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.ＰＥＮＡＮＡuQg1g1UN82
----------http range request-------199Please respect copyright.ＰＥＮＡＮＡY0FmQmIOQx
GET /zen.iso HTTP/1.1199Please respect copyright.ＰＥＮＡＮＡBh0bL4pcHx
Range: bytes=100-1023199Please respect copyright.ＰＥＮＡＮＡQ5CRBIYjW5
If-Match: "534231200_1732277116"199Please respect copyright.ＰＥＮＡＮＡWAajsQzUHq
......199Please respect copyright.ＰＥＮＡＮＡsoMrZMnZVq
-----------------------------------199Please respect copyright.ＰＥＮＡＮＡrHhCcYynjB
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.ＰＥＮＡＮＡvm2QxFDHK5
----------------html-----------------199Please respect copyright.ＰＥＮＡＮＡjAwLSPoieR
<a href="/zen.iso">zen.iso</a>199Please respect copyright.ＰＥＮＡＮＡP6kuAHfIr5
-------------------------------------199Please respect copyright.ＰＥＮＡＮＡbJUYOm1QZC
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.ＰＥＮＡＮＡsBY354B8D4
----------get request------------199Please respect copyright.ＰＥＮＡＮＡjl8Xc9aRYN
GET /zen.iso HTTP/1.1199Please respect copyright.ＰＥＮＡＮＡurl38UXmJu
......199Please respect copyright.ＰＥＮＡＮＡsWwgUsOvlR
---------------------------------

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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.ＰＥＮＡＮＡiMLvI94xo5
----------------server response--------------199Please respect copyright.ＰＥＮＡＮＡQq3dKZRgSp
HTTP/1.1 200 OK199Please respect copyright.ＰＥＮＡＮＡgX520aRjSP
Date: Sat, 23 Nov 2024 06:29:19 GMT199Please respect copyright.ＰＥＮＡＮＡxrNPrXGRUG
accept-ranges: bytes199Please respect copyright.ＰＥＮＡＮＡhk5TEg0pG8
content-range: bytes 0-534231199/534231200199Please respect copyright.ＰＥＮＡＮＡnm0UlL2l2u
ETag: "534231200_1732277116"199Please respect copyright.ＰＥＮＡＮＡx8wS3H9r5s
Content-Disposition: attachment; filename="zen.iso"199Please respect copyright.ＰＥＮＡＮＡDp3ZG1y7Dy
Content-Length: 534231200199Please respect copyright.ＰＥＮＡＮＡbf1xkOilvk
Content-Type: application/octet-stream

(http response body is the whole file content)199Please respect copyright.ＰＥＮＡＮＡYEDomwQh8X
-------------------------------------------

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.ＰＥＮＡＮＡvk6D4eIQLZ
---------------http server send big file content by multiple times--------------199Please respect copyright.ＰＥＮＡＮＡhlPm5mjzjn
const int buf_size=20000000; // buf size is about 20M199Please respect copyright.ＰＥＮＡＮＡwcgokYoXYb
int client_socket; char send_buf[buf_size]; FILE *file;

// first send http response headers199Please respect copyright.ＰＥＮＡＮＡdIx3ijHHlI
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.ＰＥＮＡＮＡOUVOs1I5kt
while(there_is_remain_file_content_to_send)199Please respect copyright.ＰＥＮＡＮＡVboRLkixsx
{ 199Please respect copyright.ＰＥＮＡＮＡyO8300pfPI
read_remain_file_content_to_send_buf(file, send_buf);199Please respect copyright.ＰＥＮＡＮＡ145GOKiTNo
send_result = send(client_socket, send_buf);199Please respect copyright.ＰＥＮＡＮＡJW4HUk4qrh
if(send_result == fail) return; else continue;199Please respect copyright.ＰＥＮＡＮＡnRNaIxBpj5
}

