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。1234 copyright protection367ＰＥＮＡＮＡwxAIJstMeW 尼

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.1234 copyright protection367ＰＥＮＡＮＡbnI613V5oI 尼

371Please respect copyright.ＰＥＮＡＮＡJwb2SaGAtw
Section A. ETag1234 copyright protection367ＰＥＮＡＮＡo7jGGDQHWN 尼

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.1234 copyright protection367ＰＥＮＡＮＡaLLYsg0a1d 尼

The server send the etag of a resource by header like this:371Please respect copyright.ＰＥＮＡＮＡnZrGeomT2x
---------------http response with etag----------------371Please respect copyright.ＰＥＮＡＮＡOjrGul30MO
HTTP/1.1 200 OK371Please respect copyright.ＰＥＮＡＮＡ5Cn5wZGCpu
Date: Mon, 25 Nov 2024 10:56:12 GMT371Please respect copyright.ＰＥＮＡＮＡvSjv8zMur8
accept-ranges: bytes371Please respect copyright.ＰＥＮＡＮＡ1HxdYMg6AT
ETag: "534231200_1732277116"371Please respect copyright.ＰＥＮＡＮＡicDiVdpQHk
Content-Length: 534231200371Please respect copyright.ＰＥＮＡＮＡXW2ncz00MM
Content-Type: application/octet-stream371Please respect copyright.ＰＥＮＡＮＡxnfpDaGthn
Content-Disposition: attachment; filename="zen.iso"1234 copyright protection367ＰＥＮＡＮＡ2sThiDcQvZ 尼

(http body for the resource)371Please respect copyright.ＰＥＮＡＮＡYwdsu9wmUC
-----------------------------------------------------1234 copyright protection367ＰＥＮＡＮＡhBLgGEKhJk 尼

371Please respect copyright.ＰＥＮＡＮＡtqUY92kX6I
Section B. if-none-match1234 copyright protection367ＰＥＮＡＮＡky8Dz0FXYn 尼

The browser send back the etag to ask server if it has been changed by header like this:371Please respect copyright.ＰＥＮＡＮＡTORX4V1m2V
-----http request for a cached resource------371Please respect copyright.ＰＥＮＡＮＡd5ElCKgOy6
GET /zen.iso HTTP/1.1371Please respect copyright.ＰＥＮＡＮＡCxU84OcnNx
if-none-match: "534231200_1732277116"371Please respect copyright.ＰＥＮＡＮＡ0lw7B7jkdI
---------------------------------------------1234 copyright protection367ＰＥＮＡＮＡP9qKsaSjBR 尼

If the etag is unchanged, the server response like this:1234 copyright protection367ＰＥＮＡＮＡxC9jfVLFLU 尼

1. for get and head requests:371Please respect copyright.ＰＥＮＡＮＡbcUMN8vFN6
-----------http response for unchanged resource------371Please respect copyright.ＰＥＮＡＮＡLQELNtMvZy
HTTP/1.1 304 Not Modified1234 copyright protection367ＰＥＮＡＮＡDenyYF6tit 尼

(http body canbe empty, the client will used its cache)371Please respect copyright.ＰＥＮＡＮＡldbLTyjNab
----------------------------------------------------1234 copyright protection367ＰＥＮＡＮＡCcdGvuzqWe 尼

2. for other methods requests:371Please respect copyright.ＰＥＮＡＮＡ0uCMGdRLSZ
-----------http response for unchanged resource------371Please respect copyright.ＰＥＮＡＮＡHuDmEUcRqP
HTTP/1.1 412 Precondition Failed 1234 copyright protection367ＰＥＮＡＮＡfHz12BfaMy 尼

(http body canbe empty, the client will used its cache)371Please respect copyright.ＰＥＮＡＮＡ2MrPmziyXR
----------------------------------------------------1234 copyright protection367ＰＥＮＡＮＡwpJuXiWX2W 尼

