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:92Please respect copyright.ＰＥＮＡＮＡECbn5fNsrX
----------http range request-------92Please respect copyright.ＰＥＮＡＮＡwRE82YriKJ
GET /zen.iso HTTP/1.192Please respect copyright.ＰＥＮＡＮＡawrUUetzjP
Range: bytes=100-102392Please respect copyright.ＰＥＮＡＮＡLlMb4TOTGc
If-Match: "534231200_1732277116"92Please respect copyright.ＰＥＮＡＮＡ5EfasVtmMa
......92Please respect copyright.ＰＥＮＡＮＡaEGLvhYtBV
-----------------------------------92Please respect copyright.ＰＥＮＡＮＡ1HlfQzq04i
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:92Please respect copyright.ＰＥＮＡＮＡUQMQ641bZc
----------------html-----------------92Please respect copyright.ＰＥＮＡＮＡxCpamwpm7Q
<a href="/zen.iso">zen.iso</a>92Please respect copyright.ＰＥＮＡＮＡ6n4VLPlGrm
-------------------------------------92Please respect copyright.ＰＥＮＡＮＡ92NXABvmTo
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:92Please respect copyright.ＰＥＮＡＮＡ9hdmaa9tpR
----------get request------------92Please respect copyright.ＰＥＮＡＮＡbKAZyRvL5r
GET /zen.iso HTTP/1.192Please respect copyright.ＰＥＮＡＮＡcX1jc24IW0
......92Please respect copyright.ＰＥＮＡＮＡHoIrTiZ5RE
---------------------------------

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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:92Please respect copyright.ＰＥＮＡＮＡMD3HDrdV84
----------------server response--------------92Please respect copyright.ＰＥＮＡＮＡfoQy0HD5tB
HTTP/1.1 200 OK92Please respect copyright.ＰＥＮＡＮＡJHT7ovOiN1
Date: Sat, 23 Nov 2024 06:29:19 GMT92Please respect copyright.ＰＥＮＡＮＡJ8F2R9Fx8H
accept-ranges: bytes92Please respect copyright.ＰＥＮＡＮＡB82YA2vKeb
content-range: bytes 0-534231199/53423120092Please respect copyright.ＰＥＮＡＮＡdYs5SL8X8d
ETag: "534231200_1732277116"92Please respect copyright.ＰＥＮＡＮＡV3flN5T9Ao
Content-Disposition: attachment; filename="zen.iso"92Please respect copyright.ＰＥＮＡＮＡjFIlBwrsM2
Content-Length: 53423120092Please respect copyright.ＰＥＮＡＮＡH2c6Kwqczy
Content-Type: application/octet-stream

(http response body is the whole file content)92Please respect copyright.ＰＥＮＡＮＡ4RDZyQS6yt
-------------------------------------------

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:92Please respect copyright.ＰＥＮＡＮＡPTQLBwu8kF
---------------http server send big file content by multiple times--------------92Please respect copyright.ＰＥＮＡＮＡzGRaxY0xjM
const int buf_size=20000000; // buf size is about 20M92Please respect copyright.ＰＥＮＡＮＡQBVqjzm6R8
int client_socket; char send_buf[buf_size]; FILE *file;

// first send http response headers92Please respect copyright.ＰＥＮＡＮＡGRjcgFYzz9
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 big92Please respect copyright.ＰＥＮＡＮＡML0subl64J
while(there_is_remain_file_content_to_send)92Please respect copyright.ＰＥＮＡＮＡVbfg7rluZq
{ 92Please respect copyright.ＰＥＮＡＮＡKPW0PtDKVz
read_remain_file_content_to_send_buf(file, send_buf);92Please respect copyright.ＰＥＮＡＮＡRKIjvxaQsB
send_result = send(client_socket, send_buf);92Please respect copyright.ＰＥＮＡＮＡm1WuLMNNrO
if(send_result == fail) return; else continue;92Please respect copyright.ＰＥＮＡＮＡfKz7cWXuI9
}

