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:129Please respect copyright.ＰＥＮＡＮＡP88SE0S21G
----------http range request-------129Please respect copyright.ＰＥＮＡＮＡN2hECv9m0r
GET /zen.iso HTTP/1.1129Please respect copyright.ＰＥＮＡＮＡKng352jUlp
Range: bytes=100-1023129Please respect copyright.ＰＥＮＡＮＡdLRz4PXyjk
If-Match: "534231200_1732277116"129Please respect copyright.ＰＥＮＡＮＡtHGvHQOpFU
......129Please respect copyright.ＰＥＮＡＮＡJu9QGEQYxa
-----------------------------------129Please respect copyright.ＰＥＮＡＮＡZBhmbQ3cph
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:129Please respect copyright.ＰＥＮＡＮＡJtEPfIYtMG
----------------html-----------------129Please respect copyright.ＰＥＮＡＮＡEAK9O4HwiB
<a href="/zen.iso">zen.iso</a>129Please respect copyright.ＰＥＮＡＮＡC3Ah1PdnVa
-------------------------------------129Please respect copyright.ＰＥＮＡＮＡ7CQ4f1yPhU
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:129Please respect copyright.ＰＥＮＡＮＡ2IBExcniY6
----------get request------------129Please respect copyright.ＰＥＮＡＮＡSbeZ2jS4Qf
GET /zen.iso HTTP/1.1129Please respect copyright.ＰＥＮＡＮＡ6eCJLIiN3C
......129Please respect copyright.ＰＥＮＡＮＡSIcj0QYDdN
---------------------------------

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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:129Please respect copyright.ＰＥＮＡＮＡX4ENz4Ao4A
----------------server response--------------129Please respect copyright.ＰＥＮＡＮＡy9KkJbYIzt
HTTP/1.1 200 OK129Please respect copyright.ＰＥＮＡＮＡYmGUusZG8Q
Date: Sat, 23 Nov 2024 06:29:19 GMT129Please respect copyright.ＰＥＮＡＮＡ0RbSuW1VrG
accept-ranges: bytes129Please respect copyright.ＰＥＮＡＮＡ0l42lGAh6h
content-range: bytes 0-534231199/534231200129Please respect copyright.ＰＥＮＡＮＡQpKgnTZoHH
ETag: "534231200_1732277116"129Please respect copyright.ＰＥＮＡＮＡnkctHTUy0z
Content-Disposition: attachment; filename="zen.iso"129Please respect copyright.ＰＥＮＡＮＡDPItfHDidW
Content-Length: 534231200129Please respect copyright.ＰＥＮＡＮＡA53pVikGpe
Content-Type: application/octet-stream

(http response body is the whole file content)129Please respect copyright.ＰＥＮＡＮＡY5TeZe07Y4
-------------------------------------------

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:129Please respect copyright.ＰＥＮＡＮＡZEDvQIvE0w
---------------http server send big file content by multiple times--------------129Please respect copyright.ＰＥＮＡＮＡIb01Vsg2DJ
const int buf_size=20000000; // buf size is about 20M129Please respect copyright.ＰＥＮＡＮＡpXhYmgsNMZ
int client_socket; char send_buf[buf_size]; FILE *file;

// first send http response headers129Please respect copyright.ＰＥＮＡＮＡdEyFVqspNG
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 big129Please respect copyright.ＰＥＮＡＮＡ39iP6Exw09
while(there_is_remain_file_content_to_send)129Please respect copyright.ＰＥＮＡＮＡswMfye3LDK
{ 129Please respect copyright.ＰＥＮＡＮＡXblJFNtUL4
read_remain_file_content_to_send_buf(file, send_buf);129Please respect copyright.ＰＥＮＡＮＡLmLXAjNxOa
send_result = send(client_socket, send_buf);129Please respect copyright.ＰＥＮＡＮＡObx4kTr6F9
if(send_result == fail) return; else continue;129Please respect copyright.ＰＥＮＡＮＡV8o15CmqIp
}

