Echo server with SFU ExWebRTC
Mix.install([
{:kino, "~> 0.13.1"},
{:ex_webrtc, "~> 0.3.0"},
{:ex_cmd, "~> 0.12.0"}
])SFU: ExWebRTC
The webcam is captured and displayed in a first element.
The browser and the Elixir SFU will establish a WebRTC PeerConnection via a signaling channel: the Livebook WebSocket.
They will exchange SDP and ICE candidates.
Once connected, the Elixir SFU will receive the streams from the via a UDP connection. In this Echo configuration, it will send back the streams to the connected peer via UDP and display them in the other element.
Information flow
- The signaling process (instantiate the PeerConnection):
graph LR;
B1[Browser
PeerConnection]-- ws-->L[Livebook];
L-- ws -->Ex[Elixir SFU
PeerConnection];
Ex -- ws --> L
L -- ws --> B1- The active connection:
graph LR;
B3[video 1]--stream
UDP -->ExSFU[Elixir SFU]
ExSFU --stream
UPD-->B4[video 2];Server-side WebRTC module
defmodule RtcServer do
use GenServer
alias ExWebRTC.{ICECandidate, PeerConnection, SessionDescription, MediaStreamTrack, RTPCodecParameters}
alias ExWebRTC.RTP.VP8.Depayloader
require Logger
defp ice_servers(), do: [%{urls: "stun:stun.l.google.com:19302"}]
defp video_codecs, do: [
%RTPCodecParameters{
payload_type: 96,
mime_type: "video/VP8",
clock_rate: 90_000
}
]
defp setup_transceivers(pc) do
media_stream_id = MediaStreamTrack.generate_stream_id()
video_track = MediaStreamTrack.new(:video, [media_stream_id])
{:ok, _sender} = PeerConnection.add_track(pc, video_track)
%{serv_video_track: video_track}
end
defp setup_ffmpeg do
ffmpeg = "/opt/homebrew/Cellar/ffmpeg/7.0-with-options_1/bin/ffmpeg"
#ffmpeg_cmd = ~w(#{ffmpeg} -i pipe:0 -f vpx -c:v copy -f webm pipe:1 )
ffmpeg_cmd = ~w(#{ffmpeg} -i pipe:0 -f rawvideo -vcodec vp8 -f ifv -y demo/output.ivf)
{:ok, pid} = ExCmd.Process.start_link(ffmpeg_cmd, log: true)
pid
end
# ??? -> can't find ffmpeg in Livebook?
def start_link(args), do: GenServer.start_link(__MODULE__, args, name: __MODULE__)
def connect, do: GenServer.call(__MODULE__, :connect)
def receive_signaling_msg({msg, sender}) do
GenServer.cast(__MODULE__, {:receive_signaling_msg, msg, sender})
end
def peek_state, do: GenServer.call(__MODULE__, :peek_state)
@impl true
def init(_args) do
Logger.info("ExRtc PeerConnection started")
{:ok, %{
sender: nil,
pc: nil,
client_video_track: nil,
video_depayloader: Depayloader.new(),
i: 1,
t: System.monotonic_time(:microsecond),
ffmpeg_pid: setup_ffmpeg()
}}
end
# Livebook calls
@impl true
def handle_call(:peek_state, _, state) do
{:reply, state, state}
end
def handle_call(:connect, _, state) do
{:ok, pc_pid} =
PeerConnection.start_link(ice_servers: ice_servers(), video_codecs: video_codecs())
state = %{state | pc: pc_pid} |> Map.merge(setup_transceivers(pc_pid))
{:reply, :connected, state}
end
# messages received from the client
@impl true
def handle_cast({:receive_signaling_msg, %{"type"=> "offer"} = msg, sender},state) do
with desc <-
SessionDescription.from_json(msg),
:ok <-
PeerConnection.set_remote_description(state.pc, desc),
{:ok, answer} <-
PeerConnection.create_answer(state.pc),
:ok <-
PeerConnection.set_local_description(state.pc, answer),
:ok <-
gather_candidates(state.pc) do
# the 'answer' is formatted into a struct, which can't be read by the JS client
answer = %{"type" => "answer", "sdp" => answer.sdp}
