明慈淨行記 - 第68章《Coding Art》第20篇 The Implementation of Http Range for Supporting Big File Download Pause and Re

本文描述了Http服务器使用http range支持浏览器的下载管理器下载大文件时可暂停和继续的实现方法，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。

Section A. Summary

I'm making a http server in C, in order to support big file download, I have to support Http Range in my server, as the usual way for web browsers to download big files is to use its download manager, where we can pause and resume the download progress. For the pause and resume functions of download manager to work, the http server must support Http Range requests.

Besides file download, http range is also needed for <video> element to play videos on web page, when the user click a random position of the video time line, the browser use http range requests to get the corresponding file content of the video file.

Http range is imported into http protocol from http 1.1. A http range request is like this:343Please respect copyright.ＰＥＮＡＮＡKcGzzzLDPC
----------http range request-------343Please respect copyright.ＰＥＮＡＮＡ0LShRJBDWi
GET /zen.iso HTTP/1.1343Please respect copyright.ＰＥＮＡＮＡkb7TKQxBq5
Range: bytes=100-1023343Please respect copyright.ＰＥＮＡＮＡ7YVecQ0k3X
If-Match: "534231200_1732277116"343Please respect copyright.ＰＥＮＡＮＡC7teku0cpa
......343Please respect copyright.ＰＥＮＡＮＡXSd538Pyv7
-----------------------------------343Please respect copyright.ＰＥＮＡＮＡM42LSECYVm
It means to get the content from byte 100 to byte 1023, totally 924 bytes, and it tells the http server the etag value of already downloaded part is "534231200_1732277116", the etag value is used for the web browser to determine if the requested content has been modified, it's necessory for some browsers like chrome and edge to resume the download of a file. If http server do not provide etag for range content, the resume function of web browser download manager will not work.

Section B. Detailed Steps

The following is the steps for serving file download, support pause and resume functions of web browser download manager.

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Step 1. make a link for file to download, for example:343Please respect copyright.ＰＥＮＡＮＡI4I3Pb5qwL
----------------html-----------------343Please respect copyright.ＰＥＮＡＮＡ0vxPJS4VdM
<a href="/zen.iso">zen.iso</a>343Please respect copyright.ＰＥＮＡＮＡP3NNoEkDD6
-------------------------------------343Please respect copyright.ＰＥＮＡＮＡ0GDHQ2WRZ2
When user click the link, web browser download manager will take care of the file download immediately. We cannot use javascript fetch(url) to start downloading, as in this way javascript code will take care of the download instead of download manager.

When the link is clicked, the http request sent to server is like this:343Please respect copyright.ＰＥＮＡＮＡ6dKrtyNPq4
----------get request------------343Please respect copyright.ＰＥＮＡＮＡ0t7F2QtZ6q
GET /zen.iso HTTP/1.1343Please respect copyright.ＰＥＮＡＮＡ4VcM5mfz7h
......343Please respect copyright.ＰＥＮＡＮＡrU6gCuAThm
---------------------------------

343Please respect copyright.ＰＥＮＡＮＡJUxGJKXEYy
Step 2. when http server receives above request, it sends the whole file content as response body back as usual, and indicates support of http range in the response headers, like this:343Please respect copyright.ＰＥＮＡＮＡrleXAhrutv
----------------server response--------------343Please respect copyright.ＰＥＮＡＮＡVaT2w1kKCk
HTTP/1.1 200 OK343Please respect copyright.ＰＥＮＡＮＡzXcghSLFit
Date: Sat, 23 Nov 2024 06:29:19 GMT343Please respect copyright.ＰＥＮＡＮＡDsxdPvYQkx
accept-ranges: bytes343Please respect copyright.ＰＥＮＡＮＡFKWLyxLt6s
content-range: bytes 0-534231199/534231200343Please respect copyright.ＰＥＮＡＮＡrfhnWhXtTI
ETag: "534231200_1732277116"343Please respect copyright.ＰＥＮＡＮＡLTCTWDpvnL
Content-Disposition: attachment; filename="zen.iso"343Please respect copyright.ＰＥＮＡＮＡsCBkrjRj40
Content-Length: 534231200343Please respect copyright.ＰＥＮＡＮＡNMQQiCX483
Content-Type: application/octet-stream

(http response body is the whole file content)343Please respect copyright.ＰＥＮＡＮＡoo79QTJBxu
-------------------------------------------

Http server usually use a small buf to send the whole file content by multiple times if the file size is bigger than it, like this:343Please respect copyright.ＰＥＮＡＮＡNN85av87Fp
---------------http server send big file content by multiple times--------------343Please respect copyright.ＰＥＮＡＮＡ7bK8mwakRJ
const int buf_size=20000000; // buf size is about 20M343Please respect copyright.ＰＥＮＡＮＡ0SjonZby7e
int client_socket; char send_buf[buf_size]; FILE *file;