// a way to make etag for a file using its file size and last modifed time129Please respect copyright.ＰＥＮＡＮＡHMtsI66hOD
const char *file_etag(const char *path) 129Please respect copyright.ＰＥＮＡＮＡeVDsM7mNmg
{129Please respect copyright.ＰＥＮＡＮＡ339vOkmRXi
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;129Please respect copyright.ＰＥＮＡＮＡWDJlMRJJDM
}129Please respect copyright.ＰＥＮＡＮＡjkn3ymW0Jp
-------------------------------------------------129Please respect copyright.ＰＥＮＡＮＡzKmYYJlkwB
"accept-ranges: bytes" tells client web browser that the http server support range.129Please respect copyright.ＰＥＮＡＮＡrj6eKNL7RU
"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).129Please respect copyright.ＰＥＮＡＮＡQ1EgUKUqZ5
'ETag: "534231200_1732277116"' tells client web browser the file version.129Please respect copyright.ＰＥＮＡＮＡXUOfFFNSsn
'Content-Disposition: attachment; filename="zen.iso"' tells client web browser to download the file as attachment, and its file name.129Please respect copyright.ＰＥＮＡＮＡWkjHadFz1L
"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.129Please respect copyright.ＰＥＮＡＮＡDDoTJR91tr
"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.129Please respect copyright.ＰＥＮＡＮＡD2doNSSJSO
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.)129Please respect copyright.ＰＥＮＡＮＡpW7di2ukfn
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:129Please respect copyright.ＰＥＮＡＮＡqINW8pNtX8
---------resume range request---------129Please respect copyright.ＰＥＮＡＮＡFrBNFjc5TH
GET /zen.iso HTTP/1.1129Please respect copyright.ＰＥＮＡＮＡUbIadux6J4
Range: bytes=243220480-129Please respect copyright.ＰＥＮＡＮＡlTdgwvxa0R
If-Match: "534231200_1732277116"129Please respect copyright.ＰＥＮＡＮＡGryRxGOqK2
......129Please respect copyright.ＰＥＮＡＮＡH84whw0Tu5
-------------------------------------129Please respect copyright.ＰＥＮＡＮＡpy34xTmiaJ
"Range: bytes=243220480-" tells the http server to send content from byte 243220480 to end.129Please respect copyright.ＰＥＮＡＮＡn7ZuZPliJh
'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:129Please respect copyright.ＰＥＮＡＮＡPJxvjpSDfa
-------response content from resume point-------129Please respect copyright.ＰＥＮＡＮＡRW2LTve9Ra
HTTP/1.1 206 Partial Content129Please respect copyright.ＰＥＮＡＮＡYWKZ96Sa3J
Date: Sat, 23 Nov 2024 06:56:17 GMT129Please respect copyright.ＰＥＮＡＮＡtuMiZGsFiJ
accept-ranges: bytes129Please respect copyright.ＰＥＮＡＮＡLNCcruc6Ig
content-range: bytes 243220480-534231199/534231200129Please respect copyright.ＰＥＮＡＮＡdPRPzqlbsb
Content-Length: 291010720129Please respect copyright.ＰＥＮＡＮＡOFIvV7mPsM
Content-Type: application/octet-stream129Please respect copyright.ＰＥＮＡＮＡiwGzdwKWDH
Content-Disposition: attachment; filename="zen.iso"129Please respect copyright.ＰＥＮＡＮＡXIKkvkWsEl
ETag: "534231200_1732277116"

(http response body is the file content from resume point to end)129Please respect copyright.ＰＥＮＡＮＡstMbdCHoht
------------------------------------------------129Please respect copyright.ＰＥＮＡＮＡuiFkT375yW
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:129Please respect copyright.ＰＥＮＡＮＡ0648jUyDcy
-------response content from resume point-------129Please respect copyright.ＰＥＮＡＮＡSBivbXDVxP
HTTP/1.1 200 OK129Please respect copyright.ＰＥＮＡＮＡmlwNTPrDDC
Date: Sat, 23 Nov 2024 06:29:19 GMT129Please respect copyright.ＰＥＮＡＮＡyBXAsFR9H0
accept-ranges: bytes129Please respect copyright.ＰＥＮＡＮＡC5uASHTuE2
content-range: bytes 0-634231201/634231202129Please respect copyright.ＰＥＮＡＮＡe8Q5LvHiQX
ETag: "634231202_2732277118"129Please respect copyright.ＰＥＮＡＮＡAIJa5V55lC
Content-Disposition: attachment; filename="zen.iso"129Please respect copyright.ＰＥＮＡＮＡpTnNAgOmRw
Content-Length: 634231202129Please respect copyright.ＰＥＮＡＮＡdVZDP7GGBK
Content-Type: application/octet-stream

(http response body is the whole file content)129Please respect copyright.ＰＥＮＡＮＡiP2PN6dkTq
------------------------------------------------129Please respect copyright.ＰＥＮＡＮＡbCiIdIghnc
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:129Please respect copyright.ＰＥＮＡＮＡFmwExTRWC5
https://www.zeng.dev/post/2023-http-range-and-play-mp4-in-browser/?

129Please respect copyright.ＰＥＮＡＮＡVNz0oZfFN1
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129Please respect copyright.ＰＥＮＡＮＡbHw05yfhJw
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129Please respect copyright.ＰＥＮＡＮＡjR0QzTxWgy
https://gemini.google.com/

感谢互联网上无私奉献的参考资129Please respect copyright.ＰＥＮＡＮＡ2mjDcrEEEk
https://developer.mozilla.org/en-US/docs/Web/HTTP/Range_requests129Please respect copyright.ＰＥＮＡＮＡpY5tpaPrkX
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/If-Range129Please respect copyright.ＰＥＮＡＮＡ4gXILWa4hh
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/ETag129Please respect copyright.ＰＥＮＡＮＡl7nga4t57G
https://www.rfc-editor.org/rfc/rfc7233129Please respect copyright.ＰＥＮＡＮＡG5e6EoLNKU
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]129Please respect copyright.ＰＥＮＡＮＡdvV1UmKYlg
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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