send(sender, {:signaling, answer})
Logger.warning("--> Server sends Answer to remote")
{:noreply, %{state | sender: sender}}
else
error ->
Logger.error("Server: Error creating answer: #{inspect(error)}")
{:stop, :shutdown, state}
end
end
def handle_cast({:receive_signaling_msg, %{"type"=> "ice"} = msg, _sender}, state) do
candidate = ICECandidate.from_json(msg["candidate"])
:ok = PeerConnection.add_ice_candidate(state.pc, candidate)
Logger.debug("--> Server processes remote ICE")
{:noreply, state}
end
def handle_cast({:receive_signaling_msg, {msg, _}}, state) do
Logger.warning("Server: unexpected msg: #{inspect(msg)}")
{:stop, :shutdown, state}
end
@impl true
def handle_info({:ex_webrtc, _pc, {:track, %{kind: :video} = client_video_track}}, state) do
{:noreply, %{state | client_video_track: client_video_track}}
end
# internal messages --------
def handle_info({:ex_webrtc, _pc, {:ice_candidate, candidate}}, state) do
candidate = ICECandidate.to_json(candidate)
send(state.sender, {:signaling, %{"type"=> "ice", "candidate" => candidate}})
Logger.debug("--> Server sends ICE to remote")
{:noreply, state}
end
def handle_info(
{:ex_webrtc, pc, {:rtp, client_track_id, _rid, packet}},
%{client_video_track: %{id: client_track_id, kind: :video}} = state
) do
PeerConnection.send_rtp(pc, state.serv_video_track.id, packet)
state = handle_paquet(packet, state)
{:noreply, state}
end
def handle_info({:ex_webrtc, pc, {:connection_state_change, :connected}}, state) do
#sdp = PeerConnection.get_remote_description(pc).sdp
#[codec, _] =
# Regex.scan(~r/a=rtpmap:([a-zA-Z0-9\s]+)/, sdp)
PeerConnection.get_transceivers(pc)
|> Enum.find(&(&1.kind == :video))
|> then(fn %{receiver: receiver} ->
Logger.warning("PeerConnection successfully connected, using #{inspect(receiver.codec.mime_type)}")
end)
{:noreply, state}
end
def handle_info({:ex_webrtc, _pc, _msg}, state) do
{:noreply, state}
end
defp handle_paquet(packet, state) do
case Depayloader.write(state.video_depayloader, packet) do
{:ok, d} ->
%{state | video_depayloader: d}
{:ok, frame, d} ->
# we measure around 30pfs
n = state.i + 1
t = System.monotonic_time(:microsecond)
q = 30
if (Integer.mod(n, q) == 0) do
Logger.debug(%{
size: Float.round(byte_size(frame)*8/1000,1),
fps: round(q* 1_000_000/(t-state.t))
})
:ok = ExCmd.Process.write(state.ffmpeg_pid, frame)
%{state | video_depayloader: d, i: n, t: t}
else
%{ state | video_depayloader: d, i: n}
end
end
end
defp gather_candidates(pc) do
receive do
{:ex_webrtc, ^pc, {:ice_gathering_state_change, :complete}} -> :ok
after
1000 -> {:error, :timeout}
end
end
endStart the GenServer
Supervisor.start_link([RtcServer], strategy: :one_for_one, name: MySup)and instantiate a new PeerConnection:
:connected = RtcServer.connect()
This will instantiate a ExWebRTC.PeerConnection server-side.
It is waiting for a peer to connect and receive its offer. It will respond with an “answer”, digest the peer’s Ice candidates, and send to the peer its own ICE candidates.
RtcServer.peek_state()Client-side WebRTC module
On connection, a new PeerConnection is created. Since the server is already connected, the client will send an “offer” via the signaling channel. The client expects an “answer” back. It will also send “Ice” candidates via the signaling channel, and expects to receive Ice candidates from the server.
