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:342Please respect copyright.ＰＥＮＡＮＡjEq6RxDkpj
----------http range request-------342Please respect copyright.ＰＥＮＡＮＡE2thUTctl3
GET /zen.iso HTTP/1.1342Please respect copyright.ＰＥＮＡＮＡsGphUvmKjO
Range: bytes=100-1023342Please respect copyright.ＰＥＮＡＮＡWVC8ESAz8R
If-Match: "534231200_1732277116"342Please respect copyright.ＰＥＮＡＮＡLrhiAqovGB
......342Please respect copyright.ＰＥＮＡＮＡpN3g4mDkZz
-----------------------------------342Please respect copyright.ＰＥＮＡＮＡFA2d0W8Ysv
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:342Please respect copyright.ＰＥＮＡＮＡYkAeCOcA3m
----------------html-----------------342Please respect copyright.ＰＥＮＡＮＡo6lcGxuoWk
<a href="/zen.iso">zen.iso</a>342Please respect copyright.ＰＥＮＡＮＡgBkyXkLn6h
-------------------------------------342Please respect copyright.ＰＥＮＡＮＡjkJhMTpbNP
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:342Please respect copyright.ＰＥＮＡＮＡWf1wrc8LzI
----------get request------------342Please respect copyright.ＰＥＮＡＮＡyXWZEy3sTD
GET /zen.iso HTTP/1.1342Please respect copyright.ＰＥＮＡＮＡXpTJgcwQ80
......342Please respect copyright.ＰＥＮＡＮＡ5KOIniGJK7
---------------------------------

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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:342Please respect copyright.ＰＥＮＡＮＡRQS1pDN1Nf
----------------server response--------------342Please respect copyright.ＰＥＮＡＮＡaGRsgSQ3ls
HTTP/1.1 200 OK342Please respect copyright.ＰＥＮＡＮＡeDQ8lpSGa2
Date: Sat, 23 Nov 2024 06:29:19 GMT342Please respect copyright.ＰＥＮＡＮＡQtlMTrtw7J
accept-ranges: bytes342Please respect copyright.ＰＥＮＡＮＡZEHl7OR6lj
content-range: bytes 0-534231199/534231200342Please respect copyright.ＰＥＮＡＮＡ24WSI1M0NX
ETag: "534231200_1732277116"342Please respect copyright.ＰＥＮＡＮＡPeXqjjcUxM
Content-Disposition: attachment; filename="zen.iso"342Please respect copyright.ＰＥＮＡＮＡdlim3ET55b
Content-Length: 534231200342Please respect copyright.ＰＥＮＡＮＡlHMJxACuei
Content-Type: application/octet-stream

(http response body is the whole file content)342Please respect copyright.ＰＥＮＡＮＡJB1qhlkOBK
-------------------------------------------

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:342Please respect copyright.ＰＥＮＡＮＡ6CijOTBzNZ
---------------http server send big file content by multiple times--------------342Please respect copyright.ＰＥＮＡＮＡ6hhE3bvrfP
const int buf_size=20000000; // buf size is about 20M342Please respect copyright.ＰＥＮＡＮＡbtkYgn6PUm
int client_socket; char send_buf[buf_size]; FILE *file;

// first send http response headers342Please respect copyright.ＰＥＮＡＮＡ5Q6oUl0m0o
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 big342Please respect copyright.ＰＥＮＡＮＡj28QIZjCbR
while(there_is_remain_file_content_to_send)342Please respect copyright.ＰＥＮＡＮＡ39YOap3QXC
{ 342Please respect copyright.ＰＥＮＡＮＡ214icMPMML
read_remain_file_content_to_send_buf(file, send_buf);342Please respect copyright.ＰＥＮＡＮＡhxIT6LChfo
send_result = send(client_socket, send_buf);342Please respect copyright.ＰＥＮＡＮＡXqGSfKyRfC
if(send_result == fail) return; else continue;342Please respect copyright.ＰＥＮＡＮＡsCZkmGm4T7
}

