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.
65Please respect copyright.ＰＥＮＡＮＡdJLGgYv7lp
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:65Please respect copyright.ＰＥＮＡＮＡyklmMM8CGo
---------------http response with etag----------------65Please respect copyright.ＰＥＮＡＮＡUVbpDS8vjQ
HTTP/1.1 200 OK65Please respect copyright.ＰＥＮＡＮＡakgBlOGJGt
Date: Mon, 25 Nov 2024 10:56:12 GMT65Please respect copyright.ＰＥＮＡＮＡGH3dIqKyjS
accept-ranges: bytes65Please respect copyright.ＰＥＮＡＮＡWxSRqnNDfI
ETag: "534231200_1732277116"65Please respect copyright.ＰＥＮＡＮＡhx3z5atxOc
Content-Length: 53423120065Please respect copyright.ＰＥＮＡＮＡkB8smLc6Wj
Content-Type: application/octet-stream65Please respect copyright.ＰＥＮＡＮＡetQtxTVLnd
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)65Please respect copyright.ＰＥＮＡＮＡXXB5vQLoId
-----------------------------------------------------
65Please respect copyright.ＰＥＮＡＮＡgnrm8yuVle
Section B. if-none-match
The browser send back the etag to ask server if it has been changed by header like this:65Please respect copyright.ＰＥＮＡＮＡbhplex5gc1
-----http request for a cached resource------65Please respect copyright.ＰＥＮＡＮＡUR426mOq8M
GET /zen.iso HTTP/1.165Please respect copyright.ＰＥＮＡＮＡ4gIfEmYRiV
if-none-match: "534231200_1732277116"65Please respect copyright.ＰＥＮＡＮＡ8gxvTITn34
---------------------------------------------
If the etag is unchanged, the server response like this:
1. for get and head requests:65Please respect copyright.ＰＥＮＡＮＡL5bCQl5Fsu
-----------http response for unchanged resource------65Please respect copyright.ＰＥＮＡＮＡ0DsasqoSaZ
HTTP/1.1 304 Not Modified
(http body canbe empty, the client will used its cache)65Please respect copyright.ＰＥＮＡＮＡ5FBJWj61q3
----------------------------------------------------
2. for other methods requests:65Please respect copyright.ＰＥＮＡＮＡpXrTpFr8wY
-----------http response for unchanged resource------65Please respect copyright.ＰＥＮＡＮＡt6niNdY12B
HTTP/1.1 412 Precondition Failed
(http body canbe empty, the client will used its cache)65Please respect copyright.ＰＥＮＡＮＡKBVLvLmKgA
----------------------------------------------------
65Please respect copyright.ＰＥＮＡＮＡIN0r5Yfimg
If the etag changes, the server response 200 OK with the new file content like this:65Please respect copyright.ＰＥＮＡＮＡXOQjYYPPII
---------------http response with etag----------------65Please respect copyright.ＰＥＮＡＮＡLDSr5fwYyG
HTTP/1.1 200 OK65Please respect copyright.ＰＥＮＡＮＡNlOkruWB4q
Date: Mon, 25 Nov 2024 10:56:12 GMT65Please respect copyright.ＰＥＮＡＮＡvJoUYYrevp
accept-ranges: bytes65Please respect copyright.ＰＥＮＡＮＡhJj7uTePuf
ETag: "534231200_1732277116"65Please respect copyright.ＰＥＮＡＮＡC33WRnwk25
Content-Length: 53423120065Please respect copyright.ＰＥＮＡＮＡkIx1kQAa7u
Content-Type: application/octet-stream65Please respect copyright.ＰＥＮＡＮＡuJnlzT7l9h
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)65Please respect copyright.ＰＥＮＡＮＡ77Gj2BJSFV
-----------------------------------------------------
65Please respect copyright.ＰＥＮＡＮＡLmF4M8F9wt
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:65Please respect copyright.ＰＥＮＡＮＡwSFgtswWTt
------resume download request-------65Please respect copyright.ＰＥＮＡＮＡyB9AoWKUSl
GET /zen.iso HTTP/1.165Please respect copyright.ＰＥＮＡＮＡqLNV6ZqyYr
Range: bytes=287718144-65Please respect copyright.ＰＥＮＡＮＡqCIxE9Trz7
If-Match: "534231200_1732277116"65Please respect copyright.ＰＥＮＡＮＡTRmIsPoRtq
-----------------------------------65Please respect copyright.ＰＥＮＡＮＡqY7MXAtYTx
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:65Please respect copyright.ＰＥＮＡＮＡFPPbLx2GgT