371Please respect copyright.ＰＥＮＡＮＡD693uBFbPB
If the etag changes, the server response 200 OK with the new file content like this:371Please respect copyright.ＰＥＮＡＮＡui1X4L9YIZ
---------------http response with etag----------------371Please respect copyright.ＰＥＮＡＮＡd1yVpK0h9B
HTTP/1.1 200 OK371Please respect copyright.ＰＥＮＡＮＡaa41xv1Y1t
Date: Mon, 25 Nov 2024 10:56:12 GMT371Please respect copyright.ＰＥＮＡＮＡvwZOL5FMZ2
accept-ranges: bytes371Please respect copyright.ＰＥＮＡＮＡWhEX2qsphy
ETag: "534231200_1732277116"371Please respect copyright.ＰＥＮＡＮＡ67AJFnLWcr
Content-Length: 534231200371Please respect copyright.ＰＥＮＡＮＡxlEVtKQbG8
Content-Type: application/octet-stream371Please respect copyright.ＰＥＮＡＮＡoeR5rQGcs8
Content-Disposition: attachment; filename="zen.iso"1234 copyright protection367ＰＥＮＡＮＡcpUK7vj5zh 尼

(http body for the resource)371Please respect copyright.ＰＥＮＡＮＡTYBKyD6Vs8
-----------------------------------------------------1234 copyright protection367ＰＥＮＡＮＡRWJfaOZOGF 尼

371Please respect copyright.ＰＥＮＡＮＡCI6LCuDr8t
Section C. if-match1234 copyright protection367ＰＥＮＡＮＡKhpuMesrKn 尼

When resuming a download in the browser download manager, the browser sends a range request with the if-match header like this:371Please respect copyright.ＰＥＮＡＮＡ5B81wecB4M
------resume download request-------371Please respect copyright.ＰＥＮＡＮＡjiKzR3Zvrx
GET /zen.iso HTTP/1.1371Please respect copyright.ＰＥＮＡＮＡEhvK9aHf72
Range: bytes=287718144-371Please respect copyright.ＰＥＮＡＮＡdjVrc5bAid
If-Match: "534231200_1732277116"371Please respect copyright.ＰＥＮＡＮＡJzzt9BBFGF
-----------------------------------371Please respect copyright.ＰＥＮＡＮＡLzwhgacxhx
It tells the server to check the etag in If-Match, if unchanged, resume to get the remain content; otherwise, return 412 Precondition Failed.1234 copyright protection367ＰＥＮＡＮＡ1T3LxUvbpM 尼

The server response for unchanged etag is like this:371Please respect copyright.ＰＥＮＡＮＡW5xl1hIKE2
-------------range response-------------------371Please respect copyright.ＰＥＮＡＮＡsJ0SPjJmQa
HTTP/1.1 206 Partial Content371Please respect copyright.ＰＥＮＡＮＡW0wau10m2z
accept-ranges: bytes371Please respect copyright.ＰＥＮＡＮＡMfE3TFUQj8
content-range: bytes 287718144-534231199/534231200371Please respect copyright.ＰＥＮＡＮＡH2zh2XMHFA
ETag: "534231200_1732277116"371Please respect copyright.ＰＥＮＡＮＡ45jWevNMfM
Content-Length: 246513056371Please respect copyright.ＰＥＮＡＮＡYpLTups0Gv
Content-Type: application/octet-stream371Please respect copyright.ＰＥＮＡＮＡAuWDEZIS14
Content-Disposition: attachment; filename="zen.iso"1234 copyright protection367ＰＥＮＡＮＡPCIxTktHI7 尼

(http body is the remain file content)371Please respect copyright.ＰＥＮＡＮＡQMxAreMeuP
-------------------------------------------1234 copyright protection367ＰＥＮＡＮＡVCTN7yZOLO 尼

If the etag changes, response 412 like this:371Please respect copyright.ＰＥＮＡＮＡztemE5aiib
-----------http response for changed resource------371Please respect copyright.ＰＥＮＡＮＡBnQ7Ji6ufR
HTTP/1.1 412 Precondition Failed 1234 copyright protection367ＰＥＮＡＮＡ0QUdGCXMyx 尼

(http body canbe empty, the client will stop downloading the resumed file)371Please respect copyright.ＰＥＮＡＮＡNTJzSQpbM7
---------------------------------------------------1234 copyright protection367ＰＥＮＡＮＡE6kMneCJdH 尼

371Please respect copyright.ＰＥＮＡＮＡNGb1LCZvxi
Section D. if-range1234 copyright protection367ＰＥＮＡＮＡVNjN1d4MJY 尼

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.1234 copyright protection367ＰＥＮＡＮＡOrjAWXZELj 尼

