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:150Please respect copyright.ＰＥＮＡＮＡT5SxDEUnTu
----------http range request-------150Please respect copyright.ＰＥＮＡＮＡUdy9EyeDsT
GET /zen.iso HTTP/1.1150Please respect copyright.ＰＥＮＡＮＡxvj9AT2KpR
Range: bytes=100-1023150Please respect copyright.ＰＥＮＡＮＡp2nWKswiZJ
If-Match: "534231200_1732277116"150Please respect copyright.ＰＥＮＡＮＡWUeNBCp9gK
......150Please respect copyright.ＰＥＮＡＮＡfUgkVZ2wK8
-----------------------------------150Please respect copyright.ＰＥＮＡＮＡNx43SHxn48
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:150Please respect copyright.ＰＥＮＡＮＡUIomevV65I
----------------html-----------------150Please respect copyright.ＰＥＮＡＮＡDACSGqJnTe
<a href="/zen.iso">zen.iso</a>150Please respect copyright.ＰＥＮＡＮＡkRH9cDyBsJ
-------------------------------------150Please respect copyright.ＰＥＮＡＮＡHxbOMcsNPm
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:150Please respect copyright.ＰＥＮＡＮＡQL8QZE8EZ0
----------get request------------150Please respect copyright.ＰＥＮＡＮＡofehRFAbPH
GET /zen.iso HTTP/1.1150Please respect copyright.ＰＥＮＡＮＡPlLAB0O6Xy
......150Please respect copyright.ＰＥＮＡＮＡ3SgaCm9zaB
---------------------------------

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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:150Please respect copyright.ＰＥＮＡＮＡUxIucizoLw
----------------server response--------------150Please respect copyright.ＰＥＮＡＮＡyvZpSzAxFx
HTTP/1.1 200 OK150Please respect copyright.ＰＥＮＡＮＡdzqnv8viZD
Date: Sat, 23 Nov 2024 06:29:19 GMT150Please respect copyright.ＰＥＮＡＮＡYgBhe5e3li
accept-ranges: bytes150Please respect copyright.ＰＥＮＡＮＡup1T1MZojK
content-range: bytes 0-534231199/534231200150Please respect copyright.ＰＥＮＡＮＡSMBMvP5sJt
ETag: "534231200_1732277116"150Please respect copyright.ＰＥＮＡＮＡP2J2hu2RRK
Content-Disposition: attachment; filename="zen.iso"150Please respect copyright.ＰＥＮＡＮＡ3lMm4gjvSg
Content-Length: 534231200150Please respect copyright.ＰＥＮＡＮＡqE2ZnXy7hu
Content-Type: application/octet-stream

(http response body is the whole file content)150Please respect copyright.ＰＥＮＡＮＡYhau4uiULL
-------------------------------------------

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:150Please respect copyright.ＰＥＮＡＮＡs6bzvWfQ2o
---------------http server send big file content by multiple times--------------150Please respect copyright.ＰＥＮＡＮＡkGvjI15KUz
const int buf_size=20000000; // buf size is about 20M150Please respect copyright.ＰＥＮＡＮＡx0bwk4nqBS
int client_socket; char send_buf[buf_size]; FILE *file;

// first send http response headers150Please respect copyright.ＰＥＮＡＮＡpirUZBFppH
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 big150Please respect copyright.ＰＥＮＡＮＡxCPQrnTbox
while(there_is_remain_file_content_to_send)150Please respect copyright.ＰＥＮＡＮＡNc0WsweYy5
{ 150Please respect copyright.ＰＥＮＡＮＡEtVGT3TH0S
read_remain_file_content_to_send_buf(file, send_buf);150Please respect copyright.ＰＥＮＡＮＡc6SIihyfMe
send_result = send(client_socket, send_buf);150Please respect copyright.ＰＥＮＡＮＡIwrknUzma1
if(send_result == fail) return; else continue;150Please respect copyright.ＰＥＮＡＮＡXhY4No4mPf
}

