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:199Please respect copyright.ＰＥＮＡＮＡeVo1y8tBwd
----------http range request-------199Please respect copyright.ＰＥＮＡＮＡMG6tKdx0Sk
GET /zen.iso HTTP/1.1199Please respect copyright.ＰＥＮＡＮＡcQUglfAEVl
Range: bytes=100-1023199Please respect copyright.ＰＥＮＡＮＡ8nTw5bq5Xd
If-Match: "534231200_1732277116"199Please respect copyright.ＰＥＮＡＮＡM9eCDm23qE
......199Please respect copyright.ＰＥＮＡＮＡkdjx4zZoFS
-----------------------------------199Please respect copyright.ＰＥＮＡＮＡNaAInHnd7g
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:199Please respect copyright.ＰＥＮＡＮＡYYOwVEYo8B
----------------html-----------------199Please respect copyright.ＰＥＮＡＮＡGxLaGFSaKX
<a href="/zen.iso">zen.iso</a>199Please respect copyright.ＰＥＮＡＮＡwGFMMow1hQ
-------------------------------------199Please respect copyright.ＰＥＮＡＮＡuj10rK0nIy
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:199Please respect copyright.ＰＥＮＡＮＡY4nWf9cZMC
----------get request------------199Please respect copyright.ＰＥＮＡＮＡ1MyvVGBozH
GET /zen.iso HTTP/1.1199Please respect copyright.ＰＥＮＡＮＡqCItxosrhc
......199Please respect copyright.ＰＥＮＡＮＡcbKh1pORoR
---------------------------------

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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:199Please respect copyright.ＰＥＮＡＮＡE4vo1aE5Qr
----------------server response--------------199Please respect copyright.ＰＥＮＡＮＡesCZbaM2Q2
HTTP/1.1 200 OK199Please respect copyright.ＰＥＮＡＮＡ8PGpno71Gq
Date: Sat, 23 Nov 2024 06:29:19 GMT199Please respect copyright.ＰＥＮＡＮＡQFmunHxoNU
accept-ranges: bytes199Please respect copyright.ＰＥＮＡＮＡNLO7gNlDhS
content-range: bytes 0-534231199/534231200199Please respect copyright.ＰＥＮＡＮＡgqie3hSRv5
ETag: "534231200_1732277116"199Please respect copyright.ＰＥＮＡＮＡfu3NjTrZot
Content-Disposition: attachment; filename="zen.iso"199Please respect copyright.ＰＥＮＡＮＡ2Ff8P81YCg
Content-Length: 534231200199Please respect copyright.ＰＥＮＡＮＡblezGVGjA7
Content-Type: application/octet-stream

(http response body is the whole file content)199Please respect copyright.ＰＥＮＡＮＡ3QLUQEFPVI
-------------------------------------------

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:199Please respect copyright.ＰＥＮＡＮＡ7Y7rjKusdC
---------------http server send big file content by multiple times--------------199Please respect copyright.ＰＥＮＡＮＡhOMQ0QktDq
const int buf_size=20000000; // buf size is about 20M199Please respect copyright.ＰＥＮＡＮＡCOuhXeEwbl
int client_socket; char send_buf[buf_size]; FILE *file;

// first send http response headers199Please respect copyright.ＰＥＮＡＮＡVZE2kAIOP0
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 big199Please respect copyright.ＰＥＮＡＮＡiF2gNpb0Zl
while(there_is_remain_file_content_to_send)199Please respect copyright.ＰＥＮＡＮＡePewxs9MN6
{ 199Please respect copyright.ＰＥＮＡＮＡpbQ6q29vDR
read_remain_file_content_to_send_buf(file, send_buf);199Please respect copyright.ＰＥＮＡＮＡYjIzDeueVJ
send_result = send(client_socket, send_buf);199Please respect copyright.ＰＥＮＡＮＡATOtIVCPev
if(send_result == fail) return; else continue;199Please respect copyright.ＰＥＮＡＮＡm4us995vHH
}

