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.
170Please respect copyright.ＰＥＮＡＮＡOpEQJpjYvk
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:170Please respect copyright.ＰＥＮＡＮＡySNQBB6nBx
---------------http response with etag----------------170Please respect copyright.ＰＥＮＡＮＡdygvQQx3Jr
HTTP/1.1 200 OK170Please respect copyright.ＰＥＮＡＮＡXLJVN1AdYO
Date: Mon, 25 Nov 2024 10:56:12 GMT170Please respect copyright.ＰＥＮＡＮＡpMCYjm2TxV
accept-ranges: bytes170Please respect copyright.ＰＥＮＡＮＡlzRzoJc0tg
ETag: "534231200_1732277116"170Please respect copyright.ＰＥＮＡＮＡj8Djm32f1U
Content-Length: 534231200170Please respect copyright.ＰＥＮＡＮＡ8ZL9WYf56t
Content-Type: application/octet-stream170Please respect copyright.ＰＥＮＡＮＡFPdkUQAQPz
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)170Please respect copyright.ＰＥＮＡＮＡ9DNuqqJ5D3
-----------------------------------------------------
170Please respect copyright.ＰＥＮＡＮＡKPcX30ESnz
Section B. if-none-match
The browser send back the etag to ask server if it has been changed by header like this:170Please respect copyright.ＰＥＮＡＮＡz3Q5B7c4I4
-----http request for a cached resource------170Please respect copyright.ＰＥＮＡＮＡUSPz0x3ibj
GET /zen.iso HTTP/1.1170Please respect copyright.ＰＥＮＡＮＡzc7hZaW5dL
if-none-match: "534231200_1732277116"170Please respect copyright.ＰＥＮＡＮＡuVe1JMlCZS
---------------------------------------------
If the etag is unchanged, the server response like this:
1. for get and head requests:170Please respect copyright.ＰＥＮＡＮＡUigMwSZC3J
-----------http response for unchanged resource------170Please respect copyright.ＰＥＮＡＮＡyWNDJYpm94
HTTP/1.1 304 Not Modified
(http body canbe empty, the client will used its cache)170Please respect copyright.ＰＥＮＡＮＡNSfzU06k1V
----------------------------------------------------
2. for other methods requests:170Please respect copyright.ＰＥＮＡＮＡsPX4D4dL3B
-----------http response for unchanged resource------170Please respect copyright.ＰＥＮＡＮＡmZo1Mbn7Fr
HTTP/1.1 412 Precondition Failed
(http body canbe empty, the client will used its cache)170Please respect copyright.ＰＥＮＡＮＡIk3XmmvxTQ
----------------------------------------------------
170Please respect copyright.ＰＥＮＡＮＡXNkJTkdpox
If the etag changes, the server response 200 OK with the new file content like this:170Please respect copyright.ＰＥＮＡＮＡaF2BQHsNPr
---------------http response with etag----------------170Please respect copyright.ＰＥＮＡＮＡTm5jtjnzce
HTTP/1.1 200 OK170Please respect copyright.ＰＥＮＡＮＡyBuTySRCxf
Date: Mon, 25 Nov 2024 10:56:12 GMT170Please respect copyright.ＰＥＮＡＮＡLOm4gYnJ9J
accept-ranges: bytes170Please respect copyright.ＰＥＮＡＮＡ3ud3NlHGvc
ETag: "534231200_1732277116"170Please respect copyright.ＰＥＮＡＮＡ47jTVLRTs7
Content-Length: 534231200170Please respect copyright.ＰＥＮＡＮＡMwqHUYUlVD
Content-Type: application/octet-stream170Please respect copyright.ＰＥＮＡＮＡR1MsCjZ0Qm
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)170Please respect copyright.ＰＥＮＡＮＡTUYc5USoFq
-----------------------------------------------------
170Please respect copyright.ＰＥＮＡＮＡpQjpodyyJq
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:170Please respect copyright.ＰＥＮＡＮＡuSJx70sr5u
------resume download request-------170Please respect copyright.ＰＥＮＡＮＡ0QLcXeKe9z
GET /zen.iso HTTP/1.1170Please respect copyright.ＰＥＮＡＮＡY9PziSWCvM
Range: bytes=287718144-170Please respect copyright.ＰＥＮＡＮＡ4o0spA8Lr7
If-Match: "534231200_1732277116"170Please respect copyright.ＰＥＮＡＮＡJOmgwHeO6t
-----------------------------------170Please respect copyright.ＰＥＮＡＮＡIULej9r1Ys
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:170Please respect copyright.ＰＥＮＡＮＡ3TUnBsHCUP
