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.
153Please respect copyright.ＰＥＮＡＮＡSSHiGpCrzb
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:153Please respect copyright.ＰＥＮＡＮＡlLNurursL9
---------------http response with etag----------------153Please respect copyright.ＰＥＮＡＮＡIbvTJ0wFOT
HTTP/1.1 200 OK153Please respect copyright.ＰＥＮＡＮＡRAEfogRv9O
Date: Mon, 25 Nov 2024 10:56:12 GMT153Please respect copyright.ＰＥＮＡＮＡmN7A3BgRUT
accept-ranges: bytes153Please respect copyright.ＰＥＮＡＮＡQJrSOHbGzN
ETag: "534231200_1732277116"153Please respect copyright.ＰＥＮＡＮＡNVAOlSyviy
Content-Length: 534231200153Please respect copyright.ＰＥＮＡＮＡ6m8ZMwWD12
Content-Type: application/octet-stream153Please respect copyright.ＰＥＮＡＮＡQDo8FB1M3p
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)153Please respect copyright.ＰＥＮＡＮＡWyOR3EmY9X
-----------------------------------------------------
153Please respect copyright.ＰＥＮＡＮＡsHP4dh2PAP
Section B. if-none-match
The browser send back the etag to ask server if it has been changed by header like this:153Please respect copyright.ＰＥＮＡＮＡzmW418rtWK
-----http request for a cached resource------153Please respect copyright.ＰＥＮＡＮＡsU37znjvZ7
GET /zen.iso HTTP/1.1153Please respect copyright.ＰＥＮＡＮＡNVbfpiz33Y
if-none-match: "534231200_1732277116"153Please respect copyright.ＰＥＮＡＮＡuNOZ028qF7
---------------------------------------------
If the etag is unchanged, the server response like this:
1. for get and head requests:153Please respect copyright.ＰＥＮＡＮＡhsMcVek7Xk
-----------http response for unchanged resource------153Please respect copyright.ＰＥＮＡＮＡ0AU9BlSIxb
HTTP/1.1 304 Not Modified
(http body canbe empty, the client will used its cache)153Please respect copyright.ＰＥＮＡＮＡil4BpGnZqt
----------------------------------------------------
2. for other methods requests:153Please respect copyright.ＰＥＮＡＮＡOWDFA46xZc
-----------http response for unchanged resource------153Please respect copyright.ＰＥＮＡＮＡPWGjIHCmTj
HTTP/1.1 412 Precondition Failed
(http body canbe empty, the client will used its cache)153Please respect copyright.ＰＥＮＡＮＡ19NF6Zlzdh
----------------------------------------------------
153Please respect copyright.ＰＥＮＡＮＡ41eJAlu4Gs
If the etag changes, the server response 200 OK with the new file content like this:153Please respect copyright.ＰＥＮＡＮＡeafcOHpsl5
---------------http response with etag----------------153Please respect copyright.ＰＥＮＡＮＡW0XTQSmeq5
HTTP/1.1 200 OK153Please respect copyright.ＰＥＮＡＮＡpK96LmRyVW
Date: Mon, 25 Nov 2024 10:56:12 GMT153Please respect copyright.ＰＥＮＡＮＡzS4N9XYbO5
accept-ranges: bytes153Please respect copyright.ＰＥＮＡＮＡonS8luL8gN
ETag: "534231200_1732277116"153Please respect copyright.ＰＥＮＡＮＡyBQ0NEZQrG
Content-Length: 534231200153Please respect copyright.ＰＥＮＡＮＡgzsWzMlObx
Content-Type: application/octet-stream153Please respect copyright.ＰＥＮＡＮＡkQaZ6VzOUv
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)153Please respect copyright.ＰＥＮＡＮＡ3gbk9vzDWZ
-----------------------------------------------------
153Please respect copyright.ＰＥＮＡＮＡeCSexgtXoj
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:153Please respect copyright.ＰＥＮＡＮＡFPlX9dJ43M
------resume download request-------153Please respect copyright.ＰＥＮＡＮＡPkowzJEH0i
GET /zen.iso HTTP/1.1153Please respect copyright.ＰＥＮＡＮＡwAoAsv6UYr
Range: bytes=287718144-153Please respect copyright.ＰＥＮＡＮＡrcW6Hfgk0d
If-Match: "534231200_1732277116"153Please respect copyright.ＰＥＮＡＮＡv5hVSBhyR8
-----------------------------------153Please respect copyright.ＰＥＮＡＮＡYSayZqvZd0
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:153Please respect copyright.ＰＥＮＡＮＡ9i9mOP3VBQ
