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:55Please respect copyright.ＰＥＮＡＮＡHzdEQ21vEz
----------http range request-------55Please respect copyright.ＰＥＮＡＮＡ1ZaU2oPMcX
GET /zen.iso HTTP/1.155Please respect copyright.ＰＥＮＡＮＡWMdu8Fr6I9
Range: bytes=100-102355Please respect copyright.ＰＥＮＡＮＡVmAQUxPz40
If-Match: "534231200_1732277116"55Please respect copyright.ＰＥＮＡＮＡB3BXYoiHKy
......55Please respect copyright.ＰＥＮＡＮＡO3SuCXg2xw
-----------------------------------55Please respect copyright.ＰＥＮＡＮＡghgDRgSMfk
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:55Please respect copyright.ＰＥＮＡＮＡxhinB5Qg17
----------------html-----------------55Please respect copyright.ＰＥＮＡＮＡnjK2gLm9CI
<a href="/zen.iso">zen.iso</a>55Please respect copyright.ＰＥＮＡＮＡOBlivow5Bt
-------------------------------------55Please respect copyright.ＰＥＮＡＮＡdQawMTmnYc
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:55Please respect copyright.ＰＥＮＡＮＡLzYisteWi9
----------get request------------55Please respect copyright.ＰＥＮＡＮＡK4wUtLQFqd
GET /zen.iso HTTP/1.155Please respect copyright.ＰＥＮＡＮＡYTKxAoQBf0
......55Please respect copyright.ＰＥＮＡＮＡBVrkrlo09F
---------------------------------

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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:55Please respect copyright.ＰＥＮＡＮＡnIPRRLkSkP
----------------server response--------------55Please respect copyright.ＰＥＮＡＮＡLb80iU1Mjy
HTTP/1.1 200 OK55Please respect copyright.ＰＥＮＡＮＡzn3HXq2AhG
Date: Sat, 23 Nov 2024 06:29:19 GMT55Please respect copyright.ＰＥＮＡＮＡGFQ44hyp4O
accept-ranges: bytes55Please respect copyright.ＰＥＮＡＮＡFRutlnalPi
content-range: bytes 0-534231199/53423120055Please respect copyright.ＰＥＮＡＮＡ0lkivhmMIt
ETag: "534231200_1732277116"55Please respect copyright.ＰＥＮＡＮＡ0qTfqe1NpK
Content-Disposition: attachment; filename="zen.iso"55Please respect copyright.ＰＥＮＡＮＡqF3IChqeeQ
Content-Length: 53423120055Please respect copyright.ＰＥＮＡＮＡzhNufrgoYN
Content-Type: application/octet-stream

(http response body is the whole file content)55Please respect copyright.ＰＥＮＡＮＡFUsPblHTPw
-------------------------------------------

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:55Please respect copyright.ＰＥＮＡＮＡLqq1FISHqY
---------------http server send big file content by multiple times--------------55Please respect copyright.ＰＥＮＡＮＡwfyyCPdoh0
const int buf_size=20000000; // buf size is about 20M55Please respect copyright.ＰＥＮＡＮＡnrWvC0wTR4
int client_socket; char send_buf[buf_size]; FILE *file;

// first send http response headers55Please respect copyright.ＰＥＮＡＮＡ6nsM2pl6XX
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 big55Please respect copyright.ＰＥＮＡＮＡlKZmYXlTd9
while(there_is_remain_file_content_to_send)55Please respect copyright.ＰＥＮＡＮＡxgK0N1tPHT
{ 55Please respect copyright.ＰＥＮＡＮＡIKVHO7jIIF
read_remain_file_content_to_send_buf(file, send_buf);55Please respect copyright.ＰＥＮＡＮＡToK0vv0TYD
send_result = send(client_socket, send_buf);55Please respect copyright.ＰＥＮＡＮＡIIz2QgxoXw
if(send_result == fail) return; else continue;55Please respect copyright.ＰＥＮＡＮＡUC0vaKylW8
}

