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:213Please respect copyright.ＰＥＮＡＮＡ5o5JD6wVkD
----------http range request-------213Please respect copyright.ＰＥＮＡＮＡ6PH0lauMMQ
GET /zen.iso HTTP/1.1213Please respect copyright.ＰＥＮＡＮＡ5CPuIv8BLC
Range: bytes=100-1023213Please respect copyright.ＰＥＮＡＮＡwZ5i8dUOMl
If-Match: "534231200_1732277116"213Please respect copyright.ＰＥＮＡＮＡIc28NrxAEd
......213Please respect copyright.ＰＥＮＡＮＡ0k6szfeLD7
-----------------------------------213Please respect copyright.ＰＥＮＡＮＡvTgK22MGqD
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:213Please respect copyright.ＰＥＮＡＮＡnGS8PhsDhs
----------------html-----------------213Please respect copyright.ＰＥＮＡＮＡxvcAvqh0Bb
<a href="/zen.iso">zen.iso</a>213Please respect copyright.ＰＥＮＡＮＡ0udDLpiqXS
-------------------------------------213Please respect copyright.ＰＥＮＡＮＡHzZjKl4AJB
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:213Please respect copyright.ＰＥＮＡＮＡdlRNCra7iX
----------get request------------213Please respect copyright.ＰＥＮＡＮＡ7Qn6Y1Xcli
GET /zen.iso HTTP/1.1213Please respect copyright.ＰＥＮＡＮＡ8g5X4q2Snp
......213Please respect copyright.ＰＥＮＡＮＡyUrMRE7CJ5
---------------------------------

213Please respect copyright.ＰＥＮＡＮＡqEs4IhZEb5
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:213Please respect copyright.ＰＥＮＡＮＡ0hL15k2G2e
----------------server response--------------213Please respect copyright.ＰＥＮＡＮＡ45SUJMZpqY
HTTP/1.1 200 OK213Please respect copyright.ＰＥＮＡＮＡVx22Y9IdS1
Date: Sat, 23 Nov 2024 06:29:19 GMT213Please respect copyright.ＰＥＮＡＮＡTTXkygyAAC
accept-ranges: bytes213Please respect copyright.ＰＥＮＡＮＡ9bWh2PQQOn
content-range: bytes 0-534231199/534231200213Please respect copyright.ＰＥＮＡＮＡYARH6dblPs
ETag: "534231200_1732277116"213Please respect copyright.ＰＥＮＡＮＡETSQYO8HZp
Content-Disposition: attachment; filename="zen.iso"213Please respect copyright.ＰＥＮＡＮＡrqoGIqAVLC
Content-Length: 534231200213Please respect copyright.ＰＥＮＡＮＡqahriVooqt
Content-Type: application/octet-stream

(http response body is the whole file content)213Please respect copyright.ＰＥＮＡＮＡeXvNs23nAN
-------------------------------------------

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:213Please respect copyright.ＰＥＮＡＮＡka80vwxwo2
---------------http server send big file content by multiple times--------------213Please respect copyright.ＰＥＮＡＮＡa10YnxD1Dx
const int buf_size=20000000; // buf size is about 20M213Please respect copyright.ＰＥＮＡＮＡkngfbzWW95
int client_socket; char send_buf[buf_size]; FILE *file;

// first send http response headers213Please respect copyright.ＰＥＮＡＮＡnUpBuJGMex
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 big213Please respect copyright.ＰＥＮＡＮＡEMBRo6k9AH
while(there_is_remain_file_content_to_send)213Please respect copyright.ＰＥＮＡＮＡ6QTd1PUHq1
{ 213Please respect copyright.ＰＥＮＡＮＡ9ySN267Q57
read_remain_file_content_to_send_buf(file, send_buf);213Please respect copyright.ＰＥＮＡＮＡMNxXDx6Ilv
send_result = send(client_socket, send_buf);213Please respect copyright.ＰＥＮＡＮＡ8UCK0FWH1m
if(send_result == fail) return; else continue;213Please respect copyright.ＰＥＮＡＮＡ7weBWEYhL7
}