// first send http response headers343Please respect copyright.ＰＥＮＡＮＡnWfNdA6uOT
send_http_response_headers(client_socket, http_response_headers);

// then send the whole file content as response body by multiple times if the file is big343Please respect copyright.ＰＥＮＡＮＡASGFxn8iQ0
while(there_is_remain_file_content_to_send)343Please respect copyright.ＰＥＮＡＮＡzwoyPYXAkO
{ 343Please respect copyright.ＰＥＮＡＮＡRvcOSEFpZY
read_remain_file_content_to_send_buf(file, send_buf);343Please respect copyright.ＰＥＮＡＮＡIFSNrlDUbl
send_result = send(client_socket, send_buf);343Please respect copyright.ＰＥＮＡＮＡEneyUT7LKn
if(send_result == fail) return; else continue;343Please respect copyright.ＰＥＮＡＮＡncfGrkNuWD
}

// a way to make etag for a file using its file size and last modifed time343Please respect copyright.ＰＥＮＡＮＡoxFNQCuKbS
const char *file_etag(const char *path) 343Please respect copyright.ＰＥＮＡＮＡkXibiDlNBS
{343Please respect copyright.ＰＥＮＡＮＡw1maYlGwir
struct stat st; if (stat(path, &st) < 0) return NULL; char* data=malloc(64); snprintf(data, 63, "%ld_%ld", (long)st.st_size, (long)st.st_mtime); return data;343Please respect copyright.ＰＥＮＡＮＡBYiEP99g4M
}343Please respect copyright.ＰＥＮＡＮＡFWgynigqSC
-------------------------------------------------343Please respect copyright.ＰＥＮＡＮＡfXzBri2elC
"accept-ranges: bytes" tells client web browser that the http server support range.343Please respect copyright.ＰＥＮＡＮＡo8vJ8m1C39
"content-range: bytes 0-534231199/534231200" tells client web browser the content range(0-534231199) of the response body and the total file size(534231200).343Please respect copyright.ＰＥＮＡＮＡZUk0MZW6gl
'ETag: "534231200_1732277116"' tells client web browser the file version.343Please respect copyright.ＰＥＮＡＮＡpdL0ZLaTWo
'Content-Disposition: attachment; filename="zen.iso"' tells client web browser to download the file as attachment, and its file name.343Please respect copyright.ＰＥＮＡＮＡiWGZF2GvSd
"Content-Length: 534231200" tells client web browser the total size of the response body, which is the size of the whole file in this example.343Please respect copyright.ＰＥＮＡＮＡo23tsYDR9F
"Content-Type: application/octet-stream" tells client web browser to treat the response body as binary stream.

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step 3. When the web browser receives above response, it knows the server support http range, and knows the total file size by "content-range: bytes 0-534231199/534231200", it also saves the etag for later comparation.343Please respect copyright.ＰＥＮＡＮＡU8BeUHLUhc
If the file is big, the above sends may take several minutes, during this time, the user can click pause in the download manager of web browser(for firefox, you have to press ctrl and right click the file to show the pause command.)343Please respect copyright.ＰＥＮＡＮＡEAZG31XzG1
When pause is pressed, the web browser disconnects with http server, and the "send_result = send(client_socket, send_buf);" of http server will be fail, then the http server stops sending anymore.

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step 4. Later the user can click resume to continue the download in download manager, and the download manager will send a http range request to tell the server the resume start point, like this:343Please respect copyright.ＰＥＮＡＮＡEaGUUXHQuE
---------resume range request---------343Please respect copyright.ＰＥＮＡＮＡ0JQXvHdNEo
GET /zen.iso HTTP/1.1343Please respect copyright.ＰＥＮＡＮＡ0D69CkLGgN
Range: bytes=243220480-343Please respect copyright.ＰＥＮＡＮＡ9AyxscBArY
If-Match: "534231200_1732277116"343Please respect copyright.ＰＥＮＡＮＡYqEYgCb1wo
......343Please respect copyright.ＰＥＮＡＮＡGnTGQcqnz3
-------------------------------------343Please respect copyright.ＰＥＮＡＮＡUmJwqvO8UL
"Range: bytes=243220480-" tells the http server to send content from byte 243220480 to end.343Please respect copyright.ＰＥＮＡＮＡMzpH2hznwY
'If-Match: "534231200_1732277116"' tells the http server to check the file etag.