-------------range response-------------------65Please respect copyright.ＰＥＮＡＮＡr7ZZrGQUpo
HTTP/1.1 206 Partial Content65Please respect copyright.ＰＥＮＡＮＡzfiVxWVKwn
accept-ranges: bytes65Please respect copyright.ＰＥＮＡＮＡvPYKTZsmKg
content-range: bytes 287718144-534231199/53423120065Please respect copyright.ＰＥＮＡＮＡtNsNMNE9rP
ETag: "534231200_1732277116"65Please respect copyright.ＰＥＮＡＮＡTEm4Ym47tK
Content-Length: 24651305665Please respect copyright.ＰＥＮＡＮＡkVYMj0Rp7S
Content-Type: application/octet-stream65Please respect copyright.ＰＥＮＡＮＡ78apK5UapN
Content-Disposition: attachment; filename="zen.iso"
(http body is the remain file content)65Please respect copyright.ＰＥＮＡＮＡvGHlUojsKo
-------------------------------------------
If the etag changes, response 412 like this:65Please respect copyright.ＰＥＮＡＮＡvNCu8De0Gz
-----------http response for changed resource------65Please respect copyright.ＰＥＮＡＮＡFp7VOELXqy
HTTP/1.1 412 Precondition Failed
(http body canbe empty, the client will stop downloading the resumed file)65Please respect copyright.ＰＥＮＡＮＡqwEJTLRex3
---------------------------------------------------
65Please respect copyright.ＰＥＮＡＮＡT7O3PNKIB7
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---65Please respect copyright.ＰＥＮＡＮＡjlCUC5C8Re
GET /zen.iso HTTP/1.165Please respect copyright.ＰＥＮＡＮＡ2hEiGiCObG
Range: bytes=287718144-65Please respect copyright.ＰＥＮＡＮＡZVkZI3dXbz
If-Range: "534231200_1732277116"65Please respect copyright.ＰＥＮＡＮＡIUUORhw1jL
------------------------------------65Please respect copyright.ＰＥＮＡＮＡ16FEekQ0eM
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:65Please respect copyright.ＰＥＮＡＮＡC9NMhRAqoV
-------------range response-------------------65Please respect copyright.ＰＥＮＡＮＡt3PHL0Q7JB
HTTP/1.1 206 Partial Content65Please respect copyright.ＰＥＮＡＮＡeKER5uRnqM
accept-ranges: bytes65Please respect copyright.ＰＥＮＡＮＡTkmZftA6F4
content-range: bytes 287718144-534231199/53423120065Please respect copyright.ＰＥＮＡＮＡWK2pWlKXVi
ETag: "534231200_1732277116"65Please respect copyright.ＰＥＮＡＮＡe2sCR064fD
Content-Length: 24651305665Please respect copyright.ＰＥＮＡＮＡxt5ihfhzVx
Content-Type: application/octet-stream65Please respect copyright.ＰＥＮＡＮＡwFeMCWscDi
Content-Disposition: attachment; filename="zen.iso"
(http body is the remain file content)65Please respect copyright.ＰＥＮＡＮＡTbeVQKX8EO
-------------------------------------------
If the etag changes, response 200 from beginning like this:65Please respect copyright.ＰＥＮＡＮＡwWRSyT6rOj
-----------http response from the beginning------65Please respect copyright.ＰＥＮＡＮＡZXru2NNCzC
HTTP/1.1 200 OK65Please respect copyright.ＰＥＮＡＮＡZX5AvEYkyN
Date: Mon, 25 Nov 2024 10:56:12 GMT65Please respect copyright.ＰＥＮＡＮＡWKX3NaqY2C
accept-ranges: bytes65Please respect copyright.ＰＥＮＡＮＡIQUWpE9myx
ETag: "534231200_1732277116"65Please respect copyright.ＰＥＮＡＮＡZ859MTm8Lq
Content-Length: 53423120065Please respect copyright.ＰＥＮＡＮＡwFayIR9num
Content-Type: application/octet-stream65Please respect copyright.ＰＥＮＡＮＡWTVeoZfQDb
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)65Please respect copyright.ＰＥＮＡＮＡ43Ht6UUS8c
---------------------------------------------------
65Please respect copyright.ＰＥＮＡＮＡX0BM28FNMZ
Section X. Thanks
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/If-None-Match65Please respect copyright.ＰＥＮＡＮＡAoFcv1kaQ1
https://blog.csdn.net/catoop/article/details/13417465365Please respect copyright.ＰＥＮＡＮＡ28ECP1qX0G
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]65Please respect copyright.ＰＥＮＡＮＡ7bjAIsMleJ
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。
ns3.141.4.12da2
然後按「加至主畫面」