-------------range response-------------------170Please respect copyright.ＰＥＮＡＮＡmBeA0fo5kc
HTTP/1.1 206 Partial Content170Please respect copyright.ＰＥＮＡＮＡqXXFmkdIGF
accept-ranges: bytes170Please respect copyright.ＰＥＮＡＮＡfv72ppnicn
content-range: bytes 287718144-534231199/534231200170Please respect copyright.ＰＥＮＡＮＡnsyR6gLecK
ETag: "534231200_1732277116"170Please respect copyright.ＰＥＮＡＮＡHd5DwX3y6i
Content-Length: 246513056170Please respect copyright.ＰＥＮＡＮＡoOrZCZ7G6L
Content-Type: application/octet-stream170Please respect copyright.ＰＥＮＡＮＡjm2H9etyt9
Content-Disposition: attachment; filename="zen.iso"
(http body is the remain file content)170Please respect copyright.ＰＥＮＡＮＡl9tOnkkxhB
-------------------------------------------
If the etag changes, response 412 like this:170Please respect copyright.ＰＥＮＡＮＡKG0rKCL6wo
-----------http response for changed resource------170Please respect copyright.ＰＥＮＡＮＡujqjEF8KpH
HTTP/1.1 412 Precondition Failed
(http body canbe empty, the client will stop downloading the resumed file)170Please respect copyright.ＰＥＮＡＮＡ7CiY3YXfGd
---------------------------------------------------
170Please respect copyright.ＰＥＮＡＮＡU5A43cdje9
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---170Please respect copyright.ＰＥＮＡＮＡGPjbyssrbm
GET /zen.iso HTTP/1.1170Please respect copyright.ＰＥＮＡＮＡNqUTrUoj89
Range: bytes=287718144-170Please respect copyright.ＰＥＮＡＮＡsoqrrFk4ht
If-Range: "534231200_1732277116"170Please respect copyright.ＰＥＮＡＮＡaibQvRIBX1
------------------------------------170Please respect copyright.ＰＥＮＡＮＡGSmrZTNupj
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:170Please respect copyright.ＰＥＮＡＮＡ54dryPUsvW
-------------range response-------------------170Please respect copyright.ＰＥＮＡＮＡbbA0Gvtpto
HTTP/1.1 206 Partial Content170Please respect copyright.ＰＥＮＡＮＡkFobJ79GWB
accept-ranges: bytes170Please respect copyright.ＰＥＮＡＮＡHm9qr4qi0C
content-range: bytes 287718144-534231199/534231200170Please respect copyright.ＰＥＮＡＮＡ9kGFOS1bsB
ETag: "534231200_1732277116"170Please respect copyright.ＰＥＮＡＮＡgmv1upzPw9
Content-Length: 246513056170Please respect copyright.ＰＥＮＡＮＡooAHc5WgYI
Content-Type: application/octet-stream170Please respect copyright.ＰＥＮＡＮＡingg28AFWZ
Content-Disposition: attachment; filename="zen.iso"
(http body is the remain file content)170Please respect copyright.ＰＥＮＡＮＡmTybtwzJYp
-------------------------------------------
If the etag changes, response 200 from beginning like this:170Please respect copyright.ＰＥＮＡＮＡiI88UqakiL
-----------http response from the beginning------170Please respect copyright.ＰＥＮＡＮＡS6su64CYRc
HTTP/1.1 200 OK170Please respect copyright.ＰＥＮＡＮＡellbJdylWN
Date: Mon, 25 Nov 2024 10:56:12 GMT170Please respect copyright.ＰＥＮＡＮＡuq0Ql4hK3H
accept-ranges: bytes170Please respect copyright.ＰＥＮＡＮＡFq9lj1fyvV
ETag: "534231200_1732277116"170Please respect copyright.ＰＥＮＡＮＡEWlrITjDbi
Content-Length: 534231200170Please respect copyright.ＰＥＮＡＮＡlI9KoxpqhK
Content-Type: application/octet-stream170Please respect copyright.ＰＥＮＡＮＡxJ78RZNFht
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)170Please respect copyright.ＰＥＮＡＮＡD2Ecw6Q1Rj
---------------------------------------------------
170Please respect copyright.ＰＥＮＡＮＡLaeQMj76oh
Section X. Thanks
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/If-None-Match170Please respect copyright.ＰＥＮＡＮＡA3pDRqsyzo
https://blog.csdn.net/catoop/article/details/134174653170Please respect copyright.ＰＥＮＡＮＡKHzBrOmVRn
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]170Please respect copyright.ＰＥＮＡＮＡN2673cctPS
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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