---resume download request by chrome---371Please respect copyright.ＰＥＮＡＮＡz7V2uwGfat
GET /zen.iso HTTP/1.1371Please respect copyright.ＰＥＮＡＮＡddwL2jj9Gh
Range: bytes=287718144-371Please respect copyright.ＰＥＮＡＮＡsyj7rLAGdM
If-Range: "534231200_1732277116"371Please respect copyright.ＰＥＮＡＮＡLJQ9NE9Lq6
------------------------------------371Please respect copyright.ＰＥＮＡＮＡr3WAUyx7D8
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.1234 copyright protection367ＰＥＮＡＮＡjo3Z12t5If 尼

The server response for unchanged etag is like this:371Please respect copyright.ＰＥＮＡＮＡ6n4f827M5v
-------------range response-------------------371Please respect copyright.ＰＥＮＡＮＡyX4MXj3kxm
HTTP/1.1 206 Partial Content371Please respect copyright.ＰＥＮＡＮＡQ1aARs6F1N
accept-ranges: bytes371Please respect copyright.ＰＥＮＡＮＡ1RL4SJbTXA
content-range: bytes 287718144-534231199/534231200371Please respect copyright.ＰＥＮＡＮＡNiADqqnIls
ETag: "534231200_1732277116"371Please respect copyright.ＰＥＮＡＮＡe0CRx6Ht4l
Content-Length: 246513056371Please respect copyright.ＰＥＮＡＮＡWgvzrr5GkK
Content-Type: application/octet-stream371Please respect copyright.ＰＥＮＡＮＡ63FffpV4TP
Content-Disposition: attachment; filename="zen.iso"1234 copyright protection367ＰＥＮＡＮＡlGw0dU73pV 尼

(http body is the remain file content)371Please respect copyright.ＰＥＮＡＮＡQgw1mKTzj8
-------------------------------------------1234 copyright protection367ＰＥＮＡＮＡjYxhbA8DgO 尼

If the etag changes, response 200 from beginning like this:371Please respect copyright.ＰＥＮＡＮＡxw0N4Ge1B1
-----------http response from the beginning------371Please respect copyright.ＰＥＮＡＮＡgq81WL4YOV
HTTP/1.1 200 OK371Please respect copyright.ＰＥＮＡＮＡGoYwi2fy4a
Date: Mon, 25 Nov 2024 10:56:12 GMT371Please respect copyright.ＰＥＮＡＮＡRA8fTNl5Mm
accept-ranges: bytes371Please respect copyright.ＰＥＮＡＮＡBjfUvh9wNz
ETag: "534231200_1732277116"371Please respect copyright.ＰＥＮＡＮＡ1Xub0YEBsL
Content-Length: 534231200371Please respect copyright.ＰＥＮＡＮＡMlGoT2r0h7
Content-Type: application/octet-stream371Please respect copyright.ＰＥＮＡＮＡtw5gRoQ0jy
Content-Disposition: attachment; filename="zen.iso"1234 copyright protection367ＰＥＮＡＮＡ656H3zjQpD 尼

(http body for the resource)371Please respect copyright.ＰＥＮＡＮＡO9Ko5LNWTg
---------------------------------------------------1234 copyright protection367ＰＥＮＡＮＡr4WXI9z6x6 尼

371Please respect copyright.ＰＥＮＡＮＡJwNhBxyldB
Section X. Thanks1234 copyright protection367ＰＥＮＡＮＡspoKmRwsAH 尼

https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/If-None-Match371Please respect copyright.ＰＥＮＡＮＡJEakmZUNl8
https://blog.csdn.net/catoop/article/details/134174653371Please respect copyright.ＰＥＮＡＮＡMup9v93UOD
https://blog.csdn.net/phker/article/details/507226191234 copyright protection367ＰＥＮＡＮＡBnxfeYtivE 尼

Section Y. Contacts Me1234 copyright protection367ＰＥＮＡＮＡFbX1S1OxGN 尼

If you found any errors or have any suggestions for this article, please let me know, my wechat: si_jinmin, my email: [email protected]371Please respect copyright.ＰＥＮＡＮＡKQBZMoblaB
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]1234 copyright protection367ＰＥＮＡＮＡO1PftQCL6q 尼

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