-------------range response-------------------153Please respect copyright.ＰＥＮＡＮＡM1rnYqgfTg
HTTP/1.1 206 Partial Content153Please respect copyright.ＰＥＮＡＮＡjOwnJgLaS3
accept-ranges: bytes153Please respect copyright.ＰＥＮＡＮＡ21BCYU4EWO
content-range: bytes 287718144-534231199/534231200153Please respect copyright.ＰＥＮＡＮＡqldvQKqTEe
ETag: "534231200_1732277116"153Please respect copyright.ＰＥＮＡＮＡfuhOe3oXsA
Content-Length: 246513056153Please respect copyright.ＰＥＮＡＮＡJs7xYBnHwv
Content-Type: application/octet-stream153Please respect copyright.ＰＥＮＡＮＡruGmQxOJAk
Content-Disposition: attachment; filename="zen.iso"
(http body is the remain file content)153Please respect copyright.ＰＥＮＡＮＡlt3segDGNX
-------------------------------------------
If the etag changes, response 412 like this:153Please respect copyright.ＰＥＮＡＮＡ8LC3eQK6iW
-----------http response for changed resource------153Please respect copyright.ＰＥＮＡＮＡXdLxSRMAX2
HTTP/1.1 412 Precondition Failed
(http body canbe empty, the client will stop downloading the resumed file)153Please respect copyright.ＰＥＮＡＮＡ7Sfjx6YatR
---------------------------------------------------
153Please respect copyright.ＰＥＮＡＮＡf8nfLHrH52
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---153Please respect copyright.ＰＥＮＡＮＡS9C99E5Jff
GET /zen.iso HTTP/1.1153Please respect copyright.ＰＥＮＡＮＡzfFioGB2DS
Range: bytes=287718144-153Please respect copyright.ＰＥＮＡＮＡ7j12VUEH6L
If-Range: "534231200_1732277116"153Please respect copyright.ＰＥＮＡＮＡVxEaC9nFyn
------------------------------------153Please respect copyright.ＰＥＮＡＮＡCoQ3ocy4HS
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:153Please respect copyright.ＰＥＮＡＮＡ0AO71ek6zx
-------------range response-------------------153Please respect copyright.ＰＥＮＡＮＡuWVFBZPn4U
HTTP/1.1 206 Partial Content153Please respect copyright.ＰＥＮＡＮＡ8bZtvQfZki
accept-ranges: bytes153Please respect copyright.ＰＥＮＡＮＡO0W2ThqgJs
content-range: bytes 287718144-534231199/534231200153Please respect copyright.ＰＥＮＡＮＡxlAVH8zqtc
ETag: "534231200_1732277116"153Please respect copyright.ＰＥＮＡＮＡ3V3Qi5MMc0
Content-Length: 246513056153Please respect copyright.ＰＥＮＡＮＡBKalnSc9RN
Content-Type: application/octet-stream153Please respect copyright.ＰＥＮＡＮＡjb1CCHPzDO
Content-Disposition: attachment; filename="zen.iso"
(http body is the remain file content)153Please respect copyright.ＰＥＮＡＮＡKaN8vXccqU
-------------------------------------------
If the etag changes, response 200 from beginning like this:153Please respect copyright.ＰＥＮＡＮＡyuUb8qaXK5
-----------http response from the beginning------153Please respect copyright.ＰＥＮＡＮＡtPwYpmRY9p
HTTP/1.1 200 OK153Please respect copyright.ＰＥＮＡＮＡQ9MFQvq50e
Date: Mon, 25 Nov 2024 10:56:12 GMT153Please respect copyright.ＰＥＮＡＮＡU0V0ml5VnG
accept-ranges: bytes153Please respect copyright.ＰＥＮＡＮＡ137CkACO0f
ETag: "534231200_1732277116"153Please respect copyright.ＰＥＮＡＮＡgQAHg5B2we
Content-Length: 534231200153Please respect copyright.ＰＥＮＡＮＡ6GKgWecHrL
Content-Type: application/octet-stream153Please respect copyright.ＰＥＮＡＮＡSPVscuuHE0
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)153Please respect copyright.ＰＥＮＡＮＡtLPfUoqSiJ
---------------------------------------------------
153Please respect copyright.ＰＥＮＡＮＡTNwubZLbHd
Section X. Thanks
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/If-None-Match153Please respect copyright.ＰＥＮＡＮＡhbofcx2kuP
https://blog.csdn.net/catoop/article/details/134174653153Please respect copyright.ＰＥＮＡＮＡkxbNUfmpTC
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]153Please respect copyright.ＰＥＮＡＮＡ23frVes0uo
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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