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:97Please respect copyright.ＰＥＮＡＮＡyHXuIkFH3A
----------http range request-------97Please respect copyright.ＰＥＮＡＮＡ5TLNHwBFRX
GET /zen.iso HTTP/1.197Please respect copyright.ＰＥＮＡＮＡGrsDD94WUQ
Range: bytes=100-102397Please respect copyright.ＰＥＮＡＮＡBlugV05ITH
If-Match: "534231200_1732277116"97Please respect copyright.ＰＥＮＡＮＡvzAC5kXvjO
......97Please respect copyright.ＰＥＮＡＮＡw4NZH2E3gI
-----------------------------------97Please respect copyright.ＰＥＮＡＮＡ1sNVw96LZ6
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:97Please respect copyright.ＰＥＮＡＮＡQlddFsZ8B9
----------------html-----------------97Please respect copyright.ＰＥＮＡＮＡYc9e7junXl
<a href="/zen.iso">zen.iso</a>97Please respect copyright.ＰＥＮＡＮＡGbkH2AbFKR
-------------------------------------97Please respect copyright.ＰＥＮＡＮＡpVuPsKc6J9
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:97Please respect copyright.ＰＥＮＡＮＡrji6QjpLuX
----------get request------------97Please respect copyright.ＰＥＮＡＮＡ00RLPXg6p1
GET /zen.iso HTTP/1.197Please respect copyright.ＰＥＮＡＮＡ2ZRC8aB3f3
......97Please respect copyright.ＰＥＮＡＮＡe38RIq35Zq
---------------------------------

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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:97Please respect copyright.ＰＥＮＡＮＡCy82TwCCRk
----------------server response--------------97Please respect copyright.ＰＥＮＡＮＡzoQuWjUc2k
HTTP/1.1 200 OK97Please respect copyright.ＰＥＮＡＮＡZoot7CBAkK
Date: Sat, 23 Nov 2024 06:29:19 GMT97Please respect copyright.ＰＥＮＡＮＡoli0lu5s8E
accept-ranges: bytes97Please respect copyright.ＰＥＮＡＮＡ48HOWtWcHG
content-range: bytes 0-534231199/53423120097Please respect copyright.ＰＥＮＡＮＡ5T1sQ8p0Tc
ETag: "534231200_1732277116"97Please respect copyright.ＰＥＮＡＮＡjWVVIEbQih
Content-Disposition: attachment; filename="zen.iso"97Please respect copyright.ＰＥＮＡＮＡV2TJoLn77J
Content-Length: 53423120097Please respect copyright.ＰＥＮＡＮＡ3wG3TdV9DX
Content-Type: application/octet-stream

(http response body is the whole file content)97Please respect copyright.ＰＥＮＡＮＡUxuRP4ilhU
-------------------------------------------

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:97Please respect copyright.ＰＥＮＡＮＡ1XIO1oUlk7
---------------http server send big file content by multiple times--------------97Please respect copyright.ＰＥＮＡＮＡmJBwMaB4Iv
const int buf_size=20000000; // buf size is about 20M97Please respect copyright.ＰＥＮＡＮＡXOXbcpSI6j
int client_socket; char send_buf[buf_size]; FILE *file;

// first send http response headers97Please respect copyright.ＰＥＮＡＮＡ9QuPmEyLGW
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 big97Please respect copyright.ＰＥＮＡＮＡ2EYFWPlhL2
while(there_is_remain_file_content_to_send)97Please respect copyright.ＰＥＮＡＮＡ6i4Lnd1rHl
{ 97Please respect copyright.ＰＥＮＡＮＡUXSojqgI6Q
read_remain_file_content_to_send_buf(file, send_buf);97Please respect copyright.ＰＥＮＡＮＡCIuvWd7Yif
send_result = send(client_socket, send_buf);97Please respect copyright.ＰＥＮＡＮＡjQl91V4VCf
if(send_result == fail) return; else continue;97Please respect copyright.ＰＥＮＡＮＡcOYuCPZvfj
}

