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:58Please respect copyright.ＰＥＮＡＮＡuAAK3dMT65
----------http range request-------58Please respect copyright.ＰＥＮＡＮＡwMJLokKu4W
GET /zen.iso HTTP/1.158Please respect copyright.ＰＥＮＡＮＡqVuJOrzMHD
Range: bytes=100-102358Please respect copyright.ＰＥＮＡＮＡAOUuZwx3oh
If-Match: "534231200_1732277116"58Please respect copyright.ＰＥＮＡＮＡPeLH54bXjE
......58Please respect copyright.ＰＥＮＡＮＡkvQvieQnn9
-----------------------------------58Please respect copyright.ＰＥＮＡＮＡifQ3eHVSez
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:58Please respect copyright.ＰＥＮＡＮＡg559AhIZi7
----------------html-----------------58Please respect copyright.ＰＥＮＡＮＡP7I951RKGE
<a href="/zen.iso">zen.iso</a>58Please respect copyright.ＰＥＮＡＮＡTTQxlReM0w
-------------------------------------58Please respect copyright.ＰＥＮＡＮＡTTC1tIXlOd
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:58Please respect copyright.ＰＥＮＡＮＡQyu4ZdGTHN
----------get request------------58Please respect copyright.ＰＥＮＡＮＡHnQ6iUsaoJ
GET /zen.iso HTTP/1.158Please respect copyright.ＰＥＮＡＮＡVqqgvUd188
......58Please respect copyright.ＰＥＮＡＮＡBrAyCxU6kC
---------------------------------

58Please respect copyright.ＰＥＮＡＮＡy6A493SZG7
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:58Please respect copyright.ＰＥＮＡＮＡAGUfv13Qfy
----------------server response--------------58Please respect copyright.ＰＥＮＡＮＡ4urfHAHuyo
HTTP/1.1 200 OK58Please respect copyright.ＰＥＮＡＮＡCO4PeSzGag
Date: Sat, 23 Nov 2024 06:29:19 GMT58Please respect copyright.ＰＥＮＡＮＡifsJis87K7
accept-ranges: bytes58Please respect copyright.ＰＥＮＡＮＡVwQfBniDsu
content-range: bytes 0-534231199/53423120058Please respect copyright.ＰＥＮＡＮＡNRLhfhUybW
ETag: "534231200_1732277116"58Please respect copyright.ＰＥＮＡＮＡyYrilxHxac
Content-Disposition: attachment; filename="zen.iso"58Please respect copyright.ＰＥＮＡＮＡWX1mO5tp7n
Content-Length: 53423120058Please respect copyright.ＰＥＮＡＮＡKS2d5moPPO
Content-Type: application/octet-stream

(http response body is the whole file content)58Please respect copyright.ＰＥＮＡＮＡgCFrqmvtQJ
-------------------------------------------

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:58Please respect copyright.ＰＥＮＡＮＡlUMRX0J9uK
---------------http server send big file content by multiple times--------------58Please respect copyright.ＰＥＮＡＮＡ7Z4MUKTFvc
const int buf_size=20000000; // buf size is about 20M58Please respect copyright.ＰＥＮＡＮＡ8mTgd3IFdJ
int client_socket; char send_buf[buf_size]; FILE *file;

// first send http response headers58Please respect copyright.ＰＥＮＡＮＡ3fiXmVGHKi
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 big58Please respect copyright.ＰＥＮＡＮＡwY2QY0Fc9Z
while(there_is_remain_file_content_to_send)58Please respect copyright.ＰＥＮＡＮＡzZG9mCHF3x
{ 58Please respect copyright.ＰＥＮＡＮＡ3lBTubmFet
read_remain_file_content_to_send_buf(file, send_buf);58Please respect copyright.ＰＥＮＡＮＡegb0LcdtX4
send_result = send(client_socket, send_buf);58Please respect copyright.ＰＥＮＡＮＡnVlaexMogX
if(send_result == fail) return; else continue;58Please respect copyright.ＰＥＮＡＮＡVU5RZjMqTM
}