// a way to make etag for a file using its file size and last modifed time342Please respect copyright.ＰＥＮＡＮＡY5MU2YW4D2
const char *file_etag(const char *path) 342Please respect copyright.ＰＥＮＡＮＡ3YJIrZAXQ0
{342Please respect copyright.ＰＥＮＡＮＡnIsov0Dstj
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;342Please respect copyright.ＰＥＮＡＮＡ5ui5RgEuem
}342Please respect copyright.ＰＥＮＡＮＡy3mHftA12K
-------------------------------------------------342Please respect copyright.ＰＥＮＡＮＡipySiEu101
"accept-ranges: bytes" tells client web browser that the http server support range.342Please respect copyright.ＰＥＮＡＮＡaRvjdLnZnY
"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).342Please respect copyright.ＰＥＮＡＮＡg83R9Pl6aO
'ETag: "534231200_1732277116"' tells client web browser the file version.342Please respect copyright.ＰＥＮＡＮＡUs8pml2ij5
'Content-Disposition: attachment; filename="zen.iso"' tells client web browser to download the file as attachment, and its file name.342Please respect copyright.ＰＥＮＡＮＡHAlrpoXOYo
"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.342Please respect copyright.ＰＥＮＡＮＡdRItTHt397
"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.342Please respect copyright.ＰＥＮＡＮＡ4T9PlfDari
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.)342Please respect copyright.ＰＥＮＡＮＡVUq3MPl0ic
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:342Please respect copyright.ＰＥＮＡＮＡJTIMn3R0MB
---------resume range request---------342Please respect copyright.ＰＥＮＡＮＡWrFZTM7uG1
GET /zen.iso HTTP/1.1342Please respect copyright.ＰＥＮＡＮＡfvIF6BghoK
Range: bytes=243220480-342Please respect copyright.ＰＥＮＡＮＡGBNWG4N95O
If-Match: "534231200_1732277116"342Please respect copyright.ＰＥＮＡＮＡLaOUuTHCtF
......342Please respect copyright.ＰＥＮＡＮＡ3NZoLVo3p5
-------------------------------------342Please respect copyright.ＰＥＮＡＮＡRTsmZWJ95N
"Range: bytes=243220480-" tells the http server to send content from byte 243220480 to end.342Please respect copyright.ＰＥＮＡＮＡG3R8CJRXh9
'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:342Please respect copyright.ＰＥＮＡＮＡ32SmIFZpEj
-------response content from resume point-------342Please respect copyright.ＰＥＮＡＮＡzqpJOm7wgj
HTTP/1.1 206 Partial Content342Please respect copyright.ＰＥＮＡＮＡMQajlrkgXb
Date: Sat, 23 Nov 2024 06:56:17 GMT342Please respect copyright.ＰＥＮＡＮＡeCYNC6Hb3K
accept-ranges: bytes342Please respect copyright.ＰＥＮＡＮＡrCuQXvrdsF
content-range: bytes 243220480-534231199/534231200342Please respect copyright.ＰＥＮＡＮＡbmPxBeQ8il
Content-Length: 291010720342Please respect copyright.ＰＥＮＡＮＡ23zWvmNhSf
Content-Type: application/octet-stream342Please respect copyright.ＰＥＮＡＮＡU5aTcC1aTG
Content-Disposition: attachment; filename="zen.iso"342Please respect copyright.ＰＥＮＡＮＡ3lzyNyN7bI
ETag: "534231200_1732277116"

(http response body is the file content from resume point to end)342Please respect copyright.ＰＥＮＡＮＡ2GwOJAiFzK
------------------------------------------------342Please respect copyright.ＰＥＮＡＮＡcTgeK3XLMj
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:342Please respect copyright.ＰＥＮＡＮＡEIZJOTFk02
-------response content from resume point-------342Please respect copyright.ＰＥＮＡＮＡ5Hroq5nsgw
HTTP/1.1 200 OK342Please respect copyright.ＰＥＮＡＮＡKdqBsODgKn
Date: Sat, 23 Nov 2024 06:29:19 GMT342Please respect copyright.ＰＥＮＡＮＡdozFyr7OGp
accept-ranges: bytes342Please respect copyright.ＰＥＮＡＮＡAk9WI4flUm
content-range: bytes 0-634231201/634231202342Please respect copyright.ＰＥＮＡＮＡDMJusYS63g
ETag: "634231202_2732277118"342Please respect copyright.ＰＥＮＡＮＡ8HcKFXuzK0
Content-Disposition: attachment; filename="zen.iso"342Please respect copyright.ＰＥＮＡＮＡuUBJ1H3Fjw
Content-Length: 634231202342Please respect copyright.ＰＥＮＡＮＡXFuQAMdjnt
Content-Type: application/octet-stream

(http response body is the whole file content)342Please respect copyright.ＰＥＮＡＮＡcNcECnIDy9
------------------------------------------------342Please respect copyright.ＰＥＮＡＮＡmpgGXYHCeB
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:342Please respect copyright.ＰＥＮＡＮＡLJuvS45odb
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342Please respect copyright.ＰＥＮＡＮＡXzPyhIReSY
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342Please respect copyright.ＰＥＮＡＮＡhGOKZDNm91
https://gemini.google.com/

感谢互联网上无私奉献的参考资342Please respect copyright.ＰＥＮＡＮＡ2vAP2uphNz
https://developer.mozilla.org/en-US/docs/Web/HTTP/Range_requests342Please respect copyright.ＰＥＮＡＮＡPAblXOkPwj
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/If-Range342Please respect copyright.ＰＥＮＡＮＡRh4E8cvVqW
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/ETag342Please respect copyright.ＰＥＮＡＮＡJnxeYrar2u
https://www.rfc-editor.org/rfc/rfc7233342Please respect copyright.ＰＥＮＡＮＡkDWL18xliz
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]342Please respect copyright.ＰＥＮＡＮＡP5K8xSILPS
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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