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:154Please respect copyright.ＰＥＮＡＮＡE8w6SnPB55
----------http range request-------154Please respect copyright.ＰＥＮＡＮＡZOAN1uGQUi
GET /zen.iso HTTP/1.1154Please respect copyright.ＰＥＮＡＮＡ1tswBWuyQE
Range: bytes=100-1023154Please respect copyright.ＰＥＮＡＮＡFcPvNEZDzT
If-Match: "534231200_1732277116"154Please respect copyright.ＰＥＮＡＮＡYKPiKpSRJS
......154Please respect copyright.ＰＥＮＡＮＡL2kW3tdqUg
-----------------------------------154Please respect copyright.ＰＥＮＡＮＡlfGGjL0BO2
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:154Please respect copyright.ＰＥＮＡＮＡINQP31Ukdz
----------------html-----------------154Please respect copyright.ＰＥＮＡＮＡfpdLwtPv2y
<a href="/zen.iso">zen.iso</a>154Please respect copyright.ＰＥＮＡＮＡTnR7Mepd2D
-------------------------------------154Please respect copyright.ＰＥＮＡＮＡfTFxcqJO8S
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:154Please respect copyright.ＰＥＮＡＮＡLVYLSBnuQs
----------get request------------154Please respect copyright.ＰＥＮＡＮＡw9nD5OuwR4
GET /zen.iso HTTP/1.1154Please respect copyright.ＰＥＮＡＮＡpsTTtIkOgv
......154Please respect copyright.ＰＥＮＡＮＡUkNEvOCgBu
---------------------------------

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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:154Please respect copyright.ＰＥＮＡＮＡWTiz1VzRMH
----------------server response--------------154Please respect copyright.ＰＥＮＡＮＡOJ5IihFpbd
HTTP/1.1 200 OK154Please respect copyright.ＰＥＮＡＮＡnPFeXMlOhp
Date: Sat, 23 Nov 2024 06:29:19 GMT154Please respect copyright.ＰＥＮＡＮＡh9zUtsKA5l
accept-ranges: bytes154Please respect copyright.ＰＥＮＡＮＡV5F64Cut4x
content-range: bytes 0-534231199/534231200154Please respect copyright.ＰＥＮＡＮＡJF4F6hykVz
ETag: "534231200_1732277116"154Please respect copyright.ＰＥＮＡＮＡMXQOWnZc0r
Content-Disposition: attachment; filename="zen.iso"154Please respect copyright.ＰＥＮＡＮＡiKkgBLkXYw
Content-Length: 534231200154Please respect copyright.ＰＥＮＡＮＡT7I2HJ2Nsy
Content-Type: application/octet-stream

(http response body is the whole file content)154Please respect copyright.ＰＥＮＡＮＡsCLcwULy9W
-------------------------------------------

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:154Please respect copyright.ＰＥＮＡＮＡ3WiD3Qr0Cc
---------------http server send big file content by multiple times--------------154Please respect copyright.ＰＥＮＡＮＡi0tRjwIXtK
const int buf_size=20000000; // buf size is about 20M154Please respect copyright.ＰＥＮＡＮＡsDl1HMY62t
int client_socket; char send_buf[buf_size]; FILE *file;

// first send http response headers154Please respect copyright.ＰＥＮＡＮＡvUXZ71oxTS
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 big154Please respect copyright.ＰＥＮＡＮＡ59ZwKv9Ebw
while(there_is_remain_file_content_to_send)154Please respect copyright.ＰＥＮＡＮＡ5oFwMMlSKk
{ 154Please respect copyright.ＰＥＮＡＮＡx5tjmsaVNc
read_remain_file_content_to_send_buf(file, send_buf);154Please respect copyright.ＰＥＮＡＮＡaTe32U7doN
send_result = send(client_socket, send_buf);154Please respect copyright.ＰＥＮＡＮＡWFLVte376H
if(send_result == fail) return; else continue;154Please respect copyright.ＰＥＮＡＮＡDCcg95yUTZ
}

// a way to make etag for a file using its file size and last modifed time154Please respect copyright.ＰＥＮＡＮＡRd4w5m5FSb
const char *file_etag(const char *path) 154Please respect copyright.ＰＥＮＡＮＡ9GVnv9nF0b
{154Please respect copyright.ＰＥＮＡＮＡreS6WEh8Mn
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;154Please respect copyright.ＰＥＮＡＮＡcui0zSOosa
}154Please respect copyright.ＰＥＮＡＮＡT3GOnwAM4z
-------------------------------------------------154Please respect copyright.ＰＥＮＡＮＡBjDxa90Tvz
"accept-ranges: bytes" tells client web browser that the http server support range.154Please respect copyright.ＰＥＮＡＮＡ1uaZnTZmEg
"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).154Please respect copyright.ＰＥＮＡＮＡJPFNB34rJv
'ETag: "534231200_1732277116"' tells client web browser the file version.154Please respect copyright.ＰＥＮＡＮＡkUcntqWyRp
'Content-Disposition: attachment; filename="zen.iso"' tells client web browser to download the file as attachment, and its file name.154Please respect copyright.ＰＥＮＡＮＡUUfbPtEgmO
"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.154Please respect copyright.ＰＥＮＡＮＡdmxjH5MbB0
"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.154Please respect copyright.ＰＥＮＡＮＡkYTszU9qtD
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.)154Please respect copyright.ＰＥＮＡＮＡLrtBrnj8vm
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:154Please respect copyright.ＰＥＮＡＮＡ0WyVGdzqIO
---------resume range request---------154Please respect copyright.ＰＥＮＡＮＡkEgiDys06L
GET /zen.iso HTTP/1.1154Please respect copyright.ＰＥＮＡＮＡEIHJiA1Lho
Range: bytes=243220480-154Please respect copyright.ＰＥＮＡＮＡbjZBvZi5ff
If-Match: "534231200_1732277116"154Please respect copyright.ＰＥＮＡＮＡz39i2W6c8n
......154Please respect copyright.ＰＥＮＡＮＡDxyjUlqdXM
-------------------------------------154Please respect copyright.ＰＥＮＡＮＡl2cfhHABjf
"Range: bytes=243220480-" tells the http server to send content from byte 243220480 to end.154Please respect copyright.ＰＥＮＡＮＡTPIpaNhLbM
'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:154Please respect copyright.ＰＥＮＡＮＡyjPnlo4kEi
-------response content from resume point-------154Please respect copyright.ＰＥＮＡＮＡvkljOQUmLN
HTTP/1.1 206 Partial Content154Please respect copyright.ＰＥＮＡＮＡbNBuHuRo3z
Date: Sat, 23 Nov 2024 06:56:17 GMT154Please respect copyright.ＰＥＮＡＮＡE2zxUgVKhC
accept-ranges: bytes154Please respect copyright.ＰＥＮＡＮＡQ1rr3YbFaI
content-range: bytes 243220480-534231199/534231200154Please respect copyright.ＰＥＮＡＮＡUOP8BPKFRL
Content-Length: 291010720154Please respect copyright.ＰＥＮＡＮＡRCuQ3ojv4b
Content-Type: application/octet-stream154Please respect copyright.ＰＥＮＡＮＡc9FU4JY7DD
Content-Disposition: attachment; filename="zen.iso"154Please respect copyright.ＰＥＮＡＮＡ0CrzVMr3ea
ETag: "534231200_1732277116"

(http response body is the file content from resume point to end)154Please respect copyright.ＰＥＮＡＮＡZVwnalk97q
------------------------------------------------154Please respect copyright.ＰＥＮＡＮＡMUfiRdX7PL
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:154Please respect copyright.ＰＥＮＡＮＡx823npceTo
-------response content from resume point-------154Please respect copyright.ＰＥＮＡＮＡNYJKfe0Nes
HTTP/1.1 200 OK154Please respect copyright.ＰＥＮＡＮＡCx0qyVvmdp
Date: Sat, 23 Nov 2024 06:29:19 GMT154Please respect copyright.ＰＥＮＡＮＡzeDGdDePZC
accept-ranges: bytes154Please respect copyright.ＰＥＮＡＮＡaZEjPVxXQa
content-range: bytes 0-634231201/634231202154Please respect copyright.ＰＥＮＡＮＡR8rQ2B3Bb4
ETag: "634231202_2732277118"154Please respect copyright.ＰＥＮＡＮＡau6fECQBIN
Content-Disposition: attachment; filename="zen.iso"154Please respect copyright.ＰＥＮＡＮＡ1hPg2Vdg7P
Content-Length: 634231202154Please respect copyright.ＰＥＮＡＮＡeW1yb4BoRA
Content-Type: application/octet-stream

(http response body is the whole file content)154Please respect copyright.ＰＥＮＡＮＡLudu7z5RI7
------------------------------------------------154Please respect copyright.ＰＥＮＡＮＡYR91PcPqlt
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:154Please respect copyright.ＰＥＮＡＮＡfOIbxo4U7T
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154Please respect copyright.ＰＥＮＡＮＡiXpqGy7hnn
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154Please respect copyright.ＰＥＮＡＮＡrRsl6nh0q7
https://gemini.google.com/

感谢互联网上无私奉献的参考资154Please respect copyright.ＰＥＮＡＮＡflDol7YuXj
https://developer.mozilla.org/en-US/docs/Web/HTTP/Range_requests154Please respect copyright.ＰＥＮＡＮＡ043YANoh7i
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/If-Range154Please respect copyright.ＰＥＮＡＮＡ1ZncXqDbmN
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/ETag154Please respect copyright.ＰＥＮＡＮＡa5SD6GZtrU
https://www.rfc-editor.org/rfc/rfc7233154Please respect copyright.ＰＥＮＡＮＡ0YJrdWUyzu
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]154Please respect copyright.ＰＥＮＡＮＡm5XWmYszVd
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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