If the etag remains the same, it means the file is not changed, so the server can send from the resume point 243220480, and its response status should be 206 partial content, like this:343Please respect copyright.ＰＥＮＡＮＡQrNYlsMxHX
-------response content from resume point-------343Please respect copyright.ＰＥＮＡＮＡ6owIMjp2GV
HTTP/1.1 206 Partial Content343Please respect copyright.ＰＥＮＡＮＡKI67Ed13oO
Date: Sat, 23 Nov 2024 06:56:17 GMT343Please respect copyright.ＰＥＮＡＮＡL2dlXJlUzv
accept-ranges: bytes343Please respect copyright.ＰＥＮＡＮＡB6xPnCRBZf
content-range: bytes 243220480-534231199/534231200343Please respect copyright.ＰＥＮＡＮＡiNOJsUFPgE
Content-Length: 291010720343Please respect copyright.ＰＥＮＡＮＡUpGSHQFWOJ
Content-Type: application/octet-stream343Please respect copyright.ＰＥＮＡＮＡWpAzWj9V4L
Content-Disposition: attachment; filename="zen.iso"343Please respect copyright.ＰＥＮＡＮＡrbrCHuiDzG
ETag: "534231200_1732277116"

(http response body is the file content from resume point to end)343Please respect copyright.ＰＥＮＡＮＡqPQUnX75Z7
------------------------------------------------343Please respect copyright.ＰＥＮＡＮＡDFhiNEWMg1
When client download manager receives this response, it will keep the already downloaded content and append the new respone body content to it.

If the etag diffs, it means the file has been changed, then the server should send the file content from beginning to end, with a 200 OK response status, like this:343Please respect copyright.ＰＥＮＡＮＡAcK8eGxKz1
-------response content from resume point-------343Please respect copyright.ＰＥＮＡＮＡ48kcakRjoo
HTTP/1.1 200 OK343Please respect copyright.ＰＥＮＡＮＡwuFRrzTI43
Date: Sat, 23 Nov 2024 06:29:19 GMT343Please respect copyright.ＰＥＮＡＮＡozYhYd9jdN
accept-ranges: bytes343Please respect copyright.ＰＥＮＡＮＡC4DSOqFlsP
content-range: bytes 0-634231201/634231202343Please respect copyright.ＰＥＮＡＮＡDESvcKvhBY
ETag: "634231202_2732277118"343Please respect copyright.ＰＥＮＡＮＡ1SLoIszAcE
Content-Disposition: attachment; filename="zen.iso"343Please respect copyright.ＰＥＮＡＮＡgqLKtRPgbo
Content-Length: 634231202343Please respect copyright.ＰＥＮＡＮＡHQ9LXWs2UH
Content-Type: application/octet-stream

(http response body is the whole file content)343Please respect copyright.ＰＥＮＡＮＡl2RyBgyda4
------------------------------------------------343Please respect copyright.ＰＥＮＡＮＡAyqLjlnJLE
When client download manager receives this response, it will clear the already downloaded content and start download from beginning.

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step 5. when a <video src="/video_file.mp4"> is used to play videos on web page, the browser also used http range requests, as Zeng Xu's article describes:343Please respect copyright.ＰＥＮＡＮＡSggExZgcc9
https://www.zeng.dev/post/2023-http-range-and-play-mp4-in-browser/?

343Please respect copyright.ＰＥＮＡＮＡJK3xoxgIC1
Section C. Thanks

感謝曾旭的幫助，曾旭的文章对http range有非常好的描述。

感谢stackoverflow对使用etag的提示343Please respect copyright.ＰＥＮＡＮＡXCSSBpjBWB
https://stackoverflow.com/questions/66998172/does-google-chrome-and-similar-browsers-support-range-headers-for-standard-downl

感谢gemini等ai对写代码的辅助343Please respect copyright.ＰＥＮＡＮＡ6wp0mEtBKz
https://gemini.google.com/

感谢互联网上无私奉献的参考资343Please respect copyright.ＰＥＮＡＮＡNHAF9qXW3Y
https://developer.mozilla.org/en-US/docs/Web/HTTP/Range_requests343Please respect copyright.ＰＥＮＡＮＡqaQfYCvq04
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/If-Range343Please respect copyright.ＰＥＮＡＮＡFh8PRGbRqk
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/ETag343Please respect copyright.ＰＥＮＡＮＡHKILVrCBpi
https://www.rfc-editor.org/rfc/rfc7233343Please respect copyright.ＰＥＮＡＮＡuiGP9wwbEe
https://mirrors.tuna.tsinghua.edu.cn/ubuntu-releases/24.10/

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Section D. 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]343Please respect copyright.ＰＥＮＡＮＡIBCEFS1OdU
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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