// a way to make etag for a file using its file size and last modifed time97Please respect copyright.ＰＥＮＡＮＡICYHRiSO25
const char *file_etag(const char *path) 97Please respect copyright.ＰＥＮＡＮＡHm4zZBaN2N
{97Please respect copyright.ＰＥＮＡＮＡVqtAT9beQW
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;97Please respect copyright.ＰＥＮＡＮＡnDqtJxaLnI
}97Please respect copyright.ＰＥＮＡＮＡbc0G2OPMz6
-------------------------------------------------97Please respect copyright.ＰＥＮＡＮＡHvCIx4dgnU
"accept-ranges: bytes" tells client web browser that the http server support range.97Please respect copyright.ＰＥＮＡＮＡGq5qFnbqoP
"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).97Please respect copyright.ＰＥＮＡＮＡsWYBj3Fb0N
'ETag: "534231200_1732277116"' tells client web browser the file version.97Please respect copyright.ＰＥＮＡＮＡl8qu2dscFA
'Content-Disposition: attachment; filename="zen.iso"' tells client web browser to download the file as attachment, and its file name.97Please respect copyright.ＰＥＮＡＮＡ71nVghJIin
"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.97Please respect copyright.ＰＥＮＡＮＡURFC3AKC8v
"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.97Please respect copyright.ＰＥＮＡＮＡ8ZE7fM94qB
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.)97Please respect copyright.ＰＥＮＡＮＡQcF0gMbj4r
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:97Please respect copyright.ＰＥＮＡＮＡYDGI0Ku0hy
---------resume range request---------97Please respect copyright.ＰＥＮＡＮＡohFI2XTE0N
GET /zen.iso HTTP/1.197Please respect copyright.ＰＥＮＡＮＡ8rjbibANmc
Range: bytes=243220480-97Please respect copyright.ＰＥＮＡＮＡ4IdDn5ZblY
If-Match: "534231200_1732277116"97Please respect copyright.ＰＥＮＡＮＡ84kmwxtjnV
......97Please respect copyright.ＰＥＮＡＮＡsuQP1EdtcF
-------------------------------------97Please respect copyright.ＰＥＮＡＮＡowaA7gbY6y
"Range: bytes=243220480-" tells the http server to send content from byte 243220480 to end.97Please respect copyright.ＰＥＮＡＮＡ3g2YnMY851
'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:97Please respect copyright.ＰＥＮＡＮＡ73w8p6w3nr
-------response content from resume point-------97Please respect copyright.ＰＥＮＡＮＡ4hdlSqMM5Q
HTTP/1.1 206 Partial Content97Please respect copyright.ＰＥＮＡＮＡRxk7uSx86b
Date: Sat, 23 Nov 2024 06:56:17 GMT97Please respect copyright.ＰＥＮＡＮＡfCMpW3BUZr
accept-ranges: bytes97Please respect copyright.ＰＥＮＡＮＡQs0xNwyBw4
content-range: bytes 243220480-534231199/53423120097Please respect copyright.ＰＥＮＡＮＡH6anpq0JgG
Content-Length: 29101072097Please respect copyright.ＰＥＮＡＮＡtCkTS8NW72
Content-Type: application/octet-stream97Please respect copyright.ＰＥＮＡＮＡDxF2SzluCi
Content-Disposition: attachment; filename="zen.iso"97Please respect copyright.ＰＥＮＡＮＡkX16z5xOot
ETag: "534231200_1732277116"

(http response body is the file content from resume point to end)97Please respect copyright.ＰＥＮＡＮＡfO1QpKE6CE
------------------------------------------------97Please respect copyright.ＰＥＮＡＮＡYJGmbMCXAl
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:97Please respect copyright.ＰＥＮＡＮＡpQKx2GHLyx
-------response content from resume point-------97Please respect copyright.ＰＥＮＡＮＡE9hfW6U0Mn
HTTP/1.1 200 OK97Please respect copyright.ＰＥＮＡＮＡRGqFTENTd7
Date: Sat, 23 Nov 2024 06:29:19 GMT97Please respect copyright.ＰＥＮＡＮＡHN0hgcx2Ap
accept-ranges: bytes97Please respect copyright.ＰＥＮＡＮＡl65Tfsiy2S
content-range: bytes 0-634231201/63423120297Please respect copyright.ＰＥＮＡＮＡaqmEJpXv8T
ETag: "634231202_2732277118"97Please respect copyright.ＰＥＮＡＮＡynnUfORIRd
Content-Disposition: attachment; filename="zen.iso"97Please respect copyright.ＰＥＮＡＮＡp7pTM6FJ5V
Content-Length: 63423120297Please respect copyright.ＰＥＮＡＮＡXUWGAWBut3
Content-Type: application/octet-stream

(http response body is the whole file content)97Please respect copyright.ＰＥＮＡＮＡzjIFxjnZal
------------------------------------------------97Please respect copyright.ＰＥＮＡＮＡvheSQXfKJx
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:97Please respect copyright.ＰＥＮＡＮＡyXkkopG1Kx
https://www.zeng.dev/post/2023-http-range-and-play-mp4-in-browser/?

97Please respect copyright.ＰＥＮＡＮＡoFUsSlleCQ
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97Please respect copyright.ＰＥＮＡＮＡrjBHXyt1Pr
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97Please respect copyright.ＰＥＮＡＮＡhLFZDQjEuW
https://gemini.google.com/

感谢互联网上无私奉献的参考资97Please respect copyright.ＰＥＮＡＮＡCaD61gXCFo
https://developer.mozilla.org/en-US/docs/Web/HTTP/Range_requests97Please respect copyright.ＰＥＮＡＮＡCe0MndhP6y
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/If-Range97Please respect copyright.ＰＥＮＡＮＡrRmNV0yeUu
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/ETag97Please respect copyright.ＰＥＮＡＮＡnfvunIrs2e
https://www.rfc-editor.org/rfc/rfc723397Please respect copyright.ＰＥＮＡＮＡhvAStsvwEg
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]97Please respect copyright.ＰＥＮＡＮＡbUZ8pMPn5V
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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