// a way to make etag for a file using its file size and last modifed time55Please respect copyright.ＰＥＮＡＮＡeoYtbvnfFE
const char *file_etag(const char *path) 55Please respect copyright.ＰＥＮＡＮＡ3xo74mTTy6
{55Please respect copyright.ＰＥＮＡＮＡFSzKoZ7RT6
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;55Please respect copyright.ＰＥＮＡＮＡXBOuqe5iqB
}55Please respect copyright.ＰＥＮＡＮＡEJW9xpqKlp
-------------------------------------------------55Please respect copyright.ＰＥＮＡＮＡJeXr5MJ85K
"accept-ranges: bytes" tells client web browser that the http server support range.55Please respect copyright.ＰＥＮＡＮＡx58yGlauQY
"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).55Please respect copyright.ＰＥＮＡＮＡBxaoJhAika
'ETag: "534231200_1732277116"' tells client web browser the file version.55Please respect copyright.ＰＥＮＡＮＡ2BNHEKcaGh
'Content-Disposition: attachment; filename="zen.iso"' tells client web browser to download the file as attachment, and its file name.55Please respect copyright.ＰＥＮＡＮＡ9kBhKltCuS
"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.55Please respect copyright.ＰＥＮＡＮＡtzle4ZWvts
"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.55Please respect copyright.ＰＥＮＡＮＡGHxHTajWoB
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.)55Please respect copyright.ＰＥＮＡＮＡNEc6mO614z
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:55Please respect copyright.ＰＥＮＡＮＡo7JqeMLlPn
---------resume range request---------55Please respect copyright.ＰＥＮＡＮＡ8OclRC1SQr
GET /zen.iso HTTP/1.155Please respect copyright.ＰＥＮＡＮＡFti1n1bbPt
Range: bytes=243220480-55Please respect copyright.ＰＥＮＡＮＡ5CyDSprDpX
If-Match: "534231200_1732277116"55Please respect copyright.ＰＥＮＡＮＡofyRtNmSeF
......55Please respect copyright.ＰＥＮＡＮＡ4O74Ldx6uP
-------------------------------------55Please respect copyright.ＰＥＮＡＮＡca8fFbHSdG
"Range: bytes=243220480-" tells the http server to send content from byte 243220480 to end.55Please respect copyright.ＰＥＮＡＮＡQZ0X3SdDoI
'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:55Please respect copyright.ＰＥＮＡＮＡziUQOpvVhH
-------response content from resume point-------55Please respect copyright.ＰＥＮＡＮＡ3jZDKoLpEa
HTTP/1.1 206 Partial Content55Please respect copyright.ＰＥＮＡＮＡKIELOeLay0
Date: Sat, 23 Nov 2024 06:56:17 GMT55Please respect copyright.ＰＥＮＡＮＡKAq0O2bRUc
accept-ranges: bytes55Please respect copyright.ＰＥＮＡＮＡjg1PIY9nLx
content-range: bytes 243220480-534231199/53423120055Please respect copyright.ＰＥＮＡＮＡNEk4nxrDcN
Content-Length: 29101072055Please respect copyright.ＰＥＮＡＮＡykAQnjit2S
Content-Type: application/octet-stream55Please respect copyright.ＰＥＮＡＮＡoETGv78mwz
Content-Disposition: attachment; filename="zen.iso"55Please respect copyright.ＰＥＮＡＮＡHWd2kGoBgH
ETag: "534231200_1732277116"

(http response body is the file content from resume point to end)55Please respect copyright.ＰＥＮＡＮＡA6DsYk94aL
------------------------------------------------55Please respect copyright.ＰＥＮＡＮＡ9gGVq7uNHK
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:55Please respect copyright.ＰＥＮＡＮＡL9lqZX9PHH
-------response content from resume point-------55Please respect copyright.ＰＥＮＡＮＡkFKluqsWri
HTTP/1.1 200 OK55Please respect copyright.ＰＥＮＡＮＡWKonVwb08v
Date: Sat, 23 Nov 2024 06:29:19 GMT55Please respect copyright.ＰＥＮＡＮＡrHfIsQQFbY
accept-ranges: bytes55Please respect copyright.ＰＥＮＡＮＡmJHqV4czbF
content-range: bytes 0-634231201/63423120255Please respect copyright.ＰＥＮＡＮＡDYAy3FgwyC
ETag: "634231202_2732277118"55Please respect copyright.ＰＥＮＡＮＡ1KybmWEaXf
Content-Disposition: attachment; filename="zen.iso"55Please respect copyright.ＰＥＮＡＮＡl9tT1aHSEb
Content-Length: 63423120255Please respect copyright.ＰＥＮＡＮＡOZkYh9l2Jg
Content-Type: application/octet-stream

(http response body is the whole file content)55Please respect copyright.ＰＥＮＡＮＡgHcaV2FhR9
------------------------------------------------55Please respect copyright.ＰＥＮＡＮＡIWOa9fX3mr
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:55Please respect copyright.ＰＥＮＡＮＡQpZieOExDS
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55Please respect copyright.ＰＥＮＡＮＡKL6oXvZpaN
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55Please respect copyright.ＰＥＮＡＮＡQzSoPhu2sI
https://gemini.google.com/

感谢互联网上无私奉献的参考资55Please respect copyright.ＰＥＮＡＮＡWCF5X16J24
https://developer.mozilla.org/en-US/docs/Web/HTTP/Range_requests55Please respect copyright.ＰＥＮＡＮＡ1AIQLhK3gH
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/If-Range55Please respect copyright.ＰＥＮＡＮＡUSEU3Lt2w6
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/ETag55Please respect copyright.ＰＥＮＡＮＡt3tOt1l9II
https://www.rfc-editor.org/rfc/rfc723355Please respect copyright.ＰＥＮＡＮＡDkFYvN6P8n
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]55Please respect copyright.ＰＥＮＡＮＡ13jniw6TzA
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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