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:325Please respect copyright.ＰＥＮＡＮＡcsUlnzmEBU
----------http range request-------325Please respect copyright.ＰＥＮＡＮＡiYV9L5EcLj
GET /zen.iso HTTP/1.1325Please respect copyright.ＰＥＮＡＮＡChg3wRqG2h
Range: bytes=100-1023325Please respect copyright.ＰＥＮＡＮＡibIvCstR6K
If-Match: "534231200_1732277116"325Please respect copyright.ＰＥＮＡＮＡFcM135Uats
......325Please respect copyright.ＰＥＮＡＮＡUPqA9a918Q
-----------------------------------325Please respect copyright.ＰＥＮＡＮＡKd0FUqUE1P
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:325Please respect copyright.ＰＥＮＡＮＡ83G80nNOyn
----------------html-----------------325Please respect copyright.ＰＥＮＡＮＡv4LxozdLYK
<a href="/zen.iso">zen.iso</a>325Please respect copyright.ＰＥＮＡＮＡaR8quO4lkl
-------------------------------------325Please respect copyright.ＰＥＮＡＮＡu4BdroWXQj
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:325Please respect copyright.ＰＥＮＡＮＡPQLVm1WsRM
----------get request------------325Please respect copyright.ＰＥＮＡＮＡ4SLV6scw6o
GET /zen.iso HTTP/1.1325Please respect copyright.ＰＥＮＡＮＡYtPVP8EqZd
......325Please respect copyright.ＰＥＮＡＮＡd9A1SGggrY
---------------------------------

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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:325Please respect copyright.ＰＥＮＡＮＡDiKPeOdVwL
----------------server response--------------325Please respect copyright.ＰＥＮＡＮＡ3vvs7fbxmJ
HTTP/1.1 200 OK325Please respect copyright.ＰＥＮＡＮＡlN8y3US3p8
Date: Sat, 23 Nov 2024 06:29:19 GMT325Please respect copyright.ＰＥＮＡＮＡyQDKuQV02J
accept-ranges: bytes325Please respect copyright.ＰＥＮＡＮＡB666Sr3Jnz
content-range: bytes 0-534231199/534231200325Please respect copyright.ＰＥＮＡＮＡaMJqtGrdK8
ETag: "534231200_1732277116"325Please respect copyright.ＰＥＮＡＮＡYfI1oNGdEq
Content-Disposition: attachment; filename="zen.iso"325Please respect copyright.ＰＥＮＡＮＡ1RYtaXfnkD
Content-Length: 534231200325Please respect copyright.ＰＥＮＡＮＡo2J8gPH1M8
Content-Type: application/octet-stream

(http response body is the whole file content)325Please respect copyright.ＰＥＮＡＮＡsUILNjUKse
-------------------------------------------

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:325Please respect copyright.ＰＥＮＡＮＡkAFJo0fkeH
---------------http server send big file content by multiple times--------------325Please respect copyright.ＰＥＮＡＮＡc9AOBLIlea
const int buf_size=20000000; // buf size is about 20M325Please respect copyright.ＰＥＮＡＮＡJlqNWWtwOZ
int client_socket; char send_buf[buf_size]; FILE *file;

// first send http response headers325Please respect copyright.ＰＥＮＡＮＡU3gijvp3Vg
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 big325Please respect copyright.ＰＥＮＡＮＡztCQm892Zj
while(there_is_remain_file_content_to_send)325Please respect copyright.ＰＥＮＡＮＡ4Gs9soEl52
{ 325Please respect copyright.ＰＥＮＡＮＡyfXSvrhzbC
read_remain_file_content_to_send_buf(file, send_buf);325Please respect copyright.ＰＥＮＡＮＡqIRaKkaqkr
send_result = send(client_socket, send_buf);325Please respect copyright.ＰＥＮＡＮＡeNjCh9vwRE
if(send_result == fail) return; else continue;325Please respect copyright.ＰＥＮＡＮＡMQewvV4kSs
}

