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:85Please respect copyright.ＰＥＮＡＮＡ7ylbrea05o
----------http range request-------85Please respect copyright.ＰＥＮＡＮＡvyMwgN3C8A
GET /zen.iso HTTP/1.185Please respect copyright.ＰＥＮＡＮＡdNoIouQQL3
Range: bytes=100-102385Please respect copyright.ＰＥＮＡＮＡm4TaXWVqxV
If-Match: "534231200_1732277116"85Please respect copyright.ＰＥＮＡＮＡWPz1VyHfPg
......85Please respect copyright.ＰＥＮＡＮＡbp0HUCi5eR
-----------------------------------85Please respect copyright.ＰＥＮＡＮＡckdFU9HI63
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:85Please respect copyright.ＰＥＮＡＮＡr8HCk6oahm
----------------html-----------------85Please respect copyright.ＰＥＮＡＮＡQc37TUe1O4
<a href="/zen.iso">zen.iso</a>85Please respect copyright.ＰＥＮＡＮＡ20OMDiJ5cK
-------------------------------------85Please respect copyright.ＰＥＮＡＮＡQ3XfsfsnOT
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:85Please respect copyright.ＰＥＮＡＮＡ3MvtRkxIyi
----------get request------------85Please respect copyright.ＰＥＮＡＮＡ4hZ4THUnpo
GET /zen.iso HTTP/1.185Please respect copyright.ＰＥＮＡＮＡYKrjauCeDq
......85Please respect copyright.ＰＥＮＡＮＡieu3cobCFv
---------------------------------

85Please respect copyright.ＰＥＮＡＮＡtOupx0B70L
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:85Please respect copyright.ＰＥＮＡＮＡPQFCYkUi29
----------------server response--------------85Please respect copyright.ＰＥＮＡＮＡo8eDJ5OPiw
HTTP/1.1 200 OK85Please respect copyright.ＰＥＮＡＮＡZoS4QASdKt
Date: Sat, 23 Nov 2024 06:29:19 GMT85Please respect copyright.ＰＥＮＡＮＡonREgDnixf
accept-ranges: bytes85Please respect copyright.ＰＥＮＡＮＡkuRzV2xvfq
content-range: bytes 0-534231199/53423120085Please respect copyright.ＰＥＮＡＮＡPocfNF6N3y
ETag: "534231200_1732277116"85Please respect copyright.ＰＥＮＡＮＡBTBXHd5icA
Content-Disposition: attachment; filename="zen.iso"85Please respect copyright.ＰＥＮＡＮＡdYU5C8UiOb
Content-Length: 53423120085Please respect copyright.ＰＥＮＡＮＡmnbvNO6JNF
Content-Type: application/octet-stream

(http response body is the whole file content)85Please respect copyright.ＰＥＮＡＮＡD2cUsaegbb
-------------------------------------------

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:85Please respect copyright.ＰＥＮＡＮＡY01JTwraQD
---------------http server send big file content by multiple times--------------85Please respect copyright.ＰＥＮＡＮＡdysEkYFRcr
const int buf_size=20000000; // buf size is about 20M85Please respect copyright.ＰＥＮＡＮＡOe8xx8uazj
int client_socket; char send_buf[buf_size]; FILE *file;

// first send http response headers85Please respect copyright.ＰＥＮＡＮＡ0dJTtTtC4K
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 big85Please respect copyright.ＰＥＮＡＮＡ4SyONPddwt
while(there_is_remain_file_content_to_send)85Please respect copyright.ＰＥＮＡＮＡ8DdEv8UM3e
{ 85Please respect copyright.ＰＥＮＡＮＡNWpr1v3DFw
read_remain_file_content_to_send_buf(file, send_buf);85Please respect copyright.ＰＥＮＡＮＡQVYogoRE6z
send_result = send(client_socket, send_buf);85Please respect copyright.ＰＥＮＡＮＡ63r2ZaF4QX
if(send_result == fail) return; else continue;85Please respect copyright.ＰＥＮＡＮＡbxuzONX7lI
}

// a way to make etag for a file using its file size and last modifed time85Please respect copyright.ＰＥＮＡＮＡN6k7oe9gzB
const char *file_etag(const char *path) 85Please respect copyright.ＰＥＮＡＮＡLUUhycanYi
{85Please respect copyright.ＰＥＮＡＮＡl1xPhVumSO
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;85Please respect copyright.ＰＥＮＡＮＡeLJXYDPOpu
}85Please respect copyright.ＰＥＮＡＮＡjvh2cu7x6r
-------------------------------------------------85Please respect copyright.ＰＥＮＡＮＡwMxIdxo4Ag
"accept-ranges: bytes" tells client web browser that the http server support range.85Please respect copyright.ＰＥＮＡＮＡvQ6DTAdSma
"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).85Please respect copyright.ＰＥＮＡＮＡmGLl8fpWSJ
'ETag: "534231200_1732277116"' tells client web browser the file version.85Please respect copyright.ＰＥＮＡＮＡQBxBqCkcGQ
'Content-Disposition: attachment; filename="zen.iso"' tells client web browser to download the file as attachment, and its file name.85Please respect copyright.ＰＥＮＡＮＡRs1uNrpwou
"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.85Please respect copyright.ＰＥＮＡＮＡymsxFeDM2A
"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.85Please respect copyright.ＰＥＮＡＮＡ7Ila5lf8Wq
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.)85Please respect copyright.ＰＥＮＡＮＡVMLMLorPCW
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:85Please respect copyright.ＰＥＮＡＮＡ9A4gOc2YcE
---------resume range request---------85Please respect copyright.ＰＥＮＡＮＡUeyVqwtF5s
GET /zen.iso HTTP/1.185Please respect copyright.ＰＥＮＡＮＡtRZBgXbfqh
Range: bytes=243220480-85Please respect copyright.ＰＥＮＡＮＡXBRdpPIp0Y
If-Match: "534231200_1732277116"85Please respect copyright.ＰＥＮＡＮＡSCpQqNGGYY
......85Please respect copyright.ＰＥＮＡＮＡWq6M5Ti1C8
-------------------------------------85Please respect copyright.ＰＥＮＡＮＡjAlSmUxKw5
"Range: bytes=243220480-" tells the http server to send content from byte 243220480 to end.85Please respect copyright.ＰＥＮＡＮＡvuUKFjmJBd
'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:85Please respect copyright.ＰＥＮＡＮＡbB2ox49pPk
-------response content from resume point-------85Please respect copyright.ＰＥＮＡＮＡZ66DDWYPL9
HTTP/1.1 206 Partial Content85Please respect copyright.ＰＥＮＡＮＡIhbEx2C91V
Date: Sat, 23 Nov 2024 06:56:17 GMT85Please respect copyright.ＰＥＮＡＮＡgCh4gxE7A2
accept-ranges: bytes85Please respect copyright.ＰＥＮＡＮＡGcvhui6aFY
content-range: bytes 243220480-534231199/53423120085Please respect copyright.ＰＥＮＡＮＡvtornfLt1z
Content-Length: 29101072085Please respect copyright.ＰＥＮＡＮＡhjPorjW8rC
Content-Type: application/octet-stream85Please respect copyright.ＰＥＮＡＮＡk6ugGqQYg4
Content-Disposition: attachment; filename="zen.iso"85Please respect copyright.ＰＥＮＡＮＡlTZj7YuZXC
ETag: "534231200_1732277116"

(http response body is the file content from resume point to end)85Please respect copyright.ＰＥＮＡＮＡDBUTxBdrCo
------------------------------------------------85Please respect copyright.ＰＥＮＡＮＡ9QgcHkX2bj
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:85Please respect copyright.ＰＥＮＡＮＡwnBUSKpiVN
-------response content from resume point-------85Please respect copyright.ＰＥＮＡＮＡV78xfBg7yy
HTTP/1.1 200 OK85Please respect copyright.ＰＥＮＡＮＡMu4d1toOkl
Date: Sat, 23 Nov 2024 06:29:19 GMT85Please respect copyright.ＰＥＮＡＮＡqxS26ujxcT
accept-ranges: bytes85Please respect copyright.ＰＥＮＡＮＡVsHMTa8kWy
content-range: bytes 0-634231201/63423120285Please respect copyright.ＰＥＮＡＮＡuT6lxAJdGW
ETag: "634231202_2732277118"85Please respect copyright.ＰＥＮＡＮＡ3x0OYw1Sbd
Content-Disposition: attachment; filename="zen.iso"85Please respect copyright.ＰＥＮＡＮＡzzz6fKDx2e
Content-Length: 63423120285Please respect copyright.ＰＥＮＡＮＡeXGeuWJJe0
Content-Type: application/octet-stream

(http response body is the whole file content)85Please respect copyright.ＰＥＮＡＮＡw5MRVgllW4
------------------------------------------------85Please respect copyright.ＰＥＮＡＮＡKhxi3wDrAk
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:85Please respect copyright.ＰＥＮＡＮＡP2OtRoV8kq
https://www.zeng.dev/post/2023-http-range-and-play-mp4-in-browser/?

85Please respect copyright.ＰＥＮＡＮＡ3tBb0VNUjF
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85Please respect copyright.ＰＥＮＡＮＡNVNqtHfqEO
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85Please respect copyright.ＰＥＮＡＮＡd4YCQtOLq0
https://gemini.google.com/

感谢互联网上无私奉献的参考资85Please respect copyright.ＰＥＮＡＮＡ8isAtjva3t
https://developer.mozilla.org/en-US/docs/Web/HTTP/Range_requests85Please respect copyright.ＰＥＮＡＮＡULPvw2L4HS
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/If-Range85Please respect copyright.ＰＥＮＡＮＡVuAqKrlnjL
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/ETag85Please respect copyright.ＰＥＮＡＮＡ710Yk6kLoK
https://www.rfc-editor.org/rfc/rfc723385Please respect copyright.ＰＥＮＡＮＡR1olA47hSh
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]85Please respect copyright.ＰＥＮＡＮＡuq2JXGjOLo
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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