Once the connection is set, the client will receive RTP packets and digest them into a element.
defmodule VideoLive do
# GenServer to communicate between browser and Livebook server
use Kino.JS
use Kino.JS.Live
require Logger
@html """
Local webcam
Echo webcam
![]()
Some frames
"""
def new() do
Kino.JS.Live.new(__MODULE__, @html)
end
asset "main.css" do
"""
#elt {
display: flex;
flex-direction: column;
align-items: center
}
button {
margin-top: 1em;
margin-bottom: 1em;
padding: 1em;
background-color: bisque;
}
"""
end
asset "main.js" do
"""
export function init(ctx, html) {
ctx.importCSS("main.css");
ctx.root.innerHTML = html;
const iceConf = {
iceServers: [{ urls: "stun:stun.l.google.com:19302" }],
};
function run() {
window.navigator.mediaDevices
.getUserMedia({ video: { width: 300, height: 300 }, audio: false })
.then((stream) => {
// display the webcam in a tag
const videoIn = document.getElementById("video-in");
videoIn.srcObject = stream;
const pc = new RTCPeerConnection(iceConf);
// capture local MediaStream (from the webcam)
const tracks = stream.getTracks();
tracks.forEach((track) => pc.addTrack(track, stream));
// send offer to any peer connected on the signaling channel
pc.onicecandidate = ({candidate}) => {
if (candidate === null) { return};
ctx.pushEvent("ice", { candidate: candidate.toJSON(), type: "ice" });
};
// send offer to any peer connected on the signaling channel
pc.onnegotiationneeded = async () => {
const offer = await pc.createOffer();
await pc.setLocalDescription(offer);
console.log("--> Offer created and sent");
ctx.pushEvent("offer", { sdp: offer});
};
// received from the remote peer (Elixir SFU server here) via UDP
pc.ontrack = ({ streams }) => {
console.log("--> Received remote track");
const echo = document.querySelector("#echo");
echo.srcObject = streams[0];
};
// received from the remote peer via signaling channel (Elixir server)
ctx.handleEvent("ice", async ({candidate}) => {
await pc.addIceCandidate(candidate);
});
ctx.handleEvent("answer", async (msg) => {
console.log("--> handled Answer");
await pc.setRemoteDescription(msg);
});
// internal WebRTC listener, for information or other action...
pc.onconnectionstatechange = () => {
console.log("~~> Connection state: ", pc.connectionState);
};
});
}
run()
}
"""
end
@impl true
def init(html, ctx) do
{:ok, assign(ctx, html: html)}
end
@impl true
def handle_connect(ctx) do
{:ok, ctx.assigns.html, ctx}
end
# received from the browser via the signaling WebSocket, call server
@impl true
def handle_event("offer",%{"sdp" => sdp}, ctx) do
RtcServer.receive_signaling_msg({sdp, self()})
{:noreply, ctx}
end
def handle_event("ice",%{"candidate" => candidate}, ctx) do
RtcServer.receive_signaling_msg({%{"type" => "ice", "candidate" => candidate}, self()})
{:noreply, ctx}
end
# received from the server, send to the browser via signaling WebSocket
@impl true
def handle_info({:signaling, %{"type" => "answer"} = msg}, ctx) do
broadcast_event(ctx, "answer", msg)
{:noreply, ctx}
end
def handle_info({:signaling, %{"type" => "ice"} = msg}, ctx) do
if msg["candidate"], do: broadcast_event(ctx, "ice",msg)
{:noreply, ctx}
end
endVideoLive.new()The PeerConnection server-side is now populated:
RtcServer.peek_state()Dockerfile Livebook
Some packages are missing when you run the official image on Ubuntu.
FROM ghcr.io/livebook-dev/livebook:latest
RUN apt update && apt upgrade -y && \
apt install pkg-config libssl-dev libsrtp2-dev -y && \
apt purge -y && apt clean -y && \
rm -rf /var/lib/apt/lists/*
WORKDIR /data
ENV LIVEBOOK_HOME=/data
ENV HOME=/home/livebook
ENV LIVEBOOK_IP="::"
HEALTHCHECK CMD wget --no-verbose --tries=1 --spider http://localhost:${LIVEBOOK_PORT-8080}/public/health || exit 1
CMD ["/app/bin/server"]docker build . -t lvb:latest -f ./demo/Dockerfiledocker run --rm -p 8080:8080 -p 8181:8181 lvb:latest