// a way to make etag for a file using its file size and last modifed time150Please respect copyright.ＰＥＮＡＮＡMYOQIDXMj4
const char *file_etag(const char *path) 150Please respect copyright.ＰＥＮＡＮＡ2Eru8uCx69
{150Please respect copyright.ＰＥＮＡＮＡRaHSZERb00
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;150Please respect copyright.ＰＥＮＡＮＡmV3LJWIPUA
}150Please respect copyright.ＰＥＮＡＮＡzi3atailFT
-------------------------------------------------150Please respect copyright.ＰＥＮＡＮＡK9fguPaXOf
"accept-ranges: bytes" tells client web browser that the http server support range.150Please respect copyright.ＰＥＮＡＮＡBKVC2WKxQZ
"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).150Please respect copyright.ＰＥＮＡＮＡRiDy886SGt
'ETag: "534231200_1732277116"' tells client web browser the file version.150Please respect copyright.ＰＥＮＡＮＡNsxKTu8fIk
'Content-Disposition: attachment; filename="zen.iso"' tells client web browser to download the file as attachment, and its file name.150Please respect copyright.ＰＥＮＡＮＡednehXg0mh
"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.150Please respect copyright.ＰＥＮＡＮＡNZreNU285a
"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.150Please respect copyright.ＰＥＮＡＮＡEP4jFuLrS8
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.)150Please respect copyright.ＰＥＮＡＮＡuX8RX1Tawo
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:150Please respect copyright.ＰＥＮＡＮＡzi083PAtMd
---------resume range request---------150Please respect copyright.ＰＥＮＡＮＡGVilg13JZP
GET /zen.iso HTTP/1.1150Please respect copyright.ＰＥＮＡＮＡSvOcHDVGjK
Range: bytes=243220480-150Please respect copyright.ＰＥＮＡＮＡc2w7gJTYj5
If-Match: "534231200_1732277116"150Please respect copyright.ＰＥＮＡＮＡ49nbKt7GLJ
......150Please respect copyright.ＰＥＮＡＮＡqm6YVsHcWf
-------------------------------------150Please respect copyright.ＰＥＮＡＮＡa7C4zIlpH8
"Range: bytes=243220480-" tells the http server to send content from byte 243220480 to end.150Please respect copyright.ＰＥＮＡＮＡUqadaiqkOb
'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:150Please respect copyright.ＰＥＮＡＮＡk4KeeUB1kY
-------response content from resume point-------150Please respect copyright.ＰＥＮＡＮＡ1Y5eZdYCDz
HTTP/1.1 206 Partial Content150Please respect copyright.ＰＥＮＡＮＡeeDFpG4SHz
Date: Sat, 23 Nov 2024 06:56:17 GMT150Please respect copyright.ＰＥＮＡＮＡyAekk7nX4Z
accept-ranges: bytes150Please respect copyright.ＰＥＮＡＮＡTNNm8qYAxi
content-range: bytes 243220480-534231199/534231200150Please respect copyright.ＰＥＮＡＮＡPlH1zV3inr
Content-Length: 291010720150Please respect copyright.ＰＥＮＡＮＡU3QYSoJ9Qb
Content-Type: application/octet-stream150Please respect copyright.ＰＥＮＡＮＡuSMjwkOPDz
Content-Disposition: attachment; filename="zen.iso"150Please respect copyright.ＰＥＮＡＮＡCH7dEkIMOD
ETag: "534231200_1732277116"

(http response body is the file content from resume point to end)150Please respect copyright.ＰＥＮＡＮＡhRsq33DI25
------------------------------------------------150Please respect copyright.ＰＥＮＡＮＡGiV4hZvVG7
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:150Please respect copyright.ＰＥＮＡＮＡpNMDKwAUAw
-------response content from resume point-------150Please respect copyright.ＰＥＮＡＮＡaQu7FW0aSC
HTTP/1.1 200 OK150Please respect copyright.ＰＥＮＡＮＡQZIjEwEu50
Date: Sat, 23 Nov 2024 06:29:19 GMT150Please respect copyright.ＰＥＮＡＮＡhZBqnmr9aU
accept-ranges: bytes150Please respect copyright.ＰＥＮＡＮＡ9RXAWlDzyV
content-range: bytes 0-634231201/634231202150Please respect copyright.ＰＥＮＡＮＡ8rClO4FOE6
ETag: "634231202_2732277118"150Please respect copyright.ＰＥＮＡＮＡ92d1rokqjO
Content-Disposition: attachment; filename="zen.iso"150Please respect copyright.ＰＥＮＡＮＡ6x1kWIr3eH
Content-Length: 634231202150Please respect copyright.ＰＥＮＡＮＡAp2XpAuaA7
Content-Type: application/octet-stream

(http response body is the whole file content)150Please respect copyright.ＰＥＮＡＮＡKiOAJPZm3T
------------------------------------------------150Please respect copyright.ＰＥＮＡＮＡXp0NpnNbij
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:150Please respect copyright.ＰＥＮＡＮＡZP1bq1oRYl
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150Please respect copyright.ＰＥＮＡＮＡh3KkU5yfX9
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150Please respect copyright.ＰＥＮＡＮＡjCvGgmcBX3
https://gemini.google.com/

感谢互联网上无私奉献的参考资150Please respect copyright.ＰＥＮＡＮＡq5lNEg6dFQ
https://developer.mozilla.org/en-US/docs/Web/HTTP/Range_requests150Please respect copyright.ＰＥＮＡＮＡUvttLzi3Nb
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/If-Range150Please respect copyright.ＰＥＮＡＮＡE76SUGXF95
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/ETag150Please respect copyright.ＰＥＮＡＮＡHgdd8Hk9r1
https://www.rfc-editor.org/rfc/rfc7233150Please respect copyright.ＰＥＮＡＮＡCQACZhQ4jt
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]150Please respect copyright.ＰＥＮＡＮＡxqPHTl3LHG
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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