// a way to make etag for a file using its file size and last modifed time58Please respect copyright.ＰＥＮＡＮＡh43DQFTcN0
const char *file_etag(const char *path) 58Please respect copyright.ＰＥＮＡＮＡlbup6QJEHf
{58Please respect copyright.ＰＥＮＡＮＡvHQYATHQUu
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;58Please respect copyright.ＰＥＮＡＮＡV5hNiB2H2g
}58Please respect copyright.ＰＥＮＡＮＡIGmskbLGj6
-------------------------------------------------58Please respect copyright.ＰＥＮＡＮＡcf5m1UG7oG
"accept-ranges: bytes" tells client web browser that the http server support range.58Please respect copyright.ＰＥＮＡＮＡyNtpcVCRTu
"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).58Please respect copyright.ＰＥＮＡＮＡMlk01fetPZ
'ETag: "534231200_1732277116"' tells client web browser the file version.58Please respect copyright.ＰＥＮＡＮＡeeLayOfdpu
'Content-Disposition: attachment; filename="zen.iso"' tells client web browser to download the file as attachment, and its file name.58Please respect copyright.ＰＥＮＡＮＡdjEOe9OMOb
"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.58Please respect copyright.ＰＥＮＡＮＡYNhOuIEsLU
"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.58Please respect copyright.ＰＥＮＡＮＡckEwjFIu0H
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.)58Please respect copyright.ＰＥＮＡＮＡ5R2AGhg7up
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:58Please respect copyright.ＰＥＮＡＮＡS416P0RYGG
---------resume range request---------58Please respect copyright.ＰＥＮＡＮＡLF3qKYNoGq
GET /zen.iso HTTP/1.158Please respect copyright.ＰＥＮＡＮＡ2fHT0BKtId
Range: bytes=243220480-58Please respect copyright.ＰＥＮＡＮＡFb3JkJz873
If-Match: "534231200_1732277116"58Please respect copyright.ＰＥＮＡＮＡwkRmFpw87D
......58Please respect copyright.ＰＥＮＡＮＡVWhf5ic0VR
-------------------------------------58Please respect copyright.ＰＥＮＡＮＡmo5El1isCo
"Range: bytes=243220480-" tells the http server to send content from byte 243220480 to end.58Please respect copyright.ＰＥＮＡＮＡctFhwEWDaj
'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:58Please respect copyright.ＰＥＮＡＮＡ4csQGwyF7p
-------response content from resume point-------58Please respect copyright.ＰＥＮＡＮＡLTOsQKM5yz
HTTP/1.1 206 Partial Content58Please respect copyright.ＰＥＮＡＮＡPifaG62typ
Date: Sat, 23 Nov 2024 06:56:17 GMT58Please respect copyright.ＰＥＮＡＮＡxK7gRpwuNf
accept-ranges: bytes58Please respect copyright.ＰＥＮＡＮＡ2WFWWTs11A
content-range: bytes 243220480-534231199/53423120058Please respect copyright.ＰＥＮＡＮＡCwGejybRbQ
Content-Length: 29101072058Please respect copyright.ＰＥＮＡＮＡzrQkC0qEVX
Content-Type: application/octet-stream58Please respect copyright.ＰＥＮＡＮＡ9c3QVZmQqc
Content-Disposition: attachment; filename="zen.iso"58Please respect copyright.ＰＥＮＡＮＡc6gBU9CGhT
ETag: "534231200_1732277116"

(http response body is the file content from resume point to end)58Please respect copyright.ＰＥＮＡＮＡFEzfph6Ovs
------------------------------------------------58Please respect copyright.ＰＥＮＡＮＡrLq0oocEZY
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:58Please respect copyright.ＰＥＮＡＮＡ0lHankV9LO
-------response content from resume point-------58Please respect copyright.ＰＥＮＡＮＡdqNNM3nThi
HTTP/1.1 200 OK58Please respect copyright.ＰＥＮＡＮＡa9Owt14cvB
Date: Sat, 23 Nov 2024 06:29:19 GMT58Please respect copyright.ＰＥＮＡＮＡYASDLBi2Ut
accept-ranges: bytes58Please respect copyright.ＰＥＮＡＮＡeNZ4wkQeWr
content-range: bytes 0-634231201/63423120258Please respect copyright.ＰＥＮＡＮＡIrRFfdZcj4
ETag: "634231202_2732277118"58Please respect copyright.ＰＥＮＡＮＡCcytyVtecS
Content-Disposition: attachment; filename="zen.iso"58Please respect copyright.ＰＥＮＡＮＡecb5EtCeCx
Content-Length: 63423120258Please respect copyright.ＰＥＮＡＮＡ1Kugm8e6gP
Content-Type: application/octet-stream

(http response body is the whole file content)58Please respect copyright.ＰＥＮＡＮＡzaSE8oe1R1
------------------------------------------------58Please respect copyright.ＰＥＮＡＮＡwl68dW3cOZ
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:58Please respect copyright.ＰＥＮＡＮＡu6A7eZKPih
https://www.zeng.dev/post/2023-http-range-and-play-mp4-in-browser/?

58Please respect copyright.ＰＥＮＡＮＡ8Sm7QZfjxa
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58Please respect copyright.ＰＥＮＡＮＡ1bYrIhYVtv
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58Please respect copyright.ＰＥＮＡＮＡvDBFyOIJOD
https://gemini.google.com/

感谢互联网上无私奉献的参考资58Please respect copyright.ＰＥＮＡＮＡU97FbuY9n6
https://developer.mozilla.org/en-US/docs/Web/HTTP/Range_requests58Please respect copyright.ＰＥＮＡＮＡwVsnqFbDWD
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/If-Range58Please respect copyright.ＰＥＮＡＮＡEDyNs0dFag
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/ETag58Please respect copyright.ＰＥＮＡＮＡPzHBKHSiGd
https://www.rfc-editor.org/rfc/rfc723358Please respect copyright.ＰＥＮＡＮＡ8CAUfYdWx9
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]58Please respect copyright.ＰＥＮＡＮＡy1XTVMcHKQ
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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