// a way to make etag for a file using its file size and last modifed time92Please respect copyright.ＰＥＮＡＮＡONIaqzUe09
const char *file_etag(const char *path) 92Please respect copyright.ＰＥＮＡＮＡB0PuH3cI6G
{92Please respect copyright.ＰＥＮＡＮＡ2sg7Ac9P6A
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;92Please respect copyright.ＰＥＮＡＮＡAnO9rF1yD5
}92Please respect copyright.ＰＥＮＡＮＡY9RQo1G6bq
-------------------------------------------------92Please respect copyright.ＰＥＮＡＮＡW4zga7bSeB
"accept-ranges: bytes" tells client web browser that the http server support range.92Please respect copyright.ＰＥＮＡＮＡwefi7tUaow
"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).92Please respect copyright.ＰＥＮＡＮＡAWV8r6uH7e
'ETag: "534231200_1732277116"' tells client web browser the file version.92Please respect copyright.ＰＥＮＡＮＡKgjAr4ZecK
'Content-Disposition: attachment; filename="zen.iso"' tells client web browser to download the file as attachment, and its file name.92Please respect copyright.ＰＥＮＡＮＡjwEqVAfEOk
"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.92Please respect copyright.ＰＥＮＡＮＡKlR3gByo9o
"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.92Please respect copyright.ＰＥＮＡＮＡBjioERhtCd
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.)92Please respect copyright.ＰＥＮＡＮＡY3waTDKvS2
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:92Please respect copyright.ＰＥＮＡＮＡyTkBNpw5Cv
---------resume range request---------92Please respect copyright.ＰＥＮＡＮＡodLaEs0PAv
GET /zen.iso HTTP/1.192Please respect copyright.ＰＥＮＡＮＡAq25cfvAnh
Range: bytes=243220480-92Please respect copyright.ＰＥＮＡＮＡWmGVRj8gPI
If-Match: "534231200_1732277116"92Please respect copyright.ＰＥＮＡＮＡiifxeZwhPA
......92Please respect copyright.ＰＥＮＡＮＡu66G54LIkR
-------------------------------------92Please respect copyright.ＰＥＮＡＮＡX6pvP7sjDD
"Range: bytes=243220480-" tells the http server to send content from byte 243220480 to end.92Please respect copyright.ＰＥＮＡＮＡ8IgPpA89Ds
'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:92Please respect copyright.ＰＥＮＡＮＡxDxs1sZcEm
-------response content from resume point-------92Please respect copyright.ＰＥＮＡＮＡF0eS7vO2Rt
HTTP/1.1 206 Partial Content92Please respect copyright.ＰＥＮＡＮＡ1Ui8Bzb8ea
Date: Sat, 23 Nov 2024 06:56:17 GMT92Please respect copyright.ＰＥＮＡＮＡlmWBFiptep
accept-ranges: bytes92Please respect copyright.ＰＥＮＡＮＡjk68nmAdEc
content-range: bytes 243220480-534231199/53423120092Please respect copyright.ＰＥＮＡＮＡpAgWtpfuKu
Content-Length: 29101072092Please respect copyright.ＰＥＮＡＮＡCgeeyTuWsg
Content-Type: application/octet-stream92Please respect copyright.ＰＥＮＡＮＡ5EPB6X9rWs
Content-Disposition: attachment; filename="zen.iso"92Please respect copyright.ＰＥＮＡＮＡgwmXe4NLmB
ETag: "534231200_1732277116"

(http response body is the file content from resume point to end)92Please respect copyright.ＰＥＮＡＮＡQ1Dh24ZpRt
------------------------------------------------92Please respect copyright.ＰＥＮＡＮＡRrT2QvRwbo
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:92Please respect copyright.ＰＥＮＡＮＡ31UqdDQrNr
-------response content from resume point-------92Please respect copyright.ＰＥＮＡＮＡ2ROaglIg4p
HTTP/1.1 200 OK92Please respect copyright.ＰＥＮＡＮＡTOkpUsQiDS
Date: Sat, 23 Nov 2024 06:29:19 GMT92Please respect copyright.ＰＥＮＡＮＡBuw9edoB0S
accept-ranges: bytes92Please respect copyright.ＰＥＮＡＮＡfXgWvjzBaq
content-range: bytes 0-634231201/63423120292Please respect copyright.ＰＥＮＡＮＡ0gO60owU7F
ETag: "634231202_2732277118"92Please respect copyright.ＰＥＮＡＮＡQ7bR8bbhXp
Content-Disposition: attachment; filename="zen.iso"92Please respect copyright.ＰＥＮＡＮＡdjsWnBcTZW
Content-Length: 63423120292Please respect copyright.ＰＥＮＡＮＡDIzDzogb5G
Content-Type: application/octet-stream

(http response body is the whole file content)92Please respect copyright.ＰＥＮＡＮＡY4MYH2IvWB
------------------------------------------------92Please respect copyright.ＰＥＮＡＮＡxRgx21rLZN
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:92Please respect copyright.ＰＥＮＡＮＡHo75dZWAYk
https://www.zeng.dev/post/2023-http-range-and-play-mp4-in-browser/?

92Please respect copyright.ＰＥＮＡＮＡMuE8RDHERt
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92Please respect copyright.ＰＥＮＡＮＡTXLV0RwEpa
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92Please respect copyright.ＰＥＮＡＮＡ5aEBmeJnDk
https://gemini.google.com/

感谢互联网上无私奉献的参考资92Please respect copyright.ＰＥＮＡＮＡ2fLXdIR9ke
https://developer.mozilla.org/en-US/docs/Web/HTTP/Range_requests92Please respect copyright.ＰＥＮＡＮＡOwY2yldja8
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/If-Range92Please respect copyright.ＰＥＮＡＮＡ8Ac4nX6FVo
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/ETag92Please respect copyright.ＰＥＮＡＮＡrGqqmLZSrQ
https://www.rfc-editor.org/rfc/rfc723392Please respect copyright.ＰＥＮＡＮＡof0RZ5T12T
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]92Please respect copyright.ＰＥＮＡＮＡ3UPDI0RRhF
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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