RIricketycricket·Jul 24, 2023
davidw is right that ports are somewhat limited, but I haven't had much trouble doing what I need with FFmpeg in particular. I used the bash wrapper from the docs for Port [^1] and it has worked well.
When a stream starts I start a supervisor that then starts a GenServer to manage the port. On init a port is started for FFmpeg (using the above bash wrapper) with args that sends 16-bit PCM audio back to the port through the `handle_info/2` callback.
When a new live HLS segment is downloaded by FFmpeg the entire segment's audio is sent to the GenServer all at once (could be a few handle_info/2 calls, but it happens quickly). Since I want to work in small fixed chunks, I send the segment's audio to an AudioBuffer GenServer (started as a sibling under the same supervisor). This buffer uses binary pattern matching to segment the audio in chunk
Read the rest
s exactly 2 seconds long while keeping any remainder in the GenServer's state for the next buffer event. I then send the chunks to another ChunkBuffer GenServer that pops chunks at 2-second intervals for processing.
Since everything is supervised, if (when...) FFmpeg crashes the supervisor just restarts it. Meanwhile, the audio in the buffer is still processing and nothing goes down. There might be a duplicate word or two in the transcription if the restarted port processes a segment again, but everything keeps running smoothly.
For even more reliability, I have the application running clustered across four locations in the US, EMEA, and APAC using libcluster[^2]. The stream supervisor is started under a Horde.DynamicSupervisor[^3] with a custom distribution strategy. The strategy prefers the region closest to the company HQ, but if it goes down, the processes will be restarted in another region.
[^1]: https://hexdocs.pm/elixir/1.13.4/Port.html#module-zombie-ope...
[^2}: https://github.com/bitwalker/libcluster
[^3]: https://github.com/derekkraan/horde