// a way to make etag for a file using its file size and last modifed time199Please respect copyright.ＰＥＮＡＮＡQ6aMb8eWJZ
const char *file_etag(const char *path) 199Please respect copyright.ＰＥＮＡＮＡB9yh2N1HlX
{199Please respect copyright.ＰＥＮＡＮＡglMgBfjubS
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.ＰＥＮＡＮＡcjeHsoCSv4
}199Please respect copyright.ＰＥＮＡＮＡzA1F7b8FiV
-------------------------------------------------199Please respect copyright.ＰＥＮＡＮＡ8vevgp1emR
"accept-ranges: bytes" tells client web browser that the http server support range.199Please respect copyright.ＰＥＮＡＮＡANz1jn3mJs
"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.ＰＥＮＡＮＡpD8v25wgsv
'ETag: "534231200_1732277116"' tells client web browser the file version.199Please respect copyright.ＰＥＮＡＮＡp92I7UYfbM
'Content-Disposition: attachment; filename="zen.iso"' tells client web browser to download the file as attachment, and its file name.199Please respect copyright.ＰＥＮＡＮＡ3O7iCbH2rC
"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.ＰＥＮＡＮＡyhDoOFe5hu
"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.ＰＥＮＡＮＡpBK3Zzxadp
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.ＰＥＮＡＮＡLgx4Ddas5g
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.ＰＥＮＡＮＡ84ptVC348w
---------resume range request---------199Please respect copyright.ＰＥＮＡＮＡPvhAh8p70A
GET /zen.iso HTTP/1.1199Please respect copyright.ＰＥＮＡＮＡaWnnOKdLNG
Range: bytes=243220480-199Please respect copyright.ＰＥＮＡＮＡU1voq8eyqw
If-Match: "534231200_1732277116"199Please respect copyright.ＰＥＮＡＮＡocZI4GRLVW
......199Please respect copyright.ＰＥＮＡＮＡKTD2gVYxZA
-------------------------------------199Please respect copyright.ＰＥＮＡＮＡBFE2sHCvC6
"Range: bytes=243220480-" tells the http server to send content from byte 243220480 to end.199Please respect copyright.ＰＥＮＡＮＡsfYQmNbpMt
'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.ＰＥＮＡＮＡpyR9c0720U
-------response content from resume point-------199Please respect copyright.ＰＥＮＡＮＡzZiiTEMJeB
HTTP/1.1 206 Partial Content199Please respect copyright.ＰＥＮＡＮＡzSKzHe0Yxw
Date: Sat, 23 Nov 2024 06:56:17 GMT199Please respect copyright.ＰＥＮＡＮＡOxjoW0ca1c
accept-ranges: bytes199Please respect copyright.ＰＥＮＡＮＡQtZpIPGnAt
content-range: bytes 243220480-534231199/534231200199Please respect copyright.ＰＥＮＡＮＡE3jWAJ94lq
Content-Length: 291010720199Please respect copyright.ＰＥＮＡＮＡe8D7AI7rCP
Content-Type: application/octet-stream199Please respect copyright.ＰＥＮＡＮＡMGxwFyWcGk
Content-Disposition: attachment; filename="zen.iso"199Please respect copyright.ＰＥＮＡＮＡnm6LWZYJFU
ETag: "534231200_1732277116"

(http response body is the file content from resume point to end)199Please respect copyright.ＰＥＮＡＮＡuydrYaYG8b
------------------------------------------------199Please respect copyright.ＰＥＮＡＮＡyG42KuQy4q
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.ＰＥＮＡＮＡ0K6hwWPRVr
-------response content from resume point-------199Please respect copyright.ＰＥＮＡＮＡnjBo2MOhOV
HTTP/1.1 200 OK199Please respect copyright.ＰＥＮＡＮＡF01YvjurzS
Date: Sat, 23 Nov 2024 06:29:19 GMT199Please respect copyright.ＰＥＮＡＮＡ06N1N4x92n
accept-ranges: bytes199Please respect copyright.ＰＥＮＡＮＡjvfN7ZtGfm
content-range: bytes 0-634231201/634231202199Please respect copyright.ＰＥＮＡＮＡlqfOpwmui1
ETag: "634231202_2732277118"199Please respect copyright.ＰＥＮＡＮＡ7elVmJF3QF
Content-Disposition: attachment; filename="zen.iso"199Please respect copyright.ＰＥＮＡＮＡoz1ppSnpMy
Content-Length: 634231202199Please respect copyright.ＰＥＮＡＮＡ1OidSWZmoi
Content-Type: application/octet-stream

(http response body is the whole file content)199Please respect copyright.ＰＥＮＡＮＡXPlMASCyqu
------------------------------------------------199Please respect copyright.ＰＥＮＡＮＡLZWn1urtVT
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.ＰＥＮＡＮＡuzX4e8iZ4H
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.ＰＥＮＡＮＡ25HxetqHhp
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.ＰＥＮＡＮＡMabqwQCnuL
https://gemini.google.com/

感谢互联网上无私奉献的参考资199Please respect copyright.ＰＥＮＡＮＡk3d7MPWmos
https://developer.mozilla.org/en-US/docs/Web/HTTP/Range_requests199Please respect copyright.ＰＥＮＡＮＡq8rvTUk9Jn
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/If-Range199Please respect copyright.ＰＥＮＡＮＡpdG1dicZ6O
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/ETag199Please respect copyright.ＰＥＮＡＮＡ143F9lNvoG
https://www.rfc-editor.org/rfc/rfc7233199Please respect copyright.ＰＥＮＡＮＡcJmoLgNnOL
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.ＰＥＮＡＮＡWYcluVDJQp
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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