
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:80Please respect copyright.ＰＥＮＡＮＡvJtAnWo2BM
-------------range response-------------------80Please respect copyright.ＰＥＮＡＮＡ8LdrE5OCrx
HTTP/1.1 206 Partial Content80Please respect copyright.ＰＥＮＡＮＡggMe4TCWsA
accept-ranges: bytes80Please respect copyright.ＰＥＮＡＮＡaSws0nzGGv
content-range: bytes 287718144-534231199/53423120080Please respect copyright.ＰＥＮＡＮＡp0wVLfWRIx
ETag: "534231200_1732277116"80Please respect copyright.ＰＥＮＡＮＡ9dtCpos1w8
Content-Length: 24651305680Please respect copyright.ＰＥＮＡＮＡIMSBEt1XUe
Content-Type: application/octet-stream80Please respect copyright.ＰＥＮＡＮＡPxFwMlSOA7
Content-Disposition: attachment; filename="zen.iso"
(http body is the remain file content)80Please respect copyright.ＰＥＮＡＮＡ2PK4vSQd7Y
-------------------------------------------
If the etag changes, response 412 like this:80Please respect copyright.ＰＥＮＡＮＡ6RYa4eYqyv
-----------http response for changed resource------80Please respect copyright.ＰＥＮＡＮＡEjr8T1ih3Y
HTTP/1.1 412 Precondition Failed
(http body canbe empty, the client will stop downloading the resumed file)80Please respect copyright.ＰＥＮＡＮＡrkyVvFbrog
---------------------------------------------------
80Please respect copyright.ＰＥＮＡＮＡvrMrPtgJAY
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---80Please respect copyright.ＰＥＮＡＮＡqku9LdTZzZ
GET /zen.iso HTTP/1.180Please respect copyright.ＰＥＮＡＮＡT3k3pc3yPh
Range: bytes=287718144-80Please respect copyright.ＰＥＮＡＮＡIqQ4KiX3TY
If-Range: "534231200_1732277116"80Please respect copyright.ＰＥＮＡＮＡngMSKAgCLm
------------------------------------80Please respect copyright.ＰＥＮＡＮＡFJdlami9ve
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:80Please respect copyright.ＰＥＮＡＮＡTaSJzpAphA
-------------range response-------------------80Please respect copyright.ＰＥＮＡＮＡFFqas9Wl4d
HTTP/1.1 206 Partial Content80Please respect copyright.ＰＥＮＡＮＡW91sIJp8uq
accept-ranges: bytes80Please respect copyright.ＰＥＮＡＮＡGAYYZlrbTb
content-range: bytes 287718144-534231199/53423120080Please respect copyright.ＰＥＮＡＮＡEVnH7QRWJg
ETag: "534231200_1732277116"80Please respect copyright.ＰＥＮＡＮＡLjVxaDwyJr
Content-Length: 24651305680Please respect copyright.ＰＥＮＡＮＡlpNak99mJ3
Content-Type: application/octet-stream80Please respect copyright.ＰＥＮＡＮＡbEGxChNvR3
Content-Disposition: attachment; filename="zen.iso"
(http body is the remain file content)80Please respect copyright.ＰＥＮＡＮＡyjBcotC701
-------------------------------------------
If the etag changes, response 200 from beginning like this:80Please respect copyright.ＰＥＮＡＮＡgghbPf7xhK
-----------http response from the beginning------80Please respect copyright.ＰＥＮＡＮＡQpkX4ka9Nn
HTTP/1.1 200 OK80Please respect copyright.ＰＥＮＡＮＡBenqcEQiem
Date: Mon, 25 Nov 2024 10:56:12 GMT80Please respect copyright.ＰＥＮＡＮＡbZfwSpZGV1
accept-ranges: bytes80Please respect copyright.ＰＥＮＡＮＡVFgIEoXpQv
ETag: "534231200_1732277116"80Please respect copyright.ＰＥＮＡＮＡpEBMAo41PY
Content-Length: 53423120080Please respect copyright.ＰＥＮＡＮＡp1TDdvWzyi
Content-Type: application/octet-stream80Please respect copyright.ＰＥＮＡＮＡhPWV7Ko2gW
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)80Please respect copyright.ＰＥＮＡＮＡ6y0wr35ygx
---------------------------------------------------
80Please respect copyright.ＰＥＮＡＮＡoCYtWEAOmY
Section X. Thanks
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/If-None-Match80Please respect copyright.ＰＥＮＡＮＡaSHKHGqbmP
https://blog.csdn.net/catoop/article/details/13417465380Please respect copyright.ＰＥＮＡＮＡIK8iR6u6L1
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]80Please respect copyright.ＰＥＮＡＮＡDJowpOaWRq
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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