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.
158Please respect copyright.ＰＥＮＡＮＡl9k75yk0t4
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:158Please respect copyright.ＰＥＮＡＮＡ5H8tKfRNgJ
---------------http response with etag----------------158Please respect copyright.ＰＥＮＡＮＡr64n1X88Rh
HTTP/1.1 200 OK158Please respect copyright.ＰＥＮＡＮＡFAxozrUKeq
Date: Mon, 25 Nov 2024 10:56:12 GMT158Please respect copyright.ＰＥＮＡＮＡsGuXYC6Cdy
accept-ranges: bytes158Please respect copyright.ＰＥＮＡＮＡNuR1R22pw9
ETag: "534231200_1732277116"158Please respect copyright.ＰＥＮＡＮＡKFkte6H4az
Content-Length: 534231200158Please respect copyright.ＰＥＮＡＮＡtIryNwkfzr
Content-Type: application/octet-stream158Please respect copyright.ＰＥＮＡＮＡe9d1erHWs0
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)158Please respect copyright.ＰＥＮＡＮＡak4CidfSUX
-----------------------------------------------------
158Please respect copyright.ＰＥＮＡＮＡiSYoHRZKJc
Section B. if-none-match
The browser send back the etag to ask server if it has been changed by header like this:158Please respect copyright.ＰＥＮＡＮＡIehYFv1W4l
-----http request for a cached resource------158Please respect copyright.ＰＥＮＡＮＡIcRQoHhdGY
GET /zen.iso HTTP/1.1158Please respect copyright.ＰＥＮＡＮＡmmoZTqf2wQ
if-none-match: "534231200_1732277116"158Please respect copyright.ＰＥＮＡＮＡuUNClPpjpI
---------------------------------------------
If the etag is unchanged, the server response like this:
1. for get and head requests:158Please respect copyright.ＰＥＮＡＮＡVJNrJAfimz
-----------http response for unchanged resource------158Please respect copyright.ＰＥＮＡＮＡcuyGXK9Kv4
HTTP/1.1 304 Not Modified
(http body canbe empty, the client will used its cache)158Please respect copyright.ＰＥＮＡＮＡXuOnWNvbHe
----------------------------------------------------
2. for other methods requests:158Please respect copyright.ＰＥＮＡＮＡsNPJu4xZub
-----------http response for unchanged resource------158Please respect copyright.ＰＥＮＡＮＡ4gqKuM70P9
HTTP/1.1 412 Precondition Failed
(http body canbe empty, the client will used its cache)158Please respect copyright.ＰＥＮＡＮＡhxRh1bweDZ
----------------------------------------------------
158Please respect copyright.ＰＥＮＡＮＡfPf8DYSFCZ
If the etag changes, the server response 200 OK with the new file content like this:158Please respect copyright.ＰＥＮＡＮＡeKi0AgOPWJ
---------------http response with etag----------------158Please respect copyright.ＰＥＮＡＮＡwc37NJ8uLp
HTTP/1.1 200 OK158Please respect copyright.ＰＥＮＡＮＡ2tkbGEGZGh
Date: Mon, 25 Nov 2024 10:56:12 GMT158Please respect copyright.ＰＥＮＡＮＡZPqBcV0FKR
accept-ranges: bytes158Please respect copyright.ＰＥＮＡＮＡHGKpAArzgZ
ETag: "534231200_1732277116"158Please respect copyright.ＰＥＮＡＮＡBkosm9OIhm
Content-Length: 534231200158Please respect copyright.ＰＥＮＡＮＡrOdXyyjlvP
Content-Type: application/octet-stream158Please respect copyright.ＰＥＮＡＮＡqXybnzmd7q
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)158Please respect copyright.ＰＥＮＡＮＡiFXL88ugAw
-----------------------------------------------------
158Please respect copyright.ＰＥＮＡＮＡknt8v67ElP
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:158Please respect copyright.ＰＥＮＡＮＡ9XE6Za0pnq
------resume download request-------158Please respect copyright.ＰＥＮＡＮＡFzw63Z4BRa
GET /zen.iso HTTP/1.1158Please respect copyright.ＰＥＮＡＮＡgslzN4vSQP
Range: bytes=287718144-158Please respect copyright.ＰＥＮＡＮＡB7gWMwTpaB
If-Match: "534231200_1732277116"158Please respect copyright.ＰＥＮＡＮＡQtbAiEiumB
-----------------------------------158Please respect copyright.ＰＥＮＡＮＡ6AcioS4mHh
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:158Please respect copyright.ＰＥＮＡＮＡvwM7h8aTFo
-------------range response-------------------158Please respect copyright.ＰＥＮＡＮＡL0apd573yq