// a way to make etag for a file using its file size and last modifed time199Please respect copyright.ＰＥＮＡＮＡjC8ZXJywbV
const char *file_etag(const char *path) 199Please respect copyright.ＰＥＮＡＮＡyYs9Qtg6qN
{199Please respect copyright.ＰＥＮＡＮＡM4Lh8Ng0ZN
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;199Please respect copyright.ＰＥＮＡＮＡBFFzftvkAu
}199Please respect copyright.ＰＥＮＡＮＡHiKapbjbN0
-------------------------------------------------199Please respect copyright.ＰＥＮＡＮＡ6PnzqI9vmw
"accept-ranges: bytes" tells client web browser that the http server support range.199Please respect copyright.ＰＥＮＡＮＡjplDXsTcoI
"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).199Please respect copyright.ＰＥＮＡＮＡ5RX4JCfcIb
'ETag: "534231200_1732277116"' tells client web browser the file version.199Please respect copyright.ＰＥＮＡＮＡj0cPiKbmfd
'Content-Disposition: attachment; filename="zen.iso"' tells client web browser to download the file as attachment, and its file name.199Please respect copyright.ＰＥＮＡＮＡo6l63MoQ1B
"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.199Please respect copyright.ＰＥＮＡＮＡNwvio1Nh3n
"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.199Please respect copyright.ＰＥＮＡＮＡBXejmzfFvH
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.)199Please respect copyright.ＰＥＮＡＮＡjaQmGmLLj6
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:199Please respect copyright.ＰＥＮＡＮＡvN8Otekeoe
---------resume range request---------199Please respect copyright.ＰＥＮＡＮＡamrnCSfQll
GET /zen.iso HTTP/1.1199Please respect copyright.ＰＥＮＡＮＡJ1kqvDj3fP
Range: bytes=243220480-199Please respect copyright.ＰＥＮＡＮＡWbm8ntK0qt
If-Match: "534231200_1732277116"199Please respect copyright.ＰＥＮＡＮＡTj0ZZ82R5M
......199Please respect copyright.ＰＥＮＡＮＡtECU93xRdk
-------------------------------------199Please respect copyright.ＰＥＮＡＮＡwJwTK5pxhE
"Range: bytes=243220480-" tells the http server to send content from byte 243220480 to end.199Please respect copyright.ＰＥＮＡＮＡFkWuRKT39Q
'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:199Please respect copyright.ＰＥＮＡＮＡQLsDGa7KmO
-------response content from resume point-------199Please respect copyright.ＰＥＮＡＮＡbdCARkELqy
HTTP/1.1 206 Partial Content199Please respect copyright.ＰＥＮＡＮＡ8QqZzOTeEY
Date: Sat, 23 Nov 2024 06:56:17 GMT199Please respect copyright.ＰＥＮＡＮＡD1VdE0Kk67
accept-ranges: bytes199Please respect copyright.ＰＥＮＡＮＡNgGkCxHjDQ
content-range: bytes 243220480-534231199/534231200199Please respect copyright.ＰＥＮＡＮＡb6MNtO9lBq
Content-Length: 291010720199Please respect copyright.ＰＥＮＡＮＡt0tYXThMGo
Content-Type: application/octet-stream199Please respect copyright.ＰＥＮＡＮＡxKLzlbviLj
Content-Disposition: attachment; filename="zen.iso"199Please respect copyright.ＰＥＮＡＮＡe0U6Js0z4I
ETag: "534231200_1732277116"

(http response body is the file content from resume point to end)199Please respect copyright.ＰＥＮＡＮＡK0ZtXST5gn
------------------------------------------------199Please respect copyright.ＰＥＮＡＮＡ8csNHwQnoZ
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:199Please respect copyright.ＰＥＮＡＮＡ4mv5nPwSQ3
-------response content from resume point-------199Please respect copyright.ＰＥＮＡＮＡsrpv5AGKcp
HTTP/1.1 200 OK199Please respect copyright.ＰＥＮＡＮＡmmYi39RrKd
Date: Sat, 23 Nov 2024 06:29:19 GMT199Please respect copyright.ＰＥＮＡＮＡevywu0aJsg
accept-ranges: bytes199Please respect copyright.ＰＥＮＡＮＡKzC4Vm3ShE
content-range: bytes 0-634231201/634231202199Please respect copyright.ＰＥＮＡＮＡc9MakTR6yL
ETag: "634231202_2732277118"199Please respect copyright.ＰＥＮＡＮＡgETjMExl8X
Content-Disposition: attachment; filename="zen.iso"199Please respect copyright.ＰＥＮＡＮＡ0GJEWaG29g
Content-Length: 634231202199Please respect copyright.ＰＥＮＡＮＡ8DkRoTxzny
Content-Type: application/octet-stream

(http response body is the whole file content)199Please respect copyright.ＰＥＮＡＮＡTrjNahuQC1
------------------------------------------------199Please respect copyright.ＰＥＮＡＮＡLV9AnHOBIt
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:199Please respect copyright.ＰＥＮＡＮＡs514yzxBQz
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199Please respect copyright.ＰＥＮＡＮＡmnyGQW8MGq
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199Please respect copyright.ＰＥＮＡＮＡSCLpwIlR7Q
https://gemini.google.com/

感谢互联网上无私奉献的参考资199Please respect copyright.ＰＥＮＡＮＡmg7NVJK2cD
https://developer.mozilla.org/en-US/docs/Web/HTTP/Range_requests199Please respect copyright.ＰＥＮＡＮＡateqfH7eQe
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/If-Range199Please respect copyright.ＰＥＮＡＮＡWb9X84WdNQ
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/ETag199Please respect copyright.ＰＥＮＡＮＡoIXtDO7tsR
https://www.rfc-editor.org/rfc/rfc7233199Please respect copyright.ＰＥＮＡＮＡieHdFX7KVr
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]199Please respect copyright.ＰＥＮＡＮＡt3tJBkyor7
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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