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:131Please respect copyright.ＰＥＮＡＮＡesDIVBnYCx
----------http range request-------131Please respect copyright.ＰＥＮＡＮＡpVfDlWTRsY
GET /zen.iso HTTP/1.1131Please respect copyright.ＰＥＮＡＮＡsHarzcwcT8
Range: bytes=100-1023131Please respect copyright.ＰＥＮＡＮＡmfpe4Uc4T6
If-Match: "534231200_1732277116"131Please respect copyright.ＰＥＮＡＮＡjGVexbMixn
......131Please respect copyright.ＰＥＮＡＮＡOJvsLlLpOE
-----------------------------------131Please respect copyright.ＰＥＮＡＮＡakbqogXfG2
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:131Please respect copyright.ＰＥＮＡＮＡxYBwUdAidJ
----------------html-----------------131Please respect copyright.ＰＥＮＡＮＡxEYqQM8Av8
<a href="/zen.iso">zen.iso</a>131Please respect copyright.ＰＥＮＡＮＡBrEQsvahX2
-------------------------------------131Please respect copyright.ＰＥＮＡＮＡ1PJHkmppqd
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:131Please respect copyright.ＰＥＮＡＮＡnhir5JJTGq
----------get request------------131Please respect copyright.ＰＥＮＡＮＡjc025ANLAR
GET /zen.iso HTTP/1.1131Please respect copyright.ＰＥＮＡＮＡpAhmvhtGCh
......131Please respect copyright.ＰＥＮＡＮＡ7SYLGm9akd
---------------------------------

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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:131Please respect copyright.ＰＥＮＡＮＡbe2rRBd8av
----------------server response--------------131Please respect copyright.ＰＥＮＡＮＡTMQhhd4o4r
HTTP/1.1 200 OK131Please respect copyright.ＰＥＮＡＮＡJxbXgaJsg6
Date: Sat, 23 Nov 2024 06:29:19 GMT131Please respect copyright.ＰＥＮＡＮＡr6iLajWdGm
accept-ranges: bytes131Please respect copyright.ＰＥＮＡＮＡAfhXb8Jtlv
content-range: bytes 0-534231199/534231200131Please respect copyright.ＰＥＮＡＮＡUcKgfRwyv7
ETag: "534231200_1732277116"131Please respect copyright.ＰＥＮＡＮＡbkv1YPxnbN
Content-Disposition: attachment; filename="zen.iso"131Please respect copyright.ＰＥＮＡＮＡXYRuuNdKKk
Content-Length: 534231200131Please respect copyright.ＰＥＮＡＮＡXk6skdX6fy
Content-Type: application/octet-stream

(http response body is the whole file content)131Please respect copyright.ＰＥＮＡＮＡblFskBd76b
-------------------------------------------

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:131Please respect copyright.ＰＥＮＡＮＡsPWHLpJ0i4
---------------http server send big file content by multiple times--------------131Please respect copyright.ＰＥＮＡＮＡiKaMJ6PUsB
const int buf_size=20000000; // buf size is about 20M131Please respect copyright.ＰＥＮＡＮＡFIcNZ6petq
int client_socket; char send_buf[buf_size]; FILE *file;

// first send http response headers131Please respect copyright.ＰＥＮＡＮＡpCQABtU0bw
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 big131Please respect copyright.ＰＥＮＡＮＡ1TlDf95kWY
while(there_is_remain_file_content_to_send)131Please respect copyright.ＰＥＮＡＮＡureXAudwBZ
{ 131Please respect copyright.ＰＥＮＡＮＡ0EEZLttOiC
read_remain_file_content_to_send_buf(file, send_buf);131Please respect copyright.ＰＥＮＡＮＡZOHeQ5Zy3h
send_result = send(client_socket, send_buf);131Please respect copyright.ＰＥＮＡＮＡFnUKEro0fj
if(send_result == fail) return; else continue;131Please respect copyright.ＰＥＮＡＮＡhRhpKzpKJu
}