HTTP/1.1 206 Partial Content158Please respect copyright.ＰＥＮＡＮＡkyt3LEirYj
accept-ranges: bytes158Please respect copyright.ＰＥＮＡＮＡRdFj6fKWRx
content-range: bytes 287718144-534231199/534231200158Please respect copyright.ＰＥＮＡＮＡDtsEHis0y0
ETag: "534231200_1732277116"158Please respect copyright.ＰＥＮＡＮＡYa8xlcUyh6
Content-Length: 246513056158Please respect copyright.ＰＥＮＡＮＡiRDyTBzDgB
Content-Type: application/octet-stream158Please respect copyright.ＰＥＮＡＮＡheUzauDn6a
Content-Disposition: attachment; filename="zen.iso"
(http body is the remain file content)158Please respect copyright.ＰＥＮＡＮＡh8uZ5874Yb
-------------------------------------------
If the etag changes, response 412 like this:158Please respect copyright.ＰＥＮＡＮＡW8fAQmtU8g
-----------http response for changed resource------158Please respect copyright.ＰＥＮＡＮＡRsxwcmDbFy
HTTP/1.1 412 Precondition Failed
(http body canbe empty, the client will stop downloading the resumed file)158Please respect copyright.ＰＥＮＡＮＡ0mX86rA0aF
---------------------------------------------------
158Please respect copyright.ＰＥＮＡＮＡ7e9Q2MHHeD
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---158Please respect copyright.ＰＥＮＡＮＡR3f0LpcoGI
GET /zen.iso HTTP/1.1158Please respect copyright.ＰＥＮＡＮＡxp0jI7ywnk
Range: bytes=287718144-158Please respect copyright.ＰＥＮＡＮＡqALzgk5Dgv
If-Range: "534231200_1732277116"158Please respect copyright.ＰＥＮＡＮＡUDQKV8jbE0
------------------------------------158Please respect copyright.ＰＥＮＡＮＡpBNIkRDvH1
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:158Please respect copyright.ＰＥＮＡＮＡ8VGi3nVRYQ
-------------range response-------------------158Please respect copyright.ＰＥＮＡＮＡKwq5eBrPcT
HTTP/1.1 206 Partial Content158Please respect copyright.ＰＥＮＡＮＡk7QjNhlrIP
accept-ranges: bytes158Please respect copyright.ＰＥＮＡＮＡ08Gn4ul3Nn
content-range: bytes 287718144-534231199/534231200158Please respect copyright.ＰＥＮＡＮＡ9TM3xfXQjy
ETag: "534231200_1732277116"158Please respect copyright.ＰＥＮＡＮＡCFaVrG7dHp
Content-Length: 246513056158Please respect copyright.ＰＥＮＡＮＡ1cWgTMpJJn
Content-Type: application/octet-stream158Please respect copyright.ＰＥＮＡＮＡMD4ITpudyW
Content-Disposition: attachment; filename="zen.iso"
(http body is the remain file content)158Please respect copyright.ＰＥＮＡＮＡcPVLqpJ19v
-------------------------------------------
If the etag changes, response 200 from beginning like this:158Please respect copyright.ＰＥＮＡＮＡzIpEjXVIY6
-----------http response from the beginning------158Please respect copyright.ＰＥＮＡＮＡLicUdvJtSp
HTTP/1.1 200 OK158Please respect copyright.ＰＥＮＡＮＡPNhC8L7PyW
Date: Mon, 25 Nov 2024 10:56:12 GMT158Please respect copyright.ＰＥＮＡＮＡqpljckOrfy
accept-ranges: bytes158Please respect copyright.ＰＥＮＡＮＡvt9kTvpjkG
ETag: "534231200_1732277116"158Please respect copyright.ＰＥＮＡＮＡTqzMXpPaPr
Content-Length: 534231200158Please respect copyright.ＰＥＮＡＮＡsWv7Wdj3JK
Content-Type: application/octet-stream158Please respect copyright.ＰＥＮＡＮＡ4NqoptPWRn
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)158Please respect copyright.ＰＥＮＡＮＡlTIP88O7Vs
---------------------------------------------------
158Please respect copyright.ＰＥＮＡＮＡsom8WRQBHB
Section X. Thanks
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/If-None-Match158Please respect copyright.ＰＥＮＡＮＡpWPxVMwNpk
https://blog.csdn.net/catoop/article/details/134174653158Please respect copyright.ＰＥＮＡＮＡjyeh6zOc2g
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]158Please respect copyright.ＰＥＮＡＮＡ9ooyUKkTUc
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]
如果您對C/C++ Programming, Linux, HTTP Protocol, Website Development, Vue, Git, VsCode感興趣，邀請您加入「Linux/C/C++ Website Development」微信群，請加我的微信（si_jinmin）以便拉您进群。
ns216.73.216.13da2