// a way to make etag for a file using its file size and last modifed time213Please respect copyright.ＰＥＮＡＮＡf8o5Tb3R9V
const char *file_etag(const char *path) 213Please respect copyright.ＰＥＮＡＮＡyqa0hbeYAN
{213Please respect copyright.ＰＥＮＡＮＡGW2zSUCLEf
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;213Please respect copyright.ＰＥＮＡＮＡBi669xiktB
}213Please respect copyright.ＰＥＮＡＮＡ15nHySOpTP
-------------------------------------------------213Please respect copyright.ＰＥＮＡＮＡhA9zl3JQLm
"accept-ranges: bytes" tells client web browser that the http server support range.213Please respect copyright.ＰＥＮＡＮＡBVF8OXiNRu
"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).213Please respect copyright.ＰＥＮＡＮＡ363gMfF2P4
'ETag: "534231200_1732277116"' tells client web browser the file version.213Please respect copyright.ＰＥＮＡＮＡ5jXM3SSXqv
'Content-Disposition: attachment; filename="zen.iso"' tells client web browser to download the file as attachment, and its file name.213Please respect copyright.ＰＥＮＡＮＡJzfZICUTEr
"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.213Please respect copyright.ＰＥＮＡＮＡtDXc3CTTNO
"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.213Please respect copyright.ＰＥＮＡＮＡVCf2Ec2dR2
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.)213Please respect copyright.ＰＥＮＡＮＡP9BzxM53VU
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:213Please respect copyright.ＰＥＮＡＮＡeVNFL0LvfD
---------resume range request---------213Please respect copyright.ＰＥＮＡＮＡCExw0bIn2V
GET /zen.iso HTTP/1.1213Please respect copyright.ＰＥＮＡＮＡSMoftIxa65
Range: bytes=243220480-213Please respect copyright.ＰＥＮＡＮＡWES27p7Rj8
If-Match: "534231200_1732277116"213Please respect copyright.ＰＥＮＡＮＡ7pMB5ZcOWh
......213Please respect copyright.ＰＥＮＡＮＡLaQzXqVH7t
-------------------------------------213Please respect copyright.ＰＥＮＡＮＡjkvWQ0c0C0
"Range: bytes=243220480-" tells the http server to send content from byte 243220480 to end.213Please respect copyright.ＰＥＮＡＮＡItjrqzuVjE
'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:213Please respect copyright.ＰＥＮＡＮＡCkwXOODEd9
-------response content from resume point-------213Please respect copyright.ＰＥＮＡＮＡ0yVIBxcRIW
HTTP/1.1 206 Partial Content213Please respect copyright.ＰＥＮＡＮＡsK34XPYMmH
Date: Sat, 23 Nov 2024 06:56:17 GMT213Please respect copyright.ＰＥＮＡＮＡWDLOpuybr3
accept-ranges: bytes213Please respect copyright.ＰＥＮＡＮＡb3pgXda2ij
content-range: bytes 243220480-534231199/534231200213Please respect copyright.ＰＥＮＡＮＡKDU7xQXD4u
Content-Length: 291010720213Please respect copyright.ＰＥＮＡＮＡiYu0MZSFbJ
Content-Type: application/octet-stream213Please respect copyright.ＰＥＮＡＮＡrwMHF8F8Fc
Content-Disposition: attachment; filename="zen.iso"213Please respect copyright.ＰＥＮＡＮＡRUUiN1JUqR
ETag: "534231200_1732277116"

(http response body is the file content from resume point to end)213Please respect copyright.ＰＥＮＡＮＡGUKyQwkG8b
------------------------------------------------213Please respect copyright.ＰＥＮＡＮＡlPVHoCgiu4
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:213Please respect copyright.ＰＥＮＡＮＡF4jEP9x39a
-------response content from resume point-------213Please respect copyright.ＰＥＮＡＮＡ8ztSK2ZvCD
HTTP/1.1 200 OK213Please respect copyright.ＰＥＮＡＮＡqVs20Js8bm
Date: Sat, 23 Nov 2024 06:29:19 GMT213Please respect copyright.ＰＥＮＡＮＡxciB25Jer9
accept-ranges: bytes213Please respect copyright.ＰＥＮＡＮＡS0s4pB5SYr
content-range: bytes 0-634231201/634231202213Please respect copyright.ＰＥＮＡＮＡNd5VCTDvXs
ETag: "634231202_2732277118"213Please respect copyright.ＰＥＮＡＮＡqozsrvAtgU
Content-Disposition: attachment; filename="zen.iso"213Please respect copyright.ＰＥＮＡＮＡQjpGiBBX6x
Content-Length: 634231202213Please respect copyright.ＰＥＮＡＮＡ5qHYQUGoXu
Content-Type: application/octet-stream

(http response body is the whole file content)213Please respect copyright.ＰＥＮＡＮＡWzycH9tShr
------------------------------------------------213Please respect copyright.ＰＥＮＡＮＡIoXzgxdqs3
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:213Please respect copyright.ＰＥＮＡＮＡJHtDR1fRR5
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213Please respect copyright.ＰＥＮＡＮＡug3U96dgsW
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213Please respect copyright.ＰＥＮＡＮＡqTosWCyJTs
https://gemini.google.com/

感谢互联网上无私奉献的参考资213Please respect copyright.ＰＥＮＡＮＡZnMw6wXRKA
https://developer.mozilla.org/en-US/docs/Web/HTTP/Range_requests213Please respect copyright.ＰＥＮＡＮＡGAOZPtR6Mp
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/If-Range213Please respect copyright.ＰＥＮＡＮＡAnmpdOaZUF
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/ETag213Please respect copyright.ＰＥＮＡＮＡ1imkNG3z5w
https://www.rfc-editor.org/rfc/rfc7233213Please respect copyright.ＰＥＮＡＮＡLu1QVIgOWD
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]213Please respect copyright.ＰＥＮＡＮＡ9bgWHtpmlF
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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