// a way to make etag for a file using its file size and last modifed time325Please respect copyright.ＰＥＮＡＮＡ0npneYhxnW
const char *file_etag(const char *path) 325Please respect copyright.ＰＥＮＡＮＡstnxeetAiC
{325Please respect copyright.ＰＥＮＡＮＡMtELrZ47vK
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;325Please respect copyright.ＰＥＮＡＮＡqcNKxPiiZO
}325Please respect copyright.ＰＥＮＡＮＡuyTVheKxK0
-------------------------------------------------325Please respect copyright.ＰＥＮＡＮＡneBIrMEX5Y
"accept-ranges: bytes" tells client web browser that the http server support range.325Please respect copyright.ＰＥＮＡＮＡ4LGvHFikAA
"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).325Please respect copyright.ＰＥＮＡＮＡe1OV2m9ddB
'ETag: "534231200_1732277116"' tells client web browser the file version.325Please respect copyright.ＰＥＮＡＮＡQzIOpCxXIk
'Content-Disposition: attachment; filename="zen.iso"' tells client web browser to download the file as attachment, and its file name.325Please respect copyright.ＰＥＮＡＮＡl7WfEHLbBk
"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.325Please respect copyright.ＰＥＮＡＮＡzmFNm9nhQt
"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.325Please respect copyright.ＰＥＮＡＮＡfOt4GmiTUL
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.)325Please respect copyright.ＰＥＮＡＮＡQtPlIAcdZ7
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:325Please respect copyright.ＰＥＮＡＮＡ2So6W1A7ks
---------resume range request---------325Please respect copyright.ＰＥＮＡＮＡzVCXy7l3hx
GET /zen.iso HTTP/1.1325Please respect copyright.ＰＥＮＡＮＡuMu6E14p9h
Range: bytes=243220480-325Please respect copyright.ＰＥＮＡＮＡ8ppvSO7EX7
If-Match: "534231200_1732277116"325Please respect copyright.ＰＥＮＡＮＡvuXP5Pmlb1
......325Please respect copyright.ＰＥＮＡＮＡohvYRaT5nC
-------------------------------------325Please respect copyright.ＰＥＮＡＮＡwMBR2SuIYC
"Range: bytes=243220480-" tells the http server to send content from byte 243220480 to end.325Please respect copyright.ＰＥＮＡＮＡyKdSKqiIu4
'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:325Please respect copyright.ＰＥＮＡＮＡl0oVUYTmgI
-------response content from resume point-------325Please respect copyright.ＰＥＮＡＮＡHEl8AYKp8q
HTTP/1.1 206 Partial Content325Please respect copyright.ＰＥＮＡＮＡvSluIkHBJT
Date: Sat, 23 Nov 2024 06:56:17 GMT325Please respect copyright.ＰＥＮＡＮＡBE2i5WN4nc
accept-ranges: bytes325Please respect copyright.ＰＥＮＡＮＡQyZGSgjGAj
content-range: bytes 243220480-534231199/534231200325Please respect copyright.ＰＥＮＡＮＡctKJmCWMmJ
Content-Length: 291010720325Please respect copyright.ＰＥＮＡＮＡB5pt9xXik3
Content-Type: application/octet-stream325Please respect copyright.ＰＥＮＡＮＡEcDhSFS196
Content-Disposition: attachment; filename="zen.iso"325Please respect copyright.ＰＥＮＡＮＡ0E2THZg8Tq
ETag: "534231200_1732277116"

(http response body is the file content from resume point to end)325Please respect copyright.ＰＥＮＡＮＡ6BZC0vbcQy
------------------------------------------------325Please respect copyright.ＰＥＮＡＮＡdaa9fi5UBl
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:325Please respect copyright.ＰＥＮＡＮＡisOQjMIHln
-------response content from resume point-------325Please respect copyright.ＰＥＮＡＮＡmrdDepAXTm
HTTP/1.1 200 OK325Please respect copyright.ＰＥＮＡＮＡSFjgE8qKqX
Date: Sat, 23 Nov 2024 06:29:19 GMT325Please respect copyright.ＰＥＮＡＮＡnGMODSblKz
accept-ranges: bytes325Please respect copyright.ＰＥＮＡＮＡz7YHKQXPgP
content-range: bytes 0-634231201/634231202325Please respect copyright.ＰＥＮＡＮＡa8byP9Hxkd
ETag: "634231202_2732277118"325Please respect copyright.ＰＥＮＡＮＡYX7tlSVi2f
Content-Disposition: attachment; filename="zen.iso"325Please respect copyright.ＰＥＮＡＮＡYOVUu7laUO
Content-Length: 634231202325Please respect copyright.ＰＥＮＡＮＡ9zcKRCktG5
Content-Type: application/octet-stream

(http response body is the whole file content)325Please respect copyright.ＰＥＮＡＮＡhCsWFfqBzO
------------------------------------------------325Please respect copyright.ＰＥＮＡＮＡUtMIdjnY7n
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:325Please respect copyright.ＰＥＮＡＮＡ7caLCDzdzX
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325Please respect copyright.ＰＥＮＡＮＡq6dKzjmlbH
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325Please respect copyright.ＰＥＮＡＮＡJsZURy6Eex
https://gemini.google.com/

感谢互联网上无私奉献的参考资325Please respect copyright.ＰＥＮＡＮＡEufxdh9Jzt
https://developer.mozilla.org/en-US/docs/Web/HTTP/Range_requests325Please respect copyright.ＰＥＮＡＮＡNRPzrJIAq0
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/If-Range325Please respect copyright.ＰＥＮＡＮＡRLYuwxKWzv
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/ETag325Please respect copyright.ＰＥＮＡＮＡNaDkfooK1k
https://www.rfc-editor.org/rfc/rfc7233325Please respect copyright.ＰＥＮＡＮＡYyky9sYeMj
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]325Please respect copyright.ＰＥＮＡＮＡCf85qPAyKx
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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