// a way to make etag for a file using its file size and last modifed time131Please respect copyright.ＰＥＮＡＮＡdcy9tRLsbN
const char *file_etag(const char *path) 131Please respect copyright.ＰＥＮＡＮＡRU3N6il4OI
{131Please respect copyright.ＰＥＮＡＮＡYMHe05ZsAm
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;131Please respect copyright.ＰＥＮＡＮＡ2PbMGlzl4b
}131Please respect copyright.ＰＥＮＡＮＡVuwdo8WmUt
-------------------------------------------------131Please respect copyright.ＰＥＮＡＮＡVHci7Pr2jz
"accept-ranges: bytes" tells client web browser that the http server support range.131Please respect copyright.ＰＥＮＡＮＡCeCHx5Zcfe
"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).131Please respect copyright.ＰＥＮＡＮＡffTDkmbUNP
'ETag: "534231200_1732277116"' tells client web browser the file version.131Please respect copyright.ＰＥＮＡＮＡjOlxcngzYJ
'Content-Disposition: attachment; filename="zen.iso"' tells client web browser to download the file as attachment, and its file name.131Please respect copyright.ＰＥＮＡＮＡNaeHN9S0Q3
"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.131Please respect copyright.ＰＥＮＡＮＡFevFEQQroW
"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.131Please respect copyright.ＰＥＮＡＮＡ7ycEjdPUQA
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.)131Please respect copyright.ＰＥＮＡＮＡ0q06QWd91L
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:131Please respect copyright.ＰＥＮＡＮＡVOBRNFGHcY
---------resume range request---------131Please respect copyright.ＰＥＮＡＮＡPPCRpD8cpN
GET /zen.iso HTTP/1.1131Please respect copyright.ＰＥＮＡＮＡNTKx5trMs1
Range: bytes=243220480-131Please respect copyright.ＰＥＮＡＮＡ3q64IKMImc
If-Match: "534231200_1732277116"131Please respect copyright.ＰＥＮＡＮＡqG0j8INBT7
......131Please respect copyright.ＰＥＮＡＮＡQ0nt1W3H57
-------------------------------------131Please respect copyright.ＰＥＮＡＮＡ2TywB9gtQE
"Range: bytes=243220480-" tells the http server to send content from byte 243220480 to end.131Please respect copyright.ＰＥＮＡＮＡkq4hziYfy7
'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:131Please respect copyright.ＰＥＮＡＮＡQFnN41eOTl
-------response content from resume point-------131Please respect copyright.ＰＥＮＡＮＡNjKgAPjoal
HTTP/1.1 206 Partial Content131Please respect copyright.ＰＥＮＡＮＡu70K5zhrbK
Date: Sat, 23 Nov 2024 06:56:17 GMT131Please respect copyright.ＰＥＮＡＮＡY2pSPt8UZl
accept-ranges: bytes131Please respect copyright.ＰＥＮＡＮＡhZYJ8D4U1V
content-range: bytes 243220480-534231199/534231200131Please respect copyright.ＰＥＮＡＮＡoiZpcC5POO
Content-Length: 291010720131Please respect copyright.ＰＥＮＡＮＡdAds0ichIi
Content-Type: application/octet-stream131Please respect copyright.ＰＥＮＡＮＡpXaCU6D4mN
Content-Disposition: attachment; filename="zen.iso"131Please respect copyright.ＰＥＮＡＮＡNf6pVKl6wM
ETag: "534231200_1732277116"

(http response body is the file content from resume point to end)131Please respect copyright.ＰＥＮＡＮＡhkTNkz7kcR
------------------------------------------------131Please respect copyright.ＰＥＮＡＮＡkLuCJ4Tf2l
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:131Please respect copyright.ＰＥＮＡＮＡgDvOhtOJf6
-------response content from resume point-------131Please respect copyright.ＰＥＮＡＮＡyLerLdb73Y
HTTP/1.1 200 OK131Please respect copyright.ＰＥＮＡＮＡccUsELHk74
Date: Sat, 23 Nov 2024 06:29:19 GMT131Please respect copyright.ＰＥＮＡＮＡ4KB4K93j8b
accept-ranges: bytes131Please respect copyright.ＰＥＮＡＮＡc8UHnH3PGx
content-range: bytes 0-634231201/634231202131Please respect copyright.ＰＥＮＡＮＡ7QAkZ6tV3Y
ETag: "634231202_2732277118"131Please respect copyright.ＰＥＮＡＮＡUvCsO3adeJ
Content-Disposition: attachment; filename="zen.iso"131Please respect copyright.ＰＥＮＡＮＡhHCxOUivg1
Content-Length: 634231202131Please respect copyright.ＰＥＮＡＮＡUhIRORwlFt
Content-Type: application/octet-stream

(http response body is the whole file content)131Please respect copyright.ＰＥＮＡＮＡUrje0bUCQy
------------------------------------------------131Please respect copyright.ＰＥＮＡＮＡgbMp4wkkqC
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:131Please respect copyright.ＰＥＮＡＮＡ3MPOwMCdIQ
https://www.zeng.dev/post/2023-http-range-and-play-mp4-in-browser/?

131Please respect copyright.ＰＥＮＡＮＡEQFEwpNtOI
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131Please respect copyright.ＰＥＮＡＮＡVwKP1VGgSH
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131Please respect copyright.ＰＥＮＡＮＡ8mC5I6huj5
https://gemini.google.com/

感谢互联网上无私奉献的参考资131Please respect copyright.ＰＥＮＡＮＡ8zuYMn0XRz
https://developer.mozilla.org/en-US/docs/Web/HTTP/Range_requests131Please respect copyright.ＰＥＮＡＮＡUVguhqPGPm
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/If-Range131Please respect copyright.ＰＥＮＡＮＡhQIAYKzHYs
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/ETag131Please respect copyright.ＰＥＮＡＮＡdr0geD9f2H
https://www.rfc-editor.org/rfc/rfc7233131Please respect copyright.ＰＥＮＡＮＡXWjQiPrwAW
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]131Please respect copyright.ＰＥＮＡＮＡp13jGAokTa
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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