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:115Please respect copyright.ＰＥＮＡＮＡrRKaLOPmzY
----------http range request-------115Please respect copyright.ＰＥＮＡＮＡeaSPp0S1IO
GET /zen.iso HTTP/1.1115Please respect copyright.ＰＥＮＡＮＡyEkPPrmyA5
Range: bytes=100-1023115Please respect copyright.ＰＥＮＡＮＡ8vGCYTPHSC
If-Match: "534231200_1732277116"115Please respect copyright.ＰＥＮＡＮＡ2QD8R41iWk
......115Please respect copyright.ＰＥＮＡＮＡk76nyDBqgO
-----------------------------------115Please respect copyright.ＰＥＮＡＮＡcZmJhEmTfn
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:115Please respect copyright.ＰＥＮＡＮＡjqZZXinrZI
----------------html-----------------115Please respect copyright.ＰＥＮＡＮＡ2dtEefyqWE
<a href="/zen.iso">zen.iso</a>115Please respect copyright.ＰＥＮＡＮＡhSyj4VO3ZM
-------------------------------------115Please respect copyright.ＰＥＮＡＮＡuyy0Vu8t1p
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:115Please respect copyright.ＰＥＮＡＮＡQozqgTXW0R
----------get request------------115Please respect copyright.ＰＥＮＡＮＡRLRzQmvWIh
GET /zen.iso HTTP/1.1115Please respect copyright.ＰＥＮＡＮＡ9XoYuWEgnT
......115Please respect copyright.ＰＥＮＡＮＡ68ObU3kZTI
---------------------------------

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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:115Please respect copyright.ＰＥＮＡＮＡ1VMhf0EPN8
----------------server response--------------115Please respect copyright.ＰＥＮＡＮＡoQJAHNRalh
HTTP/1.1 200 OK115Please respect copyright.ＰＥＮＡＮＡ4IiE2JCiar
Date: Sat, 23 Nov 2024 06:29:19 GMT115Please respect copyright.ＰＥＮＡＮＡqER4JjLqgZ
accept-ranges: bytes115Please respect copyright.ＰＥＮＡＮＡMPJmHzVKai
content-range: bytes 0-534231199/534231200115Please respect copyright.ＰＥＮＡＮＡszu3YnqZBO
ETag: "534231200_1732277116"115Please respect copyright.ＰＥＮＡＮＡqOnR1dJo2r
Content-Disposition: attachment; filename="zen.iso"115Please respect copyright.ＰＥＮＡＮＡM7DAgcBi1C
Content-Length: 534231200115Please respect copyright.ＰＥＮＡＮＡJviY6QzgCr
Content-Type: application/octet-stream

(http response body is the whole file content)115Please respect copyright.ＰＥＮＡＮＡsOhMu6ybgf
-------------------------------------------

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:115Please respect copyright.ＰＥＮＡＮＡlKFehEcV7I
---------------http server send big file content by multiple times--------------115Please respect copyright.ＰＥＮＡＮＡOBDfNWliiH
const int buf_size=20000000; // buf size is about 20M115Please respect copyright.ＰＥＮＡＮＡWxIsuEQHa6
int client_socket; char send_buf[buf_size]; FILE *file;

// first send http response headers115Please respect copyright.ＰＥＮＡＮＡwfnBhbRvbW
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 big115Please respect copyright.ＰＥＮＡＮＡ0Ihf3lqg9T
while(there_is_remain_file_content_to_send)115Please respect copyright.ＰＥＮＡＮＡ6l2kvChWdn
{ 115Please respect copyright.ＰＥＮＡＮＡ7AltxtFQR5
read_remain_file_content_to_send_buf(file, send_buf);115Please respect copyright.ＰＥＮＡＮＡgcWO1CrIta
send_result = send(client_socket, send_buf);115Please respect copyright.ＰＥＮＡＮＡU3QBWGsoWk
if(send_result == fail) return; else continue;115Please respect copyright.ＰＥＮＡＮＡHnOoyBFqlo
}

// a way to make etag for a file using its file size and last modifed time115Please respect copyright.ＰＥＮＡＮＡYbIL58yJbp
const char *file_etag(const char *path) 115Please respect copyright.ＰＥＮＡＮＡoQbqeDpM2f
{115Please respect copyright.ＰＥＮＡＮＡa8BDhoPvCV
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;115Please respect copyright.ＰＥＮＡＮＡwBvuQOgcVP
}115Please respect copyright.ＰＥＮＡＮＡAsEj9vssAk
-------------------------------------------------115Please respect copyright.ＰＥＮＡＮＡHY7gb7zmTf
"accept-ranges: bytes" tells client web browser that the http server support range.115Please respect copyright.ＰＥＮＡＮＡADYZ9GDY4w
"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).115Please respect copyright.ＰＥＮＡＮＡlDctiUkCo6
'ETag: "534231200_1732277116"' tells client web browser the file version.115Please respect copyright.ＰＥＮＡＮＡErPuj92GD0
'Content-Disposition: attachment; filename="zen.iso"' tells client web browser to download the file as attachment, and its file name.115Please respect copyright.ＰＥＮＡＮＡzuGgJc5JfB
"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.115Please respect copyright.ＰＥＮＡＮＡsw1OT6GtOI
"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.115Please respect copyright.ＰＥＮＡＮＡc9rMNTJUr5
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.)115Please respect copyright.ＰＥＮＡＮＡeesO6keX1S
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:115Please respect copyright.ＰＥＮＡＮＡcSHzHyZeRd
---------resume range request---------115Please respect copyright.ＰＥＮＡＮＡlf4EPAZ52D
GET /zen.iso HTTP/1.1115Please respect copyright.ＰＥＮＡＮＡ9eO8xgkrd7
Range: bytes=243220480-115Please respect copyright.ＰＥＮＡＮＡyMtllFvHeS
If-Match: "534231200_1732277116"115Please respect copyright.ＰＥＮＡＮＡifYPWV8WX5
......115Please respect copyright.ＰＥＮＡＮＡpWlCbTuEWt
-------------------------------------115Please respect copyright.ＰＥＮＡＮＡ8HqxR0Q36H
"Range: bytes=243220480-" tells the http server to send content from byte 243220480 to end.115Please respect copyright.ＰＥＮＡＮＡJJ9uAvxMKm
'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:115Please respect copyright.ＰＥＮＡＮＡwgzAp1MK2M
-------response content from resume point-------115Please respect copyright.ＰＥＮＡＮＡeZ8SvB92wS
HTTP/1.1 206 Partial Content115Please respect copyright.ＰＥＮＡＮＡYfke1MwM6e
Date: Sat, 23 Nov 2024 06:56:17 GMT115Please respect copyright.ＰＥＮＡＮＡLCqSBjlFDi
accept-ranges: bytes115Please respect copyright.ＰＥＮＡＮＡms4n3wqbAs
content-range: bytes 243220480-534231199/534231200115Please respect copyright.ＰＥＮＡＮＡJuO6Y8I8ju
Content-Length: 291010720115Please respect copyright.ＰＥＮＡＮＡoCCAwi9BLw
Content-Type: application/octet-stream115Please respect copyright.ＰＥＮＡＮＡx9iENXbdQ1
Content-Disposition: attachment; filename="zen.iso"115Please respect copyright.ＰＥＮＡＮＡYQfkkTwCaL
ETag: "534231200_1732277116"

(http response body is the file content from resume point to end)115Please respect copyright.ＰＥＮＡＮＡn95AYYzWxo
------------------------------------------------115Please respect copyright.ＰＥＮＡＮＡFLfnKRY0Jg
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:115Please respect copyright.ＰＥＮＡＮＡipDDzqTauV
-------response content from resume point-------115Please respect copyright.ＰＥＮＡＮＡzUPRP9dRZa
HTTP/1.1 200 OK115Please respect copyright.ＰＥＮＡＮＡ9JMPa5Apkq
Date: Sat, 23 Nov 2024 06:29:19 GMT115Please respect copyright.ＰＥＮＡＮＡd4OBx3iNBM
accept-ranges: bytes115Please respect copyright.ＰＥＮＡＮＡZUsta4UCoR
content-range: bytes 0-634231201/634231202115Please respect copyright.ＰＥＮＡＮＡB1FzQOiBfU
ETag: "634231202_2732277118"115Please respect copyright.ＰＥＮＡＮＡilC66k7mPt
Content-Disposition: attachment; filename="zen.iso"115Please respect copyright.ＰＥＮＡＮＡ06gJxQwUx4
Content-Length: 634231202115Please respect copyright.ＰＥＮＡＮＡB93MIeo7Vx
Content-Type: application/octet-stream

(http response body is the whole file content)115Please respect copyright.ＰＥＮＡＮＡYXiJBkYVXd
------------------------------------------------115Please respect copyright.ＰＥＮＡＮＡvhedx9ubRz
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:115Please respect copyright.ＰＥＮＡＮＡny00veDLKw
https://www.zeng.dev/post/2023-http-range-and-play-mp4-in-browser/?

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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115Please respect copyright.ＰＥＮＡＮＡcy5gHDOBmD
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115Please respect copyright.ＰＥＮＡＮＡnlFmgPpkhg
https://gemini.google.com/

感谢互联网上无私奉献的参考资115Please respect copyright.ＰＥＮＡＮＡRokSJlzUKu
https://developer.mozilla.org/en-US/docs/Web/HTTP/Range_requests115Please respect copyright.ＰＥＮＡＮＡNYkQ19uP2N
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/If-Range115Please respect copyright.ＰＥＮＡＮＡuYz6Z0BiWZ
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/ETag115Please respect copyright.ＰＥＮＡＮＡ1P2vEwdn67
https://www.rfc-editor.org/rfc/rfc7233115Please respect copyright.ＰＥＮＡＮＡtaMGRznR97
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]115Please respect copyright.ＰＥＮＡＮＡrmp9c3rrBN
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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