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.
166Please respect copyright.ＰＥＮＡＮＡ3axtTLHjJA
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:166Please respect copyright.ＰＥＮＡＮＡewZCX64hgj
---------------http response with etag----------------166Please respect copyright.ＰＥＮＡＮＡvGqgYJvW6Y
HTTP/1.1 200 OK166Please respect copyright.ＰＥＮＡＮＡI4x25Veq0O
Date: Mon, 25 Nov 2024 10:56:12 GMT166Please respect copyright.ＰＥＮＡＮＡcxszZYohIC
accept-ranges: bytes166Please respect copyright.ＰＥＮＡＮＡ4KeDeaGS7p
ETag: "534231200_1732277116"166Please respect copyright.ＰＥＮＡＮＡOsRvAphI2x
Content-Length: 534231200166Please respect copyright.ＰＥＮＡＮＡkD3cWJUbYV
Content-Type: application/octet-stream166Please respect copyright.ＰＥＮＡＮＡh4Tt2XGFug
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)166Please respect copyright.ＰＥＮＡＮＡhRWzxoAnJ2
-----------------------------------------------------
166Please respect copyright.ＰＥＮＡＮＡG7ACioo9Cb
Section B. if-none-match
The browser send back the etag to ask server if it has been changed by header like this:166Please respect copyright.ＰＥＮＡＮＡu7lNzXWCfo
-----http request for a cached resource------166Please respect copyright.ＰＥＮＡＮＡp0KcPm8lAl
GET /zen.iso HTTP/1.1166Please respect copyright.ＰＥＮＡＮＡQccAv2gsDu
if-none-match: "534231200_1732277116"166Please respect copyright.ＰＥＮＡＮＡ2Q0GnT2Rgb
---------------------------------------------
If the etag is unchanged, the server response like this:
1. for get and head requests:166Please respect copyright.ＰＥＮＡＮＡ3z5jwhR0l2
-----------http response for unchanged resource------166Please respect copyright.ＰＥＮＡＮＡu3FHC90oSq
HTTP/1.1 304 Not Modified
(http body canbe empty, the client will used its cache)166Please respect copyright.ＰＥＮＡＮＡe1YOLVH1t2
----------------------------------------------------
2. for other methods requests:166Please respect copyright.ＰＥＮＡＮＡ82Bx4HzoLm
-----------http response for unchanged resource------166Please respect copyright.ＰＥＮＡＮＡH9YOzWljXo
HTTP/1.1 412 Precondition Failed
(http body canbe empty, the client will used its cache)166Please respect copyright.ＰＥＮＡＮＡdJTAbecT9y
----------------------------------------------------
166Please respect copyright.ＰＥＮＡＮＡ70gnUNV7Qv
If the etag changes, the server response 200 OK with the new file content like this:166Please respect copyright.ＰＥＮＡＮＡZBBnxErtmH
---------------http response with etag----------------166Please respect copyright.ＰＥＮＡＮＡiunMlJZNl3
HTTP/1.1 200 OK166Please respect copyright.ＰＥＮＡＮＡa1CnrEeinw
Date: Mon, 25 Nov 2024 10:56:12 GMT166Please respect copyright.ＰＥＮＡＮＡv3tuLBQaTE
accept-ranges: bytes166Please respect copyright.ＰＥＮＡＮＡ0wTBej65eo
ETag: "534231200_1732277116"166Please respect copyright.ＰＥＮＡＮＡ3VBdloJzyb
Content-Length: 534231200166Please respect copyright.ＰＥＮＡＮＡady95HeDZq
Content-Type: application/octet-stream166Please respect copyright.ＰＥＮＡＮＡxmAyAivYG6
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)166Please respect copyright.ＰＥＮＡＮＡRZno4XO1H9
-----------------------------------------------------
166Please respect copyright.ＰＥＮＡＮＡAXDSbxd54f
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:166Please respect copyright.ＰＥＮＡＮＡ7MO2BZq6me
------resume download request-------166Please respect copyright.ＰＥＮＡＮＡLf1DUV5Wcq
GET /zen.iso HTTP/1.1166Please respect copyright.ＰＥＮＡＮＡvy9nMLB26Y
Range: bytes=287718144-166Please respect copyright.ＰＥＮＡＮＡnCMVl4BsSe
If-Match: "534231200_1732277116"166Please respect copyright.ＰＥＮＡＮＡjVFj5C8Mcn
-----------------------------------166Please respect copyright.ＰＥＮＡＮＡwNShBw78zb
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:166Please respect copyright.ＰＥＮＡＮＡNwg9GfssOH
-------------range response-------------------166Please respect copyright.ＰＥＮＡＮＡIS1L2Lf8jK
HTTP/1.1 206 Partial Content166Please respect copyright.ＰＥＮＡＮＡWX1ZNAbtNw
accept-ranges: bytes166Please respect copyright.ＰＥＮＡＮＡABxWm2DK5F
content-range: bytes 287718144-534231199/534231200166Please respect copyright.ＰＥＮＡＮＡ2tnebmFF7J
ETag: "534231200_1732277116"166Please respect copyright.ＰＥＮＡＮＡbaVoBqa9sU
Content-Length: 246513056166Please respect copyright.ＰＥＮＡＮＡ4gMSheEfWH
Content-Type: application/octet-stream166Please respect copyright.ＰＥＮＡＮＡmsMQma56rM
Content-Disposition: attachment; filename="zen.iso"
(http body is the remain file content)166Please respect copyright.ＰＥＮＡＮＡeKO9TNrSLG
-------------------------------------------
If the etag changes, response 412 like this:166Please respect copyright.ＰＥＮＡＮＡG0iYcKqvEf
-----------http response for changed resource------166Please respect copyright.ＰＥＮＡＮＡtqpByYcRH9
HTTP/1.1 412 Precondition Failed
(http body canbe empty, the client will stop downloading the resumed file)166Please respect copyright.ＰＥＮＡＮＡlIbBDDtQwL
---------------------------------------------------
166Please respect copyright.ＰＥＮＡＮＡPcoiV06HBZ
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---166Please respect copyright.ＰＥＮＡＮＡxCktESIBuy
GET /zen.iso HTTP/1.1166Please respect copyright.ＰＥＮＡＮＡ3wJA1EQFLR
Range: bytes=287718144-166Please respect copyright.ＰＥＮＡＮＡobRJvYZ4Yq
If-Range: "534231200_1732277116"166Please respect copyright.ＰＥＮＡＮＡZEaRarbVPK
------------------------------------166Please respect copyright.ＰＥＮＡＮＡ6SPLaye9hu
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:166Please respect copyright.ＰＥＮＡＮＡXVQAtGHJXc
-------------range response-------------------166Please respect copyright.ＰＥＮＡＮＡcvmQmgTLnv
HTTP/1.1 206 Partial Content166Please respect copyright.ＰＥＮＡＮＡU3eVThC4xE
accept-ranges: bytes166Please respect copyright.ＰＥＮＡＮＡM15NqxXcX2
content-range: bytes 287718144-534231199/534231200166Please respect copyright.ＰＥＮＡＮＡt8aGIsjTAL
ETag: "534231200_1732277116"166Please respect copyright.ＰＥＮＡＮＡNN1FLHEW7h
Content-Length: 246513056166Please respect copyright.ＰＥＮＡＮＡZun6nDiCwa
Content-Type: application/octet-stream166Please respect copyright.ＰＥＮＡＮＡAbtdIw67qR
Content-Disposition: attachment; filename="zen.iso"
(http body is the remain file content)166Please respect copyright.ＰＥＮＡＮＡMSc0JE7utq
-------------------------------------------
If the etag changes, response 200 from beginning like this:166Please respect copyright.ＰＥＮＡＮＡybZA7vuup3
-----------http response from the beginning------166Please respect copyright.ＰＥＮＡＮＡlAcd7YTWIl
HTTP/1.1 200 OK166Please respect copyright.ＰＥＮＡＮＡHUXsdMhnDZ
Date: Mon, 25 Nov 2024 10:56:12 GMT166Please respect copyright.ＰＥＮＡＮＡ07FTXreCic
accept-ranges: bytes166Please respect copyright.ＰＥＮＡＮＡ2U2D6AOdsl
ETag: "534231200_1732277116"166Please respect copyright.ＰＥＮＡＮＡJM5fvF28Jn
Content-Length: 534231200166Please respect copyright.ＰＥＮＡＮＡZGGBL7NT1G
Content-Type: application/octet-stream166Please respect copyright.ＰＥＮＡＮＡ1PT8LwhRzi
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)166Please respect copyright.ＰＥＮＡＮＡJWiKbferZh
---------------------------------------------------
166Please respect copyright.ＰＥＮＡＮＡVI5YfeOyRS
Section X. Thanks
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/If-None-Match166Please respect copyright.ＰＥＮＡＮＡexsi7L4WUF
https://blog.csdn.net/catoop/article/details/134174653166Please respect copyright.ＰＥＮＡＮＡZizLa2yOSS
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]166Please respect copyright.ＰＥＮＡＮＡ85EMI0xsSL
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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