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.
177Please respect copyright.ＰＥＮＡＮＡsa49GAJvSD
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:177Please respect copyright.ＰＥＮＡＮＡI6BA5g4Q9U
---------------http response with etag----------------177Please respect copyright.ＰＥＮＡＮＡhDK6cAnsOO
HTTP/1.1 200 OK177Please respect copyright.ＰＥＮＡＮＡAA1KQNXPwN
Date: Mon, 25 Nov 2024 10:56:12 GMT177Please respect copyright.ＰＥＮＡＮＡsMRFSxwtCY
accept-ranges: bytes177Please respect copyright.ＰＥＮＡＮＡOjCwxUBDfc
ETag: "534231200_1732277116"177Please respect copyright.ＰＥＮＡＮＡ5N1hfi8mkp
Content-Length: 534231200177Please respect copyright.ＰＥＮＡＮＡ8m4ZCDoWeR
Content-Type: application/octet-stream177Please respect copyright.ＰＥＮＡＮＡnJLNBWCbx7
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)177Please respect copyright.ＰＥＮＡＮＡW3tmID5cUm
-----------------------------------------------------
177Please respect copyright.ＰＥＮＡＮＡ9cqJhQuJOI
Section B. if-none-match
The browser send back the etag to ask server if it has been changed by header like this:177Please respect copyright.ＰＥＮＡＮＡgbsuhU2165
-----http request for a cached resource------177Please respect copyright.ＰＥＮＡＮＡbxhq1MVKZy
GET /zen.iso HTTP/1.1177Please respect copyright.ＰＥＮＡＮＡkdABHlAyy1
if-none-match: "534231200_1732277116"177Please respect copyright.ＰＥＮＡＮＡuJWOncVrYN
---------------------------------------------
If the etag is unchanged, the server response like this:
1. for get and head requests:177Please respect copyright.ＰＥＮＡＮＡ5SxdYVpbrW
-----------http response for unchanged resource------177Please respect copyright.ＰＥＮＡＮＡxuleksJiuE
HTTP/1.1 304 Not Modified
(http body canbe empty, the client will used its cache)177Please respect copyright.ＰＥＮＡＮＡlUtwymsop3
----------------------------------------------------
2. for other methods requests:177Please respect copyright.ＰＥＮＡＮＡEQMwY0ltuY
-----------http response for unchanged resource------177Please respect copyright.ＰＥＮＡＮＡSqfSBu2soK
HTTP/1.1 412 Precondition Failed
(http body canbe empty, the client will used its cache)177Please respect copyright.ＰＥＮＡＮＡ1ikPeUS85N
----------------------------------------------------
177Please respect copyright.ＰＥＮＡＮＡKRfqWV2wZD
If the etag changes, the server response 200 OK with the new file content like this:177Please respect copyright.ＰＥＮＡＮＡGj9c5vANtR
---------------http response with etag----------------177Please respect copyright.ＰＥＮＡＮＡLqtiWymdth
HTTP/1.1 200 OK177Please respect copyright.ＰＥＮＡＮＡ46S4mr7KzL
Date: Mon, 25 Nov 2024 10:56:12 GMT177Please respect copyright.ＰＥＮＡＮＡeMsCIp2HNz
accept-ranges: bytes177Please respect copyright.ＰＥＮＡＮＡncSZnafmOL
ETag: "534231200_1732277116"177Please respect copyright.ＰＥＮＡＮＡRo1N83x1oE
Content-Length: 534231200177Please respect copyright.ＰＥＮＡＮＡURRRZIuNQt
Content-Type: application/octet-stream177Please respect copyright.ＰＥＮＡＮＡMWkN9Nzamj
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)177Please respect copyright.ＰＥＮＡＮＡ0IYvP3JCiS
-----------------------------------------------------
177Please respect copyright.ＰＥＮＡＮＡSGiKLy9FG6
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:177Please respect copyright.ＰＥＮＡＮＡh45qXpWxKW
------resume download request-------177Please respect copyright.ＰＥＮＡＮＡ8GZZ9jFAiX
GET /zen.iso HTTP/1.1177Please respect copyright.ＰＥＮＡＮＡO8MTIKfa36
Range: bytes=287718144-177Please respect copyright.ＰＥＮＡＮＡIaGoz0WSga
If-Match: "534231200_1732277116"177Please respect copyright.ＰＥＮＡＮＡTiQhbpVCGs
-----------------------------------177Please respect copyright.ＰＥＮＡＮＡKpTglOSzg6
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:177Please respect copyright.ＰＥＮＡＮＡWgwVK5Wcmh
-------------range response-------------------177Please respect copyright.ＰＥＮＡＮＡEtw1BzCMxF
HTTP/1.1 206 Partial Content177Please respect copyright.ＰＥＮＡＮＡb3BHTaMg0h
accept-ranges: bytes177Please respect copyright.ＰＥＮＡＮＡJcFYMVwzQ7
content-range: bytes 287718144-534231199/534231200177Please respect copyright.ＰＥＮＡＮＡtU0Hx3uv7E
ETag: "534231200_1732277116"177Please respect copyright.ＰＥＮＡＮＡ9npK2P47w6
Content-Length: 246513056177Please respect copyright.ＰＥＮＡＮＡpZanO4G84k
Content-Type: application/octet-stream177Please respect copyright.ＰＥＮＡＮＡrCCfKdq0Ay
Content-Disposition: attachment; filename="zen.iso"
(http body is the remain file content)177Please respect copyright.ＰＥＮＡＮＡ2xMroEiVs8
-------------------------------------------
If the etag changes, response 412 like this:177Please respect copyright.ＰＥＮＡＮＡaJ9YSf7hsL
-----------http response for changed resource------177Please respect copyright.ＰＥＮＡＮＡ0Hlkh2ylno
HTTP/1.1 412 Precondition Failed
(http body canbe empty, the client will stop downloading the resumed file)177Please respect copyright.ＰＥＮＡＮＡypzhZ6Z6Fi
---------------------------------------------------
177Please respect copyright.ＰＥＮＡＮＡXM7SWkDpHd
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---177Please respect copyright.ＰＥＮＡＮＡsOwM436q88
GET /zen.iso HTTP/1.1177Please respect copyright.ＰＥＮＡＮＡkh4CvApUlc
Range: bytes=287718144-177Please respect copyright.ＰＥＮＡＮＡCOp3LFH5BL
If-Range: "534231200_1732277116"177Please respect copyright.ＰＥＮＡＮＡZA6vAoHwmg
------------------------------------177Please respect copyright.ＰＥＮＡＮＡPRQ7msZsdK
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:177Please respect copyright.ＰＥＮＡＮＡmeqRZkC7S9
-------------range response-------------------177Please respect copyright.ＰＥＮＡＮＡJvNOKoUov1
HTTP/1.1 206 Partial Content177Please respect copyright.ＰＥＮＡＮＡAXAIl8sqQ9
accept-ranges: bytes177Please respect copyright.ＰＥＮＡＮＡ9UAXL836yX
content-range: bytes 287718144-534231199/534231200177Please respect copyright.ＰＥＮＡＮＡtd59lQLLcu
ETag: "534231200_1732277116"177Please respect copyright.ＰＥＮＡＮＡztDG3mOnB9
Content-Length: 246513056177Please respect copyright.ＰＥＮＡＮＡZzhvbvuoh0
Content-Type: application/octet-stream177Please respect copyright.ＰＥＮＡＮＡSiJN2gZqkE
Content-Disposition: attachment; filename="zen.iso"
(http body is the remain file content)177Please respect copyright.ＰＥＮＡＮＡvKUHAYdKPY
-------------------------------------------
If the etag changes, response 200 from beginning like this:177Please respect copyright.ＰＥＮＡＮＡA9Lp15pp5R
-----------http response from the beginning------177Please respect copyright.ＰＥＮＡＮＡgf77P6oIyt
HTTP/1.1 200 OK177Please respect copyright.ＰＥＮＡＮＡFwVmUbKQgb
Date: Mon, 25 Nov 2024 10:56:12 GMT177Please respect copyright.ＰＥＮＡＮＡm31JHV1EZc
accept-ranges: bytes177Please respect copyright.ＰＥＮＡＮＡJIBD4QaWQ7
ETag: "534231200_1732277116"177Please respect copyright.ＰＥＮＡＮＡbOiXs7kg1o
Content-Length: 534231200177Please respect copyright.ＰＥＮＡＮＡifmYbdJtqk
Content-Type: application/octet-stream177Please respect copyright.ＰＥＮＡＮＡcfmNMwDZEn
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)177Please respect copyright.ＰＥＮＡＮＡHzA6w4fkk6
---------------------------------------------------
177Please respect copyright.ＰＥＮＡＮＡq75IXef0Hn
Section X. Thanks
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/If-None-Match177Please respect copyright.ＰＥＮＡＮＡp42vP635Uj
https://blog.csdn.net/catoop/article/details/134174653177Please respect copyright.ＰＥＮＡＮＡbjGxlTbhOs
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]177Please respect copyright.ＰＥＮＡＮＡKHP2KQUQvT
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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