How to download Dash video files as they appear on the server?

No need to develop a wheel!

The grand tool youtube-dl knows hundreds of platforms/protocols/etc as well as MPEG DASH as well as Apple HLS and so on and so forth! Very often updates/bugfixes

Have fun!

http://rg3.github.io/youtube-dl/

PS.: If you want to keep fragments - use the option

--keep-fragments                 

Keep downloaded fragments on disk after downloading is finished; fragments are erased by default

Upd:

Your playlist has plenty of options (based on video bandwidth), eg:

youtube-dl.exe -F https://dash.akamaized.net/akamai/bbb_30fps/bbb_30fps.mpd
[generic] bbb_30fps: Requesting header
WARNING: Falling back on generic information extractor.
[generic] bbb_30fps: Downloading webpage
[generic] bbb_30fps: Extracting information
[info] Available formats for bbb_30fps:
format code                 extension  resolution note
bbb_a64k                    m4a        audio only DASH audio   67k , m4a_dash container, mp4a.40.5 (48000Hz)
bbb_30fps_320x180_200k      mp4        320x180    DASH video  254k , mp4_dash container, avc1.64000d, 30fps, video only
bbb_30fps_320x180_400k      mp4        320x180    DASH video  507k , mp4_dash container, avc1.64000d, 30fps, video only
bbb_30fps_480x270_600k      mp4        480x270    DASH video  759k , mp4_dash container, avc1.640015, 30fps, video only
bbb_30fps_640x360_800k      mp4        640x360    DASH video 1013k , mp4_dash container, avc1.64001e, 30fps, video only
bbb_30fps_640x360_1000k     mp4        640x360    DASH video 1254k , mp4_dash container, avc1.64001e, 30fps, video only
bbb_30fps_768x432_1500k     mp4        768x432    DASH video 1883k , mp4_dash container, avc1.64001e, 30fps, video only
bbb_30fps_1024x576_2500k    mp4        1024x576   DASH video 3134k , mp4_dash container, avc1.64001f, 30fps, video only
bbb_30fps_1280x720_4000k    mp4        1280x720   DASH video 4952k , mp4_dash container, avc1.64001f, 30fps, video only
bbb_30fps_1920x1080_8000k   mp4        1920x1080  DASH video 9914k , mp4_dash container, avc1.640028, 30fps, video only
bbb_30fps_3840x2160_12000k  mp4        3840x2160  DASH video 14931k , mp4_dash container, avc1.640033, 30fps, video only (best)

now, if you wish you download ALL the segments for all the bitrates, you could try to use the magic option

--all-formats

or you could do it one by one using the --format option, e.g.:

youtube-dl.exe --format bbb_30fps_320x180_200k --keep-fragments 

You may find that there is actually not an easier way readily available to you, unless someone happens to have built a tool to do this, which I am not aware of.

It worth remembering that a DASH stream will usually have multiple ABR (Adaptive Bit Rate) renditions and the original video on the server will actually be stored in multiple different bit rate versions on the server or CDN.

A normal player will select each chunk or segment of the video from whichever bit rate version is most appropriate for the device type and current network conditions.

So, if you wanted to replicate what was on the original server you would have to create a client which downloaded all the segments from all the available bit rate versions of the video. Or alternatively just download the highest quality version and transcode to the other bitrates yourself.

To actually then serve the DASH streams to somewhere else you would also need to make sure you update the manifest to point to the right URL's on your server.

And finally, you would need to check that you have the rights to share it and that the file is not encrypted, in which case you may not be able to share it with clients.

I found a project that downloads the segments as they are stored on the server: https://github.com/Viblast/dash-proxy

I had to fork it to fix a few things for myself though: https://github.